Cannabis Grow

The total seed weight was recorded, and we calculated the total seed production by weighing and averaging five groups of 100 seeds. A t-test was used to compare the differences in inflorescence growth and seed production for treatments using massive, irradiated pollen or non-irradiated pollen against the results from open pollination. This analysis was conducted using the R software. Lastly, seeds from the massive pollination experiment were used for a certation experiment. Two hundred seeds were randomly selected from female plants within the same treatment and were then planted in the greenhouse under the previously described conditions. Plant sex was recorded after anthesis. Finally, the sex ratio observed in both the irradiated massive pollination group and the non-irradiated massive pollination group was subjected to a statistical analysis for comparison against the sex ratio found in the open pollination group using a chi-square test.Two powder types were effective in reducing the seed set in Amaranthus palmeri as shown by our dose-response analysis . Data from the two tested powder types were pooled because there was no difference between the full model and the reduced model , as shown in Appendix 1. This suggests four parameters can be fixed across curves of talc powder and wheat powder without significantly reducing the goodness of fit.

The seed set in both treatments was lower than the seed set from open pollination . The effective pollen share in the mixture was 4.81 , vertical grow racks producing a seed set halfway between the lower limit and upper limit. The ED50,the ratio reducing seed set by 50%, was not estimable because the lower limit of the model was greater than half the maximum response . To minimize seed set and conserve irradiated pollen, a mixture ratio of 25%:75% is recommended; this is the smallest ratio that yielded seed set close to the lower limit while minimizing the amount of irradiated pollen required . This ratio can effectively reduce seed set in A. palmeri while conserving the limited resources of irradiated pollen. The lower seed set observed with pure powder application compared to open pollination can be attributed to the physical barrier created by the powder, which covers the stigma and prevents non-irradiated pollen from fertilizing the ovule and producing seeds . With an increase in the proportion of irradiated pollen in the mixture, there is a decrease in the seed set. This is due to the fact that while the irradiated pollen can germinate on the stigma and produce a pollen tube, it is incapable of fertilizing the egg cell, thereby failing to produce any seeds . When selecting an optimal diluent for pollen application, it is crucial to take into account factors such as non-toxicity and preventing any disruption to pollen-stigma interactions. Artificial supplementary application of pollen using non-toxic diluents like wheat flour and talc powder, has shown positive results in various plants. For instance, in raspberry , talc-diluted pollen is employed to enhance fruit production . Similarly, in Cannabis sativa, the use of cryop reserved pollen mixed with wheat flour yields seeds of comparable number, size, and morphology to those produced with untreated firesh pollen .

However, when wheat flour is combined with irradiated pollen of Palmer amaranth, it tends to clump and degrade pollen flow more than talc powder.The combined effect of sterile pollen application starting time, application firequency and application interval had a statistically significant impact on seed production . Initiating application 7 days after anthesis consistently resulted in the lowest calculated seed production per plant, reduced by about 50% relative to the open pollination as shown in Figure 3. Estimated seed production from single applications of sterile pollen at 14 or 21 days after anthesis did not show a significant difference compared to open pollination . Although not statistically significant, a trend was observed where increasing the number of applications or decreasing the interval between them tended to reduce seed production per plant. Based on these findings, the optimal application strategy for the sterile pollen technique is to begin at 7 days after anthesis and apply three times at 7-day intervals. Due to the indeterminate nature of Palmer amaranth inflorescences , flowers varied in age at the time we applied irradiated and sterile pollen, resulting in within individual variation. Additionally, not all plants within the population flower simultaneously, leading to between-individual variation where a portion of the population may not be exposed to sterile pollen. Those flowering variations among female flowers present challenges in terms of the timing for the application of the sterile pollen technique. Furthermore, the initiation of flowering occurred earlier in males than females under both water stress and control conditions . When applying a pollen-powder mixture to female flowers, our aim is to cover as many stigmas as possible while minimizing the influence of naturally occurring pollen on seed development, considering the earlier flowering of males compared to females.

Through our experiments, the optimal application strategy we identified above allows us to achieve minimal seed production while using a reduced amount of irradiated pollen. The time of flower opening in Palmer amaranth marks the onset of a period in which pollen will be released from male flowers and when pollination, fertilization, and seed production occur in female flowers. Therefore, the timing of flower opening in females plays a significant role in determining the appropriate initiation and interval for application of the sterile pollen technique. In Palmer amaranth, flowers on the same plant have a continuous opening sequence, with varying opening times among the flowers. Flower opening behavior in Palmer amaranth can be influenced by factors such as the time of day and the position of the flower within the inflorescence . Based on our observations, it appears that flowers situated in the middle lower part of the inflorescence tend to open first. In many species, including Palmer amaranth, flower opening occurs in the morning, correlated with an increase in temperature and light intensity, and with a decrease in ambient humidity . The majority of plant species, as indicated by studies utilizing the GloPL Dataset , the Konstanz Breeding System Dataset , and the Stellenbosch Breeding System Dataset , demonstrate pollination firequency plays a critical role in determining seed production . The reduced seed-set rate in early-flowering plants was associated with pollen limitation, asobserved in species like Peucedanum multivittatum and Rhododendron aureum . While the failure of pollen tubes to enter ovules is a common cause of reduced seed production, it is not the sole factor contributing to low fertility, as post fertilization ovule abortion has been observed to decrease fertility in alfalfa . Additionally, reports indicate that embryo abortion can also lead to reduced seed production in various plant species, including red clover and garden pea , as well as in plant families other than the Fabaceae .Regarding massive pollination with non-irradiated and fertile pollen in Palmer amaranth, there were no statistically significant differences in plant height, branch number, inflorescence length, dry weight, seed weight, and seed production per plant compared to open pollination . This indicates our assumed tradeoff between inflorescence outgrowth and fertilization rate is not significant. These results suggest that when plants receive a substantial amount of sterile pollen, the presumed trade-off between inflorescence growth and fertilization rate is not significant either. The activity and development of apical and lateral buds, as well as fruits, are controlled by light, temperature, hormone, carbohydrate, and nutrient signaling . These signals enable communication between the shoot apex and lateral sinks , ensuring the plant’s architecture and reproductive capacity align with available resources . In annual plants, growing tables the suppression of inflorescence growth due to fruit load typically occurs at the late stage of inflorescence development, referred to as the end of the flowering transition . For instance, in Arabidopsis , during this phase, the inhibition of inflorescence shoot growth by fruit load is regulated by auxin and carbohydrate signaling . Our findings suggest that the rate of fertilization has a minor effect on inflorescence outgrowth in Palmer amaranth, likely because the development of fruit in Palmer amaranth, which is a thin membranous structure known as an utricle, has relatively low costs. Pollen limitation has two aspects: quality limitation and quantity limitation. Quality limitation refers to the reduced effectiveness of pollination due to the inferior quality of pollen. In Palmer amaranth, irradiated pollen under 300 Gy doses is genetically inactive and cannot fertilize the egg cell to form seeds . In addition, much literature on inbreeding depression has shown that pollen quality effects associated with both self fertilization and mating between related plants can also reduce seed production .

This reduction is likely because embryos homozygous for deleterious alleles die during development. On the other hand, traditional pollen limitation is typically associated with plants receiving an insufficient quantity of pollen grains to fertilize all their ovules . Extensive reviews show that supplemental pollination often increases , and rarely decreases , seed or fruit production. Regarding sex ratio in offspring after massive pollination in Palmer amaranth, results from massive pollination with irradiated and non-irradiated consistently showed the sex ratio in the progeny population is female dominant as predicted by certation theory . It is a prezygotic mechanism of sex determination hypothesized to originate from the competition between a female-determining gamete and a male-determining gamete . As a result, when a heavy load of pollen is dusted on female flowers, the female-determining gamete would rapidly reach and sire more than half the ovules and leave a small proportion of ovules available to male-determining gametes as was found in Silene alba and Rumexspecies . Several other mechanisms have been proposed to account for female bias. In species with sex chromosomes where males are heterogametic, Y-chromosome degeneration may lead to female-biased populations . This is due to sex viability differences and sex-chromosomal genotype performance during pollination and fertilization. Studies on Rumex nivalis, a species with heteromorphic sex chromosomes , show that both certation and gender-based mortality contribute to female biased sex ratios . Research using sex-specific markers across different life stages revealed that female bias starts in pollen and intensifies from seeds to flowering. Environmental factors, like proximity of females to males, affect these ratios ; females nearer to males capture more pollen, resulting in more female biased ratios. Experiments confirm that higher pollen loads intensify this bias , supporting Correns’ certation hypothesis that larger pollen loads increase gametophytic competition, favoring fertilization by female-determining pollen tubes. In conclusion, we investigated how to improve the efficiency of the sterile pollen technique for reducing seed production in A. palmeri under greenhouse conditions. The optimal formulation is to utilize a mixture of 25% irradiated pollen to 75% talc powder by volume, which enhances pollen distribution by improving flow and uniformity. Furthermore, the efficiency of the sterile pollen technique is affected by variations in the timing of female flower opening and interference from naturally-occurring pollen. These factors make it challenging to further enhance the technique’s efficiency. However, through our investigations, we identified the optimal sterile pollen application strategy: initiating application 7 days after anthesis and repeating it three times at 7-day intervals. This strategy allows us to achieve reduction in seed production while also minimizing the amount of irradiated pollen required. Lastly, we found massive pollination of irradiated pollen or non-irradiated pollen did not have an effect oninflorescence growth, but it did affect sex ratio in the progeny population, resulting in slightly female-biased progeny as predicted by certation theory.The transition to agriculture in humans ~10,000 y ago is often cited as the key innovation that led to large and complex human civilizations . Yet, insects have practiced farming on a much longer timescale. For example, fungus-growing ants have farmed specific “cultivar” fungi for ~50 My . Fungus-growing ants include over 250 known species, the most complex of which, Atta leaf-cutting ants, create colonies that rival large cities in population size . Fungus-growing ants obligately rely on their cultivar fungus to digest otherwise inaccessible plant nutrients. In underground fungus gardens, the cultivar fungus breaks down recalcitrant organic material provided by the ants and in return produces specialized structures that the ants consume, making the cultivar fungus a valuable resource for the ants .

Summers and Stapleton have shown that tomatoes grown over reflective mulches averaged approximately 7 percent virus-infected plants, while plants grown over bare soil averaged in excess of 50 percent infection with the same viruses. This approach is currently the only viable means of managing virus disease in these production systems. Growers are cautioned to use only metalized reflective mulches when insect anddisease management is the primary objective. Other colors lack the high degree of UV reflectance necessary to repel incoming insects. Also, the mulch must be applied prior to seedling emergence. Plants may be inoculated with aphidborne viruses in the cotyledon stage, and any delay in applying the mulch could lead to an extensive infection. Tomatoes may suffer some leaf burn and early stunting but rapidly outgrow these problems and soon surpass plants grown over bare soil. Eggplant tends to be more sensitive and may suffer prolonged injury. We have observed no problems with any cucurbits grown over metalized mulches. Plants, particularly cucurbits, grown over metalized mulches may be more susceptible to frost injury in the late fall. More widespread adoption of the use of UV-reflective plastic mulches in California has not occurred largely because of added costs associated with their use and disposal and a general lack of familiarity with production techniques required for their successful use.Southern California’s Coachella and Imperial Valleys are major production regions for a wide variety of winter vegetables. Fields in these valleys are typically fallowed during the summer due to high temperatures that preclude the production of most vegetables.

Recently, growing rack researchers at the University of California, Riverside, in conjunction with Riverside County Cooperative Extension, the Indio USDA Natural Resources Conservation Service, and the USDA Agricultural Research Service in Beltsville, Maryland, developed a cover crop mulch production system for this region using cowpea , a warm-season legume. This system relies on the use of large, bush-type cultivars. The cowpea is seeded into preshaped soil beds in June and August. When the cover crop is chopped and converted into a mulch, the above ground biomass can be as much as 2,542 pounds per acre , providing a surface matte that has been shown to control weeds, reduce parasitic nematodes, and provide for vegetable yields comparable to the current bare soil production system. This cover crop mulch production technique has also resulted in increased soil carbon, which is an important management goal of producers in the region.Current agricultural systems rely heavily on the use of herbicides and tillage for weed management, but both have negative impacts on the environment and farm productivity in long-term use . An integrated approach to weed management which incorporates ecological principles and involves using multiple tactics that vary in timing and type of control is needed to reduce the probability of rapid weed adaptation to managemen t practices . Moreover, weed management decisions should aim to prevent soil seedbank inputs rather than just minimize current yield loss for agricultural profitability .

While there have been many studies focused on weed seed biology and seedbank management , research focused on reducing weed seed production by manipulating flowering and seed set is lacking. Flowering plants have developed a wide range of reproductive systems. Most flowering plants are hermaphrodites, having both functional sex organs in the same flower, which is believed to be the ancestral form . Plants have evolved various mechanisms to deviate from this ancestral form such as production of unisexual staminate or pistillate flowers to avoid inbreeding . However, the adoption of any sexual strategy is often accompanied by genetic and demographic trade-offs . For example, dioecy maximizes outcrossing and thereby reduces the likelihood of inbreeding depression . It further allows for more efficient resource allocation between male and female roles by reducing competition between these two functions through sexual specialization . However, successful fertilization in dioecious species relies on proximity and synchronization of male and female flowers. These limitations present an opportunity for the development of novel management strategies for dioecious weeds.Insect sterile technique , an environmentally-friendly and biologically-based method for controlling insect pest, involves sterilizing male insects by irradiation and subsequently releasing the sterile males to mate with wild females , resulting in infertile eggs and reduced insect pest population sizes. Drawing from the success of the IST in controlling insect pests, a similar approach could be effective for weed populations. There are several methods to make pollen grains functionally deficient and thereby reduce seed set .

The most commonly used is ionizing irradiation with X-rays or gamma rays due to their ease of use, effective penetration, consistent results, and minimal disposal issues . Irradiated pollen can germinate and grow pollen tubes but fails to fertilize egg cells . Moreover, when sterile pollen is artificially applied to stigmas, it can obstruct the fertilization process by fertile pollen, thus disrupting seed production. Pollinating female plants of Amaranthus palmeri with sterile pollen resulted in 40% reduction in the number of newly formed seeds . This also has been demonstrated in various plants like apple, pear, citrus, cacao, and melon . The genus Amaranthus contains about 75 species worldwide . Most Amaranthus species are monoecious whereas a few species are dioecious . Dioecious Amaranthus were originally placed into a single subgenus due to dioecy but in recent phylogenetic studies have been grouped with monoecious species in various clades . Palmer amaranth is indigenous to the southwestern United States and northern Mexico . Initially considered an insignificant weed in fields of cotton, soybean, and maize in 1974, it is now one of the top 10 most troublesome weeds in the US . This is due to a combination of human-assisted seed dispersal and new habitat creation through agricultural expansion . Amaranthus palmeri has emerged as one of the most economically damaging weeds in the United States, causing yield losses of up to 91% in corn , 79% in soybean , and 65% in cotton . Furthermore, A. palmeri has evolved resistance to nine herbicide classes and is able to produce up to one million seeds per plant . This weed is a particularly suitable candidate for exploration of the sterile pollen technique for weed control. As a dioecious species with separate male and female plants, it relies heavily on cross-pollination for successful seed production. Theoretically, this makes it feasible to collect pollen grains from male plants, sterilize them, and subsequently release pollen onto female plants.The separation of sexes is the norm in animals, but it is rare in plants . Most angiospermous plants are hermaphrodites, having both functional sex organs in the same flower, which is believed to be the ancestral form . Plants have evolved various mechanisms to deviate from this ancestral form such as production of unisexual staminate or pistillate flowers to avoid inbreeding . Only about 6% of plant species are dioecious , i.e. having male and female reproductive structures on separate individual plants. Despite its rarity, dioecy has developed from hermaphroditic ancestors independently in numerous taxonomic groups, with at least one dioecious species found in 50% of angiosperm families . Historically, two types of unisexual flowers have been defined based on the developmental mechanisms of the evolution from hermaphroditism. In type I, flowers initiate reproductive structures of both sexes but ultimately become unisexual by arresting the development of the inappropriate sex, such as most dioecious Anacardiaceae . In type II, unisexual flowers have only one set of sex organ primordia that emerges from the floral meristem, such as spinach . It is believed that sex determination of type II occurs before initiation of stamens or carpels ; hence, flowers of either sex exhibit no hermaphrodite stage. Most species can be characterized as type I where the arrest of development of stamens or carpels can be triggered by diverse mechanisms at any point from initiation of stamen and carpel primodia to post-meiosis . For example, hydroponic rack system the development of male and female structures in pistillate flowers resembles hermaphroditic flowers until the end of stage seven in Silene latifolia when stamen development is terminated after anther differentiation . In Celtis iguanaea, gynoecium development arrests before carpal elongation in staminate flowers whereas androecium development in pistillate flowers terminates beforepollen maturation . Thus, it has been speculated that the termination of carpels and stamens rarely happens by a typical procedure or at a certain stage . In angiosperms, sex chromosomes have been identified in about 150 species, with half the species having visible heteromorphic chromosomes . In species with no sex chromosomes, there are often sex-determining genes that either suppress maleness or femaleness .

For instance, in Silene latifolia , there are two important regions in sex determination: one inhibits gynoecium growth and the other supports androecium development . Additionally, programmed cell death has been proposed as a main force in driving the development of unisexuality in some angiosperms ; PCD terminates the gynoecial development in male flowers of Zea mays and anther development in Opuntia robusta . The genus Amaranthus contains about 75 species worldwide and includes crops as well as invasive weeds . Most Amaranthus species have a monoecious breeding system whereas a few species are dioecious . Dioecious Amaranthus were originally placed into a single subgenus due to dioecy but in recent phylogenetic studies have been grouped with monoecious species in various clades . Because of the widespread evolution of herbicide resistance, several weedy Amaranthus species have received increasing attention recently. Palmer amaranth , one of the most devastating weeds in the US, is a dioecious summer annual weed . It ranked as the worst weed in US corn fields in a survey conducted by the Weed Science Society of America . Furthermore,A. palmeri has evolved resistance to nine herbicide classes and is able to produce up to one million seeds per plant . To control this noxious weed, we must adopt a multi-tactic approach that incorporates ecological principles into weed management practices. Understanding the reproductive biology of this species can aid in the development of agronomic strategies for management and mitigation of herbicide resistance . Only a few species have been studied for the potential loss of sexual function in pistillate or staminate organs during floral morphogenesis and we are aware of no study of A. palmeri. Characterizing floral development in A. palmeri is extremely difficult due to the minute flower buds size . The objectives of this study were twofold: to compare floral organogenesis in pistillate and staminate flowers of A. palmeri using scanning electron microscopy and to define stages of floral development in A. palmeri. Considering the higher prevalence of type I , our hypothesis was that A. palmeri flowers produce structures of both sexes during the early stages of development and then the vestigial organs from the opposite sex are aborted as development continues.Flower buds from both male and female plants were dissected for light microscopy . The inflorescence axes from male and female plants were cut into approximately 1cm segments and these segments were further sliced longitudinally. The cut surface was visualized under scanning electron microscopy to locate axillary floral structures. Multiple buds at different angles from male and female plants were prepared to increase throughput and the likelihood of successful imaging. Bracts and tepals of flowers were removed as much as possible without damaging the reproductive structures. Following dissection, tissues were transferred into 70% Formalin-acetic acidalcohol ; 10:1:2:7 Ethanol : Glacial acetic acid: Formalin : Distilled H2O to fix for 24 hours. Samples were transferred through a dehydration series of 70%, 80%, 90%, 95%, 100%, and another 100% ethanol for at least 10 minutes each step. The tissue in 100% ethanol was critical point dried to preserve the surface structure which could be damaged due to surface tension when changing from the liquid state to gas state. Dried samples were mounted on aluminium stubs with adhesive tabs. Samples were coated with a thin gold-palladium layer by sputter coating for 60 seconds at 12mA. Lastly, samples were visualized with a ThermoFisher Quattro ESEM scanning electron microscopy operating at 5 kV and digital images captured as TIFF files.Flowers of both sexes are about 1 mm diameter, white or whitish green and are produced in dense and compact clusters on cylindrical inflorescences with smaller axillary spikes at the base . Each pistillate or staminate flower is enclosed by 1 to 3 stiff awlshaped bracts .

In addition to diseases, the cause of some disorders of sunflower appears to be varietal and physiological, with no causal diseases yet identified. In one malady observed, the female lines do not drop their heads after pollination. No management recommendations exist for this disorder given that it appears to be varietal and physiological. Foamy head rot of sunflower produces copious white frothy material that exudes from the flower heads, leaf petioles, and stalk . No wounds are obvious, though insects such as stink bugs and ants are often attracted to the sweet, alcohol-smelling frothy sap. Yields and seed quality are reduced, depending on the severity of the problem. This disorder has been observed only in female lines of select varieties, with certain genetics that predispose them to this problem. This condition usually occurs during hot, dry spells with ample soil moisture. Despite extensive testing, no pathogens have been identified as causal agents . Excessive heat may cause splitting or microwounds that allow the ingress of organisms that otherwise would not be harmful, much like Rhizopus head rot following insect or bird damage to flower heads. The foamy disorder was observed in the same female parent lines in 1993 and 2014, where it was fairly uniformly distributed throughout fields. Similar symptoms were found in some female plants in 2017, after a heat wave, harvest drying rack likely in a different parent line compared with earlier years, as damage was not as widespread as occurred in previous years.

Excessive heat waves can also injure sunflowers, especially during the early reproductive stages through bloom . For example, in 2017 in the Sacramento Valley, the high temperatures in mid June were 100o to 106o F for 8 consecutive days during the R1-R4 stage for some fields. Heat damage that followed included flower buds that did not develop or they partially opened , with some seed set, but yields were significantly reduced. Again, the heat damage was observed only in female lines and not in the males. Some parental lines showed more tolerance to the heat wave than others, with less flower damage, indicating differences in parental line susceptibility. There was some indication that heat injury to sunflowers was worse where fields had gravel streaks . This suggests that it is critical to be prepared with good irrigation management practices before heatwaves occur to ensure that sunflowers are not moisture stressed during extreme heat events. Excessively high temperatures at bloom after several heat waves in 2017 also resulted in sunburn on the fully opened female flower heads , followed by a breakdown of tissue that became gelatinous from secondary pathogens.Symptoms were uniform on plants throughout fields where heat related damage occurred, which is a clue that the cause was abiotic or environmental and not due to a disease. In other fields there were empty or undeveloped hulls due to poor pollination from a lack of honey bee activity from the heat wave . Nematodes Nematodes, microscopic, wormlike organisms that feed on plant roots, are seldom a problem in sunflower production .

However, most nematodes found in California have a wide host range, so it is possible that nematode damage could occur. Nematodes that exist in California that might parasitize sunflower include the root knot nematode , which produces galls on the roots , and the lesion nematode , which causes brown, discolored scarring on the roots . If nematodes are suspected, soil samples can be taken and sent to a lab for pest identification. The only economical management tool is rotation away from any susceptible host crop and using resistant plant varieties in crop rotations, such as certain varieties of tomatoes. Weeds Weed control is essential for maximizing sunflower seed yields and quality and for meeting seed certification standards in hybrid sunflower seed production , especially for prohibited and restricted weeds. The most common prohibited weed in California is perennial pepper weed . Canada thistle is another prohibited weed and a problem for sunflowers grown in the Intermountain area . An isolated population of branched broomrape , also a prohibited rated species, was observed in the Sacramento Valley in 2017 and eradication efforts are in progress . Restricted noxious weeds are not allowed in processed seed lots. Noxious weeds with a restricted rating include Italian thistle , slender flowered thistle , field bindweed , and Johnson grass as well as other perennial sorghum species. While the restricted noxious weed species observed in sunflower seed production are smaller seeded and can be effectively removed during seed conditioning, they should be controlled in the field to prevent risk to seed lot contamination. CCIA also requires rouging of corn and castor bean because it can be hard to remove these seeds during conditioning and there is zero tolerance for them in the seed analyses. Weed control in sunflowers begins with knowing the field history and types of weeds in the field. Post emergence herbicide options are limited for broad leaf weed control in sunflowers once the crop has emerged, so avoid fields with weed pressure, especially for the prohibited and restricted weeds listed above.

In addition, knowing the weed spectrum in the field is needed to help determine which herbicides to use at planting. Prepare a good seed bed to ensure a uniform, good stand that will help shade out and compete with weeds. Planting into moisture on subsurface drip systems, where the soil surface remains dry, helps keep weeds under control. Cultivate sunflowers at lay by to help control weeds. Seed companies may also send in roguing crews to manage weeds. Glyphosate is typically used to burn down winter weeds prior to planting, then preplant herbicides are applied for spring and summer weed control. Preplant herbicides are mechanically mixed into the top 2 to 3 inches of the soil with a cultivator. The choice of herbicide depends on the weed complex present in the field. Mixtures of herbicides are often used, with rates highly dependent on the soil texture, as defined in the pesticide label. Sonalan HFP is often used in sunflower production because it provides good control of nightshade . A tank mix of Sonalan and Medal or Treflan provides excellent control of most annual grasses, fair to good control of small-seeded broad leaves including pigweeds and lambsquarters , and fair control of wild oat , but does not control wild sunflower, cocklebur , or wild mustard . Treflan has some soil residual activity and may damage oats or sorghum the following year. Prowl H2O is sometimes used as preplant incorporated herbicide for weed control in sunflower production, depending on the weeds present in the field. For post emergence weed control, Poast and Select Max can be used on grass weeds including cereal grains, corn, and sorghum. Surfactants and AMS adjuvants are recommended to enhance the efficacy of these herbicides. For broad leaf weed control, Shark can be used only as a hooded sprayer application . Clearfield sunflower with imazamox tolerance and ExpressSun with tribenuron tolerance allow for the control of difficult broad leaf weeds, including cocklebur, puncturevine , and wild mustard. When growing varieties with these traits, be aware of plant back restrictions for subsequent crops, such as tomatoes. Volunteer sunflowers can be a problem in subsequent crop rotations, such as wheat. A frost often kills sunflower plants out of season, vertical growing racks but late-germinating seedlings that get buried deep during tillage and grow slowly can escape these events. Destroying the male rows soon after pollination is complete results in less viable seed remaining in the field. However, destroying them too early after pollination will result in any late-blooming females in the field having the potential to receive sunflower pollen from foraging bees exclusively from outside sources, increasing the risk of varietal contamination. In addition, avoid deep tillage if possible after harvest to reduce the frequency of late emerging sunflower volunteers. In small grains, 2,4-D or MCPA can be used for sunflower control. Watch for Clearfield and ExpressSun technology in crop rotations, as these sunflowers will not controlled by imazamox or tribenuron herbicides.Sunflowers are vulnerable to damage by birds because the seeds are exposed and the large head serves as a perch during feeding, and birds favor them.

Although many species of birds feed in maturing sunflower fields, the greatest losses are caused by migrating flocks of blackbirds . Although blackbirds are protected under the Migratory Bird Treaty Act, cultural practices can be used to manage blackbirds, including bird-scaring cannons that produce an extremely loud explosion . Most growers elect to avoid planting in areas where blackbirds frequent, including the Sacramento bypass, which serves as a wildlife area. Some insectivorous birds are beneficial in sunflower fields when they prey on pests such as sunflower head moth. Also watch for bird damage on newly emerged seedlings.Only the female plant rows are harvested; males are chopped and disked after pollination and seed set. At maturity , females are often treated with defoliants or desiccants to dry them down more quickly in preparation for harvest. This reduces exposure to vertebrate pests, such as birds, weathering , variable maturity due to multiple soil types, and harvest issues due to live, green weeds.Harvest aid products used include the herbicide paraquat, a desiccant, as well as sodium chlorate–based defoliant materials . Sodium chlorate materials are contact only and are very effective as defoliants during hot, dry conditions. Paraquat is most effective during cool or cloudy weather when it is better absorbed into the stem and leaf material, allowing it to diffuse more readily into the plant before being activated by sunlight. Never use glyphosate , as this herbicide will affect seed germination when applied to seed production fields. Sodium chlorate is best applied by ground for maximum coverage. Harvesting is subject to the supervision of the county agricultural commissioner, who must be contacted prior to harvest. After the CCIA completes the three required field inspections, they issue a field inspection report. Once the seed company has the report and is ready to harvest, they notify the grower to contact the agricultural commissioner for the harvester inspection. The agricultural commissioner’s office will not inspect a harvester or authorize harvest until they confirm that CCIA has issued the field inspection report. A county agricultural commissioner biologist must inspect the harvester to verify that the harvester has been properly cleaned of any contaminating seed sources . When approved, harvesting is usually done by growers using their own combine and header, when the seed moisture is less than 12 percent. Proper combine setup is essential to prevent seed loss and injury, excess foreign material, and weed seeds in the harvested product.Sunflower seeds are hauled to the warehouse for a precleaning, also known as scalping, to reduce the foreign material to 5 to 15 percent of the volume by the contracting seed company. The goal is to reduce the level of foreign matter to 3 percent or less to obtain 97 percent pure seed and to remove nonmarketable seeds prior to sizing. After scalping, the seed is further cleaned to bring the product to 98 percent pure seed and to ensure desirable seed size. Growers will be docked if extra conditioning is needed, such as clean out fees for low germination or impurities such as weed seeds. All seed must be sampled and tested after conditioning and lots are certified as required by export markets. For certification, all samples drawn for seed analyses must be drawn by an Association of American Seed Control Officials accredited seed sampler. Currently, CCIA trains AASCO accredited seed samplers. Every 3 years, all accredited samplers must renew their accreditation by attending training seminars that CCIA conducts. If a company or conditioner does not have an accredited sampler on staff, an accredited individual from the agricultural commissioner’s office will draw samples for them. After the seed company submits information on the seed lot and seed analysis to CCIA, CCIA reviews it. When standards are met, CCIA issues a seed inspection report, after which, companies may request tags. If the seed is intended for shipment to a country that is a part of the OECD Seed Schemes , a company must also apply through CCIA for an OECD certificate. The tags, OECD certificate, and phytosanitary certificate must accompany the shipment.

Although we sampled populations from Uruguay, we sampled them from the Uruguay River, in‐between Argentina and Uruguay, and speculate that the sample is likely genetically similar to those weevils in Santa Fe, Argentina. Thus, rather than due to multiple introductions, the higher allelic richness in Florida, Texas, and California, USA, may have been due to the temporal proximity of these populations to the initial imported population from the native range . In addition to clarifying the introduction pathways, our population genetic analyses demonstrated the presence of several distinct and broad genetic clusters for each N. bruchi and N. eichhorniae. In the case of N. bruchi, FLOCK and DAPC indicated two main genetic clusters and 11 sub‐clusters For N. eichhorniae, FLOCK and DAPC signified four to six main genetic clusters and 23 sub‐clusters. In comparison, the STRUCTURE program detected two to six distinct broad populations for each weevil species, but did not detect sub‐clustering within these populations . This indicates that significant divergence occurred among and between several of the introduced populations and the native population since the initial introductions in the 1970s. This supports previous studies on invasive species and biological control agents that demonstrate the divergence of populations from the native range but see Franks et al. . Divergence of introduced populations from the native populations likely depends on the time since the initial introduction. For example, we sampled populations almost 50 years after the initial introductions, harvest drying rack whereas Franks et al. sampled in the introduced range just 2 years after the initial releases.

One caveat that we acknowledge is that the genetic divergence between the introduced and native range may have been due to the fact we sampled from Uruguay rather than Argentina, where the actual initial source populations were exported from. However, based on the DAPC and FLOCK analyses, the populations from Uruguay for both species appear to be genetic sources for several of our populations. Thus, we feel confident that the genetic composition from weevils in Argentina compared to those in Uruguay is not very different. In addition to the results demonstrating that genetic drift and inbreeding occurred in several populations, we speculate that divergence has also occurred due to local adaptation to some of the regions of introduction. Many of the introduced regions that we tested in this study have colder climates than that occurring in South America. Recently, Reddy et al. tested the cold‐temperature tolerance and life‐history performance of N. eichhorniae under cool temperature conditions simulating the fall season in Sacramento– San Joaquin River Delta, California. Reddy et al. tested the same populations of N. eichhorniae used in the present study and found that weevils from the population in Australia had a higher fecundity under these cool temperature conditions compared to weevils from California and Uruguay, SA: Kubusi River. These results were surprising as the population in Australia had lower genetic diversity than the other populations, thus suggesting that populations can still adapt to local areas even with moderate levels of genetic diversity.

Furthermore, the present study combined with that of Reddy et al. demonstrates that both genetic composition and life‐history performance may have diverged among these populations. We support the recommendation that population genetic analyses be performed prior to the selection and release of biological control agents . The genetic diversity and genetic composition may have implications for the population growth of the biological control agents and their success in controlling the target weed or pest. Although these weevils have shown tremendous success in reducing water hyacinth in a number of countries , less than optimal levels of control has been found in regions with cooler temperatures, including some of the high altitude areas in South Africa and in the Sacramento–San Joaquin River Delta, in northern California . The lower efficacy of biological control in these regions could be due to climatic mismatch and/or the inability to thrive and adapt to the local area based on the genetic diversity and composition as influenced by importation methods and the selected source populations.Hybrid sunflower seed production involves planting male-fertile parental lines and malesterile parental lines in separate rows within a field. Cross-pollination between the two lines produces a hybrid seed that contains the best traits of each parent. Males generally have multiple flowers per plant, compared with female plants with a single composite flower. Honey bees are relied on to move pollen from the male to female lines. After pollination, when the seeds are set, growers chop the male rows to prevent any male seeds from contaminating the female lines at harvest.

When the female sunflower stalks dry down at the end of the growing season, they are harvested, with yields averaging 1,200 pounds per acre, depending on the yield potential of the female parent . Prices paid to seed growers averaged $1.30 per pound over the past 5-years, depending on the hybrid, with lower yielding types having higher prices . The value of planting seed is about five to ten times that of a commercial oil or confectionary seed crop. Sunflower, and other members of this genus, are native to North America. They were domesticated thousands of years ago and likely favored as an important high-energy food source by Native Americans. For the oil-seed market, sunflower oil is often preferred by the food processing industry because it is stable at high cooking temperatures. The oil also supplies more vitamin E than any other vegetable oil, and the varieties with high levels of oleic oil deliver low levels of saturated fat. The small-seeded oil types are also important for birdseed markets. For the confectionary market, the non-oil, large-seeded white-stripe sunflower types are important for the snack food industry. The United States grows about 1.7 million acres of sunflower, mostly for oil, primarily in the Dakotas, although the production area stretches south to the panhandle of Texas. In most years, about 25 percent of Californiahybrid sunflower seed goes to international markets, though in some years it can be as high as 60 percent. Russia and Ukraine are California’s largest export markets, where more than 50 million acres of sunflowers is grown for oil, followed by China and the European Union.Management practices for sunflower production depend on the growth stage of the plants . Sunflowers reach physiological maturity at the R9 stage and can be harvested once the desired seed moisture content is reached. In the Sacramento Valley, for a March planting time, maturity is reached in 130 to 140 days from the planting date. For April and May plantings maturity is typically reached in 120 days. In the Imperial Valley sunflowers are typically planted in February, with 130 to 140 days to harvest from the planting date. In general, sunflower fields initiate bloom about 60 to 70 days after planting, bloom for about 10 days, and then are harvested about 60 days after bloom. Sunflowers are in the Compositae family. The sunflower head is actually not a single flower but a flower head consisting of numerous small individual florets. The outside florets resembling petals are called ray flowers and are sterile. The numerous disk flowers in the center produce seed and normally are considered ‘perfect’ . However, in hybrid seed production the female lines have cytoplasmic male sterility, so they produce only female florets, enabling cross-pollination with a male line .Sunflower breeding programs for hybrid seed development are primarily driven by private seed companies. Hybrid sunflower varieties are first produced when plant breeders cross-pollinate two different inbred selection lines where each of the parents contain complimentary traits . The first generation of a hybridized plant cross tends to grow with more vigor and produce higher yields and oil content than the parental lines. The hybrids also display better disease resistance, self-compatibility , vertical growing racks and uniformity in height, maturity, and moisture content, which facilitates harvest. The two main types of sunflower varieties are oilseed and non-oilseed . Oilseed varieties have a very high oil content ; they are processed into sunfloweroil, and the resulting meal is used as a high-protein animal feed. Small-seeded oil types are also used for birdseed.

The confectionary types usually produce a larger black and white striped seed and are used in a variety of food products ranging from snacks to bread. Most of the oilseed sunflower hybrids planted in North America fall in the mid to high oleic fatty acid content range. These edible oils are stable, with no trans fats, are naturally low in saturated fats, and resist rancidity in long-term storage. Hybrids with high linoleic acid content produce a polyunsaturated oil, which is considered to be less healthy and not preferred by the U.S. food manufacturing industry. However, the bulk of the rest of the world produces sunflower oils high in linoleic fatty acid content. In some years, 60% of the seed production acreage is for export, with a predominance of high-linoleic types. Commercial sunflower seed is virtually all proprietary and owned by companies such as NuSeed, DOW-DuPont-Pioneer, Syngenta, Limagrain, and Bayer-Monsanto, who contract with growers to produce the seed. There are no genetically modified sunflowers currently on the market. Because the Helianthus genus is native to North America, there is concern regardinguncontrolled gene flow of GMO types into wild sunflowers. Herbicide-tolerant hybrids such as Clearfield with imazamox tolerance and ExpressSun with tribenuron tolerance were derived through conventional breeding, using wild sunflowers that already exhibited these genetic traits.Hybrid sunflower seed production must follow strict standards to meet California state export mandates for all certified sunflower fields. These include crop rotation, weed and disease control, simultaneous bloom of male and female lines, and field isolation in time and space to ensure varietal purity . Standards are set by the University of California Division of Agriculture and Natural Resources California Crop Improvement Association and the California Department of Food and Agriculture . To meet the standards, fields are certified by two agencies, CCIA and the county agricultural commissioner in the county where the sunflower seed is produced. CCIA inspects fields three times during the season for genetic purity, appropriate field isolation including no wild or volunteer sunflowers, and weed control. The first inspection occurs during prebloom , the second during early bloom , and the third during mid to late bloom . When “off types” are found, the seed companies will send in crews to rogue the fields for other varieties, to within prescribed tolerances. Any pollen-shedding females, called “shedders,” will also be rogued. Additional roguing may be required if either the seed company agronomist or the CCIA inspector continue to find excessive levels of off-types, shedders, or diseased plants in the field. CCIA inspects fields for noxious weeds that fall into two classifications: prohibited and restricted. The list of plants prohibited under the California Seed Law , including seeds and propagules, are updated periodically and listed on the CCIA website . There is zero tolerance in certified seed production for either prohibited or restricted weeds in both the field and seed lots . CCIA may reject fields and refuse certification due to unsatisfactory appearance caused by weeds, poor growth, poor stand, disease, insect damage, and any other condition that prevents accurate inspection or creates doubt as to the identity of the variety. For non-certified seed production, restricted weeds, as well as weed seeds that are difficult to separate from sunflower seed, must be controlled and are subject to tolerance standards after harvest. Certification standards for hybrid sunflower seed production require a 1.25-mile isolation from volunteer sunflowers, wild sunflowers, ornamental sunflowers, and sunflower fields containing a different male line. Failure to accomplish adequate isolation within the prescribed tolerances prior to bloom will subject harvested seed to a mandatory grow-out and possible rejection of seed, depending on the grow-out results for identifying seed purity. To ensure isolation during the growing season, seed companies coordinate planting of seed fields off season, using an electronic isolation map hosted by CCIA . This map, however, does not maintain a comprehensive listing of wild sunflower populations, so companies will generally avoid planting in regions historically known to have large populations of wild sunflowers, such as the Sacramento Delta region and the San Joaquin Valley. County agricultural commissioner biologists inspect sunflower hybrid seed fields twice during the season for diseases. The first occurs during prebud formation , when they primarily look for virus symptoms. The second field inspection is between full bloom and seed maturity , when they inspect for a range of diseases .

Weedy rice emerging in cultivated rice populations could be explained by the quick loss of a few domestication alleles in regions of the genome that experienced relaxation of selective pressure. Based on an evolutionary dynamic described in [75], the evolutionary genetic dynamics we observe here could be those of deep divergence of non-endemic wild species at neutral loci but allele-sharing at key wild-like loci, which necessitates a closer look at candidate genes. An interesting further avenue of research will be to sequence known genetic regions that confer weedy characteristics and whose patterns have been identified in both weedy and cultivated rice to have a better understanding of the evolution of weediness as manifested in the California ecotype . The Rc locus would be a good candidate because most rice cultivated in California would carry the loss of function mutation, resulting in a white pericarp. We can determine if California weedy rice shares polymorphisms associated with the loss of function allele , but has re-evolved pigmented pericarp either by reversion or by changes at another locus. We could also test if California weedy rice has captured the ancestral functional allele through gene flow from another source such as cultivated rice with a red pigmented pericarp or wild or weedy relatives present outside of the US. In closing, weed dry rack we present compelling evidence of rapid independent origin of weedy races of rice from cultivated relatives and contribute to our understanding of adaptive evolution under domestication.

The use of divergence population genetics to track crop and weed interactions, as done in this study, is useful in understanding how weeds evolve and what approaches can be used to best control their spread. However, there is still much to learn about the extent to which contemporary populations diverge in genetic and morpho-physiological background from their non-wild progenitors during de-domestication.Site-specific weed control matches site-specific conditions with the proper herbicide and application rate. Spatially variable herbicide-rate applications can achieve the most effective application, because each part of the field receives a precise amount of herbicide based on its need. The benefits of this technology include a reduction in spray volume and consequently lower herbicide costs, time savings because of fewer stops to refill, and less non-target spraying, which reduces potential environmental risks . Reductions in herbicide use achieved with site-specific applications depend on the level of weeds in the field, but can be as high as 40% to 50% . In an evaluation of site specific, post emergence weed control of broad leaf and grass weeds in corn, Williams et al. showed a 51% reduction in rimsulfuron and an 11.5% reduction in bromoxynil plus terbuthylazine use, compared with conventional herbicide spraying. In a preliminary trial of post emergence weed-patch spraying in spring barley, a non-significant yield increase was observed when weeds were controlled in patches, but 41% less herbicide was used compared with whole-field spraying . We tested the hypothesis that weed patches present in specific locations of a field before the previous year’s harvest indicate where weeds will be present during the following growing season. Mapping these weed patches indicates where herbicides should be applied, and conversely, the absence of weeds indicates where little or no herbicide is required.

Although sampling is often performed on a larger grid than the grid used for pesticide application, geostatistics allows the estimation of weed populations between sample points, and thus the application map can be made to correspond with the width of the sprayer. Our objective was to evaluate site-specific herbicide applications of a pre-emergent herbicide using two types of weed maps developed from weed counts made the previous year, and to calculate the herbicide savings.We conducted a variable-rate experiment on an 11-acre portion of a 79-acre field located in Yolo County. The crops were processing tomato in 1999 and sun- flower in 2000. We developed weed maps from the tomato crop and used them to develop variable-rate applications the following year to sunflower. In sunflower, a pre-emergent herbicide is applied either before planting and mechanically incorporated, or after planting but before crop or weed emergence and incorporated mechanically or by irrigation. We studied the effectiveness of variable-rate application of a pre-emergent herbicide, although this technology can be used for post emergent herbicides as well. Processing-tomato seeds were planted from May 4 to 8, 1999. A preemergent herbicide, napropaminde , was applied in an 8-inch band, centered on the crop row before tomato planting. The field was hand weeded on May 26 and cultivated on June 3. A lay by postemergent herbicide,trifluralin , was applied on the sides of the bed and in furrows on June 20. Another hand-weeding followed on June 27. Furrows and sides of beds were again cultivated on July 26. The crop was harvested from Sept. 10 to 14, 1999. Using weed maps developed from the 1999 tomato crop, we developed variable-rate application maps for the following year. In 2000, sunflower was planted on March 4 and March 23 to 25 . Ethalfluralin was applied post plant, pre-emergent on March 28, followed by two cultivations . Sunflower male plants were destroyed on July 15, and female plants were harvested on July 21 to 22. Both crops were furrow-irrigated.Weed distribution was mapped in the tomato crop in 1999. The density of the weed population was assessed in two ways: by cumulative weed-seedling counts throughout the crop season or by mature-weed counts at the time of crop harvest .

Weed densities were estimated using a grid 165 feet wide and 185 feet long . The measurement unit was a 20-inchby-20-inch quadrat for seedling counts, and a 15-feet-by-17-feet grid cell for mature-plant counts. All data points were assigned north and east coordinates to allow the weed maps to be spatially analyzed in a geographic information system . Weed population densities estimated by the different methods were used to create continuous weed-density maps, utilizing an interpolation method to estimate weed densities between the sampled locations. The interpolated weed-density maps were used to create treatment maps based on weed infestation levels. The field map was divided into a matrix of cells, and the average weed infestation level was estimated for each cell. Infestation levels were defined as weed-free , medium or high . Levels were arbitrarily set to cover the range of observed densities. Herbicide treatment maps were created by assigning varying herbicide rates to each location according to infestation levels, and dividing the field into zones receiving the same herbicide rate. Zones were marked with colored flags. The three herbicide rates were 0, 0.75 and 1.50 pounds active ingredient per acre of ethal- fluralin . A portion of the field with the steepest gradient in weed infestation was selected for a split-plot experiment. The main effect was the treatment map source , and the secondary effect was ethalfluralin rate at three levels . The main plots were 15 feet wide and 2,508 feet long, and replicated four times. Each plot was split into 38 subplots of 15 feet wide and 66 feet long. One of the three herbicide rates was applied to each subplot based on the weed map. Each replication included a three-bed strip, drying rack weed which received a constant, full herbicide rate . One bed strip did not receive any herbicide application and served as an untreated control. All plots except the control were treated on March 28, 2000. Ethalfluralin was applied to the soil surface and cultivated immediately after application to incorporate the herbicide and remove any emerged weeds. The entire study area was cultivated at that time, including plots where no herbicides were applied. The variable-rate herbicide application was evaluated by density measurements of weeds that survived the treatment. Weed density measurements were made 2, 4 and 6 weeks after the herbicide application or cultivation. Measurements consisted of visual estimates of total weed cover for each subplot and counts of weed seedlings in 100-square-inch quadrats placed randomly 10 times per herbicide level in each replication. A prototype variable-rate applicator developed by the UC Davis Department of Biological and Agricultural Engineering was used in the experiment . Zones corresponding to the same treatment were marked with colored flags and rate changes were done manually. The VRA changes the application rate in about 0.1 second. The VRA traveled at a speed of 5 miles per hour , resulting in 1 to 2 feet of travel before the desired application rate was reached. A 3-foot buffer area around each change in herbicide rate was delineated and excluded from measurement after the variable-rate herbicide application.The cumulative weed-seedling density throughout the growing season was 35.6 plants per square yard, and average mature-plant density was 1.2 plants per square yard.

Eighty-five percent of all weeds were from the Solanaceae family and therefore, these were used for the subsequent development of weed density maps. The dominance of a few weed species in arable fields is characteristic of different cropping systems . Since tomato was the crop in 1999, it was reasonable to expect that weeds escaping control would be from the same family . Members of the same family of plants have similar physiology, which would make them less susceptible to herbicides used in that crop. The combination of hand-weeding, cultivation and herbicide treatment reduced the number of weeds reaching maturity. As a result, mature-weed density in the 1999 tomato field was much less than the seedling density. Two weed-density maps were used for the variable-rate herbicide experiment: one created from seedling counts and another based on mature-weed counts. For example, three field surveys were conducted during the 1999 season. Weed densities and distribution measured from the three surveys had a high degree of spatial correlation, indicating that highly infested areas of the field had high densities of nightshade weeds throughout the season. For the tomato crop, it was observed that the weed-seedling density was highest in June, 1.5 to 2 months after planting . Conditions were ideal for nightshade emergence in June given the warm temperatures and a tomato canopy that was still open enough for light to reach the soil surface. Cultivation occurred after the May seedling counts and may have moved seed into the ideal position for germination and subsequent irrigation.Where no herbicides were applied, weed cover was significantly less when using the mature-weed map, because it better-estimated weed cover the following year . There was no significant difference between map source for the 0.75 and 1.5 lb. a.i./acre herbicide rates. Based on seedling emergence 2 weeks after application, overall weed control was significantly better when the treatment maps were based on mature weeds with 58 weeds per square yard than on seedlings, which had 142 weeds per square yard. Weed cover was significantly less at 2 and 4 weeks after herbicide application when treatments were based on mature-weed maps compared with seedling maps, but did not differ at 6 weeks after treatment . Weed cover when no herbicide was applied ranged from 15% to 55%, 2 weeks after the experiment was initiated. High weed cover on some noherbicide plots points to a major pitfall of map-based, variable-rate applications of pre-emergent herbicides: Locations where no weeds were predicted to grow received no herbicide. This prediction was based on the presence or absence of weeds the previous year. The treatment map shows the no-herbicide plots in the middle of the field surrounded by plots receiving medium and high rates. Although this location was predicted to have weeds below the treatment threshold, weed seedlings emerged here the following year. Redistribution of seeds during harvest is probably the main reason for poor estimates in the no-herbicide areas, although other factors, such as seed dormancy and movement of seed by water or animals may also be a factor.There was a significant difference in weed control among herbicide rates. The no-herbicide plots had the highest number of seedlings, averaging 86 plants per square yard, 2 weeks after the experiment was initiated. The average number of surviving seedlings in the medium-rate plots was significantly lower, nine plants per square yard. The plot with ethalfluralin at the full rate had the least number of weeds , but there was no statistically significant difference between the half rate and full rate. All plots receiving the medium or full herbicide rate had weed cover below 10%, 2 and 4 weeks after application. Black nightshade and hairy nightshade were the only weed species surviving the high-rate treatment in relatively higher numbers.

Tetflupyrolimet treatments had complete or near complete control of Echinochloa spp. by 14 DAT, but by 56 DAT, complete control was observed in all treatments regardless of tetflupyrolimet application timing. The grower standard treatment had near complete control of bearded sprangletop at 14 DAT, which later increased to complete control by 56 DAT. Treatments including both PRE and POST applications of tetflupyrolimet had season long complete control of bearded sprangletop. Excellent grass control by tetflupyrolimet is consistent even when combined with other herbicides with or without activity on grasses. Mixing tetflupyrolimet with grass control herbicides does not appear to provide an antagonistic effect on weed control unlike the combination of other rice herbicides, such propanil tank mixed with ACCase inhibitors that was reported by Matzebacher et al. . Because tetflupyrolimet has activity on grasses with no activity on sedge or broadleaf weeds, a combination of tetflupyrolimet with other herbicides that control sedge and broad leaf weeds is required for complete control of the weed species that are common in California rice fields. In 2022, the grower standard treatment showed season long complete control of rice field bulrush . All other treatments showed complete control of rice field bulrush at 14 DAT, which then decreased by 56 DAT to 88 to 97% control in treatments that did not include benzobicyclon plus halosulfuron, vertical grow racks which is an herbicide treatment with known activity on rice field bulrush .

The control of small flower umbrellas edge was similar to results of rice field bulrush control where complete control was obtained by the grower standard and all other treatments at 14 DAT, followed by a minimal decrease in control by 56 DAT for tetflupyrolimet followed by carfentrazone, tetflupyrolimet followed by bensulfuron followed bypropanil, tetflupyrolimet followed by triclopyr plus propanil, and clomazone followed by tetflupyrolimet followed by propanil. The only significantly different treatment for small flower umbrellas edge control was tetflupyrolimet followed by carfentrazone, which still had 72% control of small flower umbrellas edge. The lessened control of small flower umbrellas edge for this treatment is unlikely to be from herbicide resistance because previous greenhouse trials reported that this species did not show resistance to carfentrazone . Complete season long control of duck salad was achieved by the grower standard treatment as well as all other treatments besides tetflupyrolimet followed by carfentrazone, which showed 90% control at 56 DAT. Control of redstem by the grower standard treatment shifted from complete control at 14 DAT to near complete control by 56 DAT. All other treatments not including benzobicyclon plus halosulfuron had complete season long control of redstem. PRE tetflupyrolimet followed by POST benzobicyclon plus halosulfuron had moderate control of redstem at 82%, while PRE benzobicyclon plus halosulfuron followed by POST tetflupyrolimet had minimal control of redstem at 33%. Redstem is known to emerge later in the season, missing the PRE applications . The delayed emergence explains why greater control is achieved when benzobicyclon plus halosulfuron is applied POST rather than PRE, since it has slight control of this species while tetflupyrolimet has no control of redstem. Therefore, herbicides, such as propanil and triclopyr, are needed to successfully control redstem.

In 2023, control of ricefield bulrush by the grower standard at 14 DAT was 98% and 100% at 56 DAT . There was a wide range of 40 to 100% control of ricefield bulrush for all other treatments. Ricefield bulrush control was 65, 50, and 40% with tetflupyrolimet followed by triclopyr and propanil, clomazone followed by tetflupyrolimet followed by propanil,and tetflupyrolimet followed by benzobicyclon plus halosulfuron, respectively. The low rice field bulrush control by these treatments was largely because none of the PRE herbicides in these treatments have good activity on sedges . Furthermore, the 14 DAT rating was only 2 to 3 days after the follow up herbicide application which was not enough time to completely control ricefield bulrush. By 56 DAT, near complete or complete control was observed in all treatments except tetflupyrolimet followed by carfentrazone, which gave 77% control. In 2023, small flower umbrellas edge control by the grower standard increased from 96% control at 14 DAT to complete control by 56 DAT. All other treatments showed either near complete or complete control of small flower umbrellas edge throughout the season. The grower standard showed complete control of duck salad throughout the entire season. In all the other treatments, there was complete control of duck salad at 14 DAT besides tetflupyrolimet followed by triclopyr and clomazone followed by tetflupyrolimet followed by propanil . This varying control is once again because of the application timings, where the PRE herbicides do not have activity on duck salad but by 56 DAT, an application of propanil or triclopyr has been made. Near complete control was shown at 56 DAT in all treatments excluding tetflupyrolimet followed by carfentrazone, which was significantly lower from most other treatments at 83%. The grower standard treatment showed no control of redstem at 14 DAT due to the weak activity of benzobicyclon plus halosulfuron on redstem , however by 56 DAT, redstem was completely controlled by the subsequent propanil and triclopyr applications. All other treatments had complete control of redstem at 14 DAT.

However, by 56 DAT, control of redstem in treatments that did not include an application of propanil decreased, ranging from 63 to 73%, while the treatments that did include an application of propanil achieved near complete control . The control of grass, sedge, and broad leaf weeds by the combination of herbicides used in this experiment showed acceptable control in both years without signs of synergistic or antagonistic effects. There have been a multitude of both herbicide synergism and antagonism cases in rice across the world, such as the synergism of barnyard grass and red rice control when mixing imazethapyr, propanil, and thiobencarb . This suggests that tetflupyrolimet may be more user friendly for applicators and growers than other available rice herbicides, however, it is crucial to understand the weed populations dynamics in a field when choosing herbicide programs to ensure the effective control of all weed species present. The grower standard treatment showed slight rice injury symptoms of stunting and stand reduction by 28 DAT, 4×4 plastic tray which completely recovered by 42 DAT . At 28 DAT, tetflupyrolimet followed by thiobencarb followed by propanil showed 16% injury compared to the non-treated control. Because of the ability of thiobencarb to reduce shoot growth, the application of thiobencarb could have had damaged the root system of the rice causing death of some plants that were not completely anchored to the seedbed . There was no chlorosis observed for any herbicide treatment at 14 DAT, however at 28 DAT, only three treatments – tetflupyrolimet followed by benzobicyclon plus halosulfuron, tetflupyrolimet followed by thiobencarb followed by propanil, and benzobicyclon plus halosulfuron followed by tetflupyrolimet – showed very slight chlorosis. Rice plants, however, completely recovered from chlorosis by 42 DAT. Moderate bleaching was observed in the clomazone followed by tetflupyrolimet followed by propanil treatment at 7 DAT but fully recovered by 14 DAT, which was not surprising because clomazone is known to cause bleachingof rice after its application . There were no bleaching symptoms observed in any other treatments. No significant stunting was observed at 14 DAT for any treatment. At 28 DAT, tetflupyrolimet followed by thiobencarb followed by propanil showed slight stunting symptoms of 10% . This response was not surprising because thiobencarb is known to show stunting in rice . By 42 DAT, however, the stunted rice had completely recovered. There were no significant or lasting necrosis symptoms observed in this study. The average yield for the non-treated control in 2022 was 3,687 kg ha-1 , which was significantly lower than all other treatments . The grower standard treatment yielded 9,118 kg ha-1 , which was not significantly different from any tetflupyrolimet treatment. Of the tetflupyrolimet treatments, the lowest yielding treatment was clomazone followed by tetflupyrolimet followed by propanil at 7,740 kg ha-1 and the highest yielding treatment was tetflupyrolimet followed by thiobencarb followed by propanil treatment 9,552 kg ha-1 . The reduced yield of the clomazone followed by tetflupyrolimet followed by propanil treatment could be due to early bleaching resulting from the PRE clomazone application .

The greater yield from the tetflupyrolimet followed by thiobencarb followed by propanil treatment could be from superior weed control throughout the season that caused a lower level of weed competition to rice. However, in 2023, the only significant difference detected was that the yield of the non-treated control was significantly lower than all other treatments . The difference of higher yields in 2022 and lower yields in 2023 could possibly be due to the cooler weather during the 2023 season compared to 2022 .There was significant interaction between treatment and year for necrosis and yield while there were no significant treatment by year interactions for bleaching, chlorosis, stunting, and stand reduction symptoms. Therefore, necrosis and yield data were analyzed separately by year and all other rice symptom data were combined. In general, slight chlorosis symptoms in both years were observed in the grower standard treatment for ‘CM-203,’ ‘M-206,’ and ‘M-209’ at 14 DAT but rice plants completely recovered by 28 DAT . The grower standard treatment also showed minimal stunting and stand reduction symptoms in all varieties, which fully recovered from stunting shortly thereafter. No tetflupyrolimet treatments, regardless of application timing or rate, had significant levels of chlorosis, bleaching, stunting, or stand reduction at any rating time. In 2022, consistent necrosis in the tips of the rice leaves was observed. At 14 DAT, all treatments besides PRE tetflupyrolimet at 0.125 kg ai ha-1 showed minimal necrosis, however, it does not seem to be a trend of a specific variety showing necrotic symptoms . Importantly, these necrosis symptoms also were observed in treatments that did not include tetflupyrolimet, which suggests that the necrosis symptoms were not related to application of tetflupyrolimet. These minimal symptoms persisted until the end of rice heading stage when they fully recovered before harvest. Average yield data for all tetflupyrolimet treatments were similar to the grower standard treatment within every variety . In 2023, there were no necrosis symptoms observed, which suggests that the necrosis symptoms observed in 2022 were not from tetflupyrolimet, but rather possibly abiotic factors . Furthermore, no necrosis symptoms were observed in any of the other two experiments, regardless of year or site. Genetic variations within species can contribute to differential response of herbicides, however, no varietal response to tetflupyrolimet wasobserved in this study. Rice varietal response was reported in California when clomazone and triclopyr were used . The yield data in 2023 for all tetflupyrolimet treatments were comparable to the grower standard treatment for every variety.Tetflupyrolimet provided excellent season long control of Echinochloa spp. and bearded sprangletop regardless of application timing. There was no trend of late season escapes of bearded sprangletop in tetflupyrolimet treatments unlike other bearded sprangletop control herbicides such as clomazone and thiobencarb. The introduction of tetflupyrolimet to the rice cropping system is contingent upon its ability to perform well in an herbicide program where sedge and broadleaf weeds can be controlled as well. Tetflupyrolimet applied in combination with benzobicyclon plus halosulfuron, thiobencarb followed by propanil, bensulfuron followed by propanil, triclopyr and propanil, or clomazone followed by propanil provided near perfect season long weed control. There were neither synergistic nor antagonistic effects when tetflupyrolimet was applied with any of the herbicides in this experiment. Excellent crop safety was displayed across each experiment, regardless of rate or timing. Tetflupyrolimet gave excellent grass control as both a pre-emergent and post-emergent herbicide incorporated into a weed management program, however, if tetflupyrolimet is applied later than day of seeding, a higher rate is likely needed for the same grass control results. California rice varieties – ‘M-105,’ ‘M-206,’ ‘M-209,’ ‘M-211,’ ‘L-208,’ and ‘CM-203’ – did not show any trend of crop injury caused by tetflupyrolimet. Tetflupyrolimet could be a valuable addition to weed control programs in California water seeded rice, regardless of variety grown.Understanding the evolutionary genetics of adaptation to human-mediated practices like small and large-scale production agriculture is critical to address global challenges including the security of food, fuel, bio-product, and fiber production.

Glyphosate was suggested to cause inhibition of photosynthetic CO2 assimilation as well as a decrease in intermediates of the photosynthetic carbon reduction cycle. Shikimic acid, one of the main products in the EPSPS pathway, is a precursor of pigments, defense compounds, lignin and other important molecules in plants. Interestingly, glyphosate injury was also found to be correlated with chlorophyll content. This research was conducted to examine the joint effects of increased temperature and elevated CO2 level on the sensitivity of weeds to glyphosate. To accomplish this objective, we chose two weed species, C. canadensis and C. album, that differ in leaf surface characteristics, flowering phenology and plant architecture. The specific research objectives were to examine the influence of increased temperatures, elevated CO2 levels, and the combination of both factors on the sensitivity of C. canadensis and C. album to glyphosate and to investigate the mechanistic basis of plant response to glyphosate treatment under these environmental conditions. For both species and all populations, plant survival was highest under the combined high temperature and elevated CO2 treatment. Two out of four C. canadensis populations had a considerably higher percentage of plants surviving treatment with glyphosate under the combination of low temperature and ambient CO2 level than all others . Thus, differences in plant survival between the LT/ACO2 and the HT/ECO2 were not statistically significant for these two populations. However, vertical grow racks for the remaining six populations of both C. album and C. canadensis, the survival percentage differed significantly between the LT/ACO2 and HT/ECO2 treatments .

Large differences in plant survival between current and projected environmental conditions were recorded for populations CA1, CA3 and CCS in which no plants survived glyphosate treatment under LT/ACO2 but 61.1%, 69.0% and 64.0% of the plants tested, respectively, survived under HT/ECO2 conditions . In addition, a higher percentage of glyphosate-treated plants survived under high temperature than under elevated CO2 level . Loss of apical dominance and outgrowth of multiple lateral shoots were observed in glyphosate-treated plants grown under high temperature alone and the combination of both high temperature and elevated CO2 level . This phenotype was consistently observed for C. album , but only approximately 10% of the C. canadensis plants exhibited a loss of apical dominance under HT/ACO2 and HT/ECO2. Despite using the same photoperiod for all treatments, variation in flowering phenology among C. album plants under different temperatures was detected. At the end of the experiment, 21 days after glyphosate treatment, both treated and untreated CA1 plants grown under HT/ACO2 or HT/ECO2 conditions had flower buds or flowers while plants grown under LT/ACO2 and LT/ECO2 did not have visible reproductive structures .Over the four days following herbicide treatment, leaves of glyphosate-treated plants grown under HT/ECO2 exhibited more rapid reduction in chlorophyll content than leaves of plants grown under LT/ACO2 . Te differences in SPAD measurements between the two environmental treatments were statistically significant for both C. album and C. canadensis . Interestingly, differences between species were also observed as glyphosate-treated C. album plants grown under HT/ECO2 exhibited faster reduction in chlorophyll than C. canadensis plants treated and grown under the same conditions . Five days after glyphosate application, leaves of treated plants grown under HT/ECO2 exhibited severe chlorosis and turgor loss thus preventing further measurements. Phosphor images of 14C-glyphosate translocation from the treated leaf to the rest of the plant revealed differences in the distribution of glyphosate within plants grown under different environmental conditions . For both species, differences in glyphosate translocation were mainly observed at 12, 24 and 48 hours after treatment . Higher 14C-glyphosate signal intensity was detected in the shoot and roots of C. album plants grown under HT/ECO2 than plants grown under LT/ACO2 conditions at both 12 and 24 HAT .

A similar pattern of glyphosate distribution was observed in C. canadensis although the differences in glyphosate distribution among plants grown under the different environmental conditions were not as visually distinguishable as in C. album plants . For both species, apparent differences in glyphosate translocation were also observed at 48 HAT. Based on the phosphor imaging results described above, which indicate that the largest differences in 14C-glyphosate translocation between plants grown under different environmental conditions, occur at 12, 24 and 48 HAT, we investigated the absorption and quantified the distribution of 14C-glyphosate in different plant parts of C. album and C. canadensis under different environmental conditions at these time points. Glyphosate absorption differed markedly between the two species . C. album plants grown under HT/ECO2 conditions absorbed 14C-glyphosate in a significantly greater amount than plants grown under LT/ ACO2 within 12 and 24 HAT . However, at 48 HAT, no statistically significant difference in glyphosate absorption was observed between plants grown under the different environmental conditions. Although less 14C-glyphosate was absorbed by C. canadensis plants grown under HT/ECO2, differences in absorption between plants grown under different environmental conditions were not statistically significant . Overall, C. album absorbed substantially more 14C-glyphosate than C. canadensis. Quantification of 14C-glyphosate translocation into different plant parts revealed that significantly more glyphosate was retained in the treated leaf of C. album plants grown under LT/ACO2 than HT/ECO2 conditions at both 24 and 48 HAT . In foliage leaves , low amounts of 14C-glyphosate were found in plants grown under both LT/ACO2 and HT/ECO2 with no statistically significant differences between the environmental conditions . Higher amounts of 14C-glyphosate were found in plant stems under HT/ECO2 than LT/ACO2 although a statistically significant difference between treatments was only observed at 24 HAT . More glyphosate was found in both the shoot apical meristems and the roots of plants grown under HT/ECO2 compared with plants grown under LT/ACO2. For both shoot apical meristems and roots, differences between environmental conditions were statistically significant at 24 and 48 HAT . In C. canadensis, significantly more 14C-glyphosate was translocated out of the treated leaf of plants grown under HT/ECO2 at all harvest time points . No significant differences were observed in the amount of 14C-glyphosate found in the rosette leaves of plants grown under different environmental conditions . However, more 14C-glyphosate was observed in both shoot meristems and the roots of plants grown under HT/ECO2 compared with plants grown under LT/ACO2 at all harvest time points . Significant differences in the quantity of 14C-glyphosate between environmental conditions were observed at both 12 and 24 HAT for shoot meristems , whereas in the roots, significant differences were observed for all harvest time points .Taken together, the results of our study clearly indicate that the control of two major weeds in California agriculture by glyphosate could be reduced under the projected changes in climatic conditions. Compared to current conditions, both C. canadensis and C. album plants were less sensitive to glyphosate under the higher temperatures, elevated CO2 levels and the combination of both environmental conditions, which are predicted for the future. To the best of our knowledge, 4×4 plastic tray this research provides the first experimental evidence of the joint effects of both high temperatures and elevated CO2 levels on weed sensitivity to glyphosate. Reduced glyphosate sensitivity under high temperature and CO2 conditions was observed for all four populations of each species. Although the populations used for this study were primarily chosen from herbicide-free habitats, two C. canadensis populations exhibited a higher percentage of plants surviving glyphosate treatment at low temperature combined with ambient CO2 level than all others.

The wind-mediated seed dispersal, combined with the evolution and spread of glyphosate resistant C. canadensis populations across the Central valley of California, may account for the higher percentage of plants surviving glyphosate under current conditions. The rapid reduction in chlorophyll content , loss of apical dominance, and early initiation of reproductive structures observed in glyphosate-treated plants grown under high temperature combined with elevated CO2 level provide insights into the mechanistic basis of the reduced plant sensitivity to glyphosate under climate change scenarios. It is generally claimed that glyphosate controls weedy plants by binding to, and inhibiting the EPSPS enzyme, which is essential for the biosynthesis of branched-chain amino acids. Interestingly, several recent studies, in addition to this study, have revealed changes in phenological and physiological plant traits caused by glyphosate. Outgrowth of lateral shoots, delayed flower development and reduced stomatal conductance, have been observed in response to glyphosate treatment. Additionally, as a phloem-mobile herbicide, glyphosate exhibits a classic source-to-sink translocation pattern. The influence of glyphosate on photosynthesis-related processes, such as carbon fixation, starch accumulation and general carbohydrate formation, can eventually lead to self-induced limitation of glyphosate translocation. Our fndings suggest that most of the glyphosate that was not retained in the treated leaves was translocated into shoot apical meristems and young leaves , which caused rapid leaf decay and thus reduced glyphosate translocation to other plant organs. Glyphosate absorption differed between the two species. In C. album, significantly higher 14C-glyphosate absorption was observed in plants grown under HT/ECO2 compared to LT/ACO2 conditions. In C. canadensis, 14C-glyphosate absorption was marginally higher, but not significantly, under LT/ACO2 conditions. However, despite the differences in glyphosate absorption between species, the translocation and distribution pattern of 14C-glyphosate within plants, once absorbed, was similar. In glyphosate-treated plants grown under HT/ECO2, glyphosate was translocated more quickly out of the treated leaf to other plant tissues than in plants grown under LT/ACO2 conditions. Moreover, in plants grown under HT/ECO2, glyphosate translocation from the treated leaf into strong sinks was rapid for both C. album and C. canadensis . The rapid movement of glyphosate into shoot apical meristems and roots may reduce the mobility of the herbicide to other parts of the plant thereby reducing the overall sensitivity of plants to glyphosate under higher temperature and CO2 conditions. It has been hypothesized for many glyphosate-resistant weeds that less glyphosate is translocated from the treated leaf to other plant parts compared to glyphosate-sensitive plants. Interestingly, our results suggest a mechanistic basis for reduced plant sensitivity to glyphosate that differs from the altered glyphosate translocation mechanism hypothesized for many glyphosate-resistant weeds. For both C. album and C. canadensis, reduced translocation of glyphosate from the treated leaf was proposed as the mechanism for glyphosate tolerance. Our results suggest that the mechanism leading to reduced glyphosate sensitivity under high temperatures and elevated CO2 levels may differ from that conferring evolved glyphosate resistance in weeds. The pattern of glyphosate translocation observed in C. canadensis and C. album in this study can also explain the loss of apical dominance and the initiation of lateral shoots in glyphosate-treated plants grown under HT/ECO2. It is well-known that auxin moves basipetally from the apical shoot in order to suppress lateral bud growth. Glyphosate translocation into the shoot apical meristem may cause severe damage to this tissue and, as a result, constrain auxin production. Low quantities of auxin and glyphosate at the whole plant level may enable the outgrowth of lateral shoots which, in turn, could lead to increased plant survival after glyphosate treatment and the phenotype observed in this study. In conclusion, we have shown that glyphosate-treated plants grown under increased temperature and elevated CO2 level exhibit reduced glyphosate sensitivity. Thus, the continued over reliance on glyphosate for weed control under changing climatic conditions may result in more weed control failures. In addition, from a practical point of view, the loss of apical dominance and early initiation of reproductive structures, as observed in glyphosate-treated plants grown under high temperature in this study, could further exacerbate weed problems by resulting in an unexpected increase in seed production per plant and rapid replenishment of the soil seed bank. Our translocation studies have revealed variation in glyphosate distribution pattern between plants grown under different environmental treatments. Thissue-specific glyphosate sequestration may be the leading cause for sub-lethal glyphosate quantities at the whole plant level reducing the overall efficacy of the herbicide. Further research is required to determine the exact mechanism leading to the reduced plant sensitivity to glyphosate under altered environmental conditions.Farmers in the region of the central Valley of California usually treat C. album and C. canadensis with glyphosate after seeds germinate and seedlings emerge in February or March during which the daily current temperatures averaged 18°C and current maximum temperatures averaged 27°C.Based on Intergovernmental Panel on Climate Change predictions, future projected extreme temperatures are estimated to be 3–5 °C higher than current maximum temperatures. Thus, the two temperature treatments chosen for this study were 18/12 °C as the current average and 32/26 °C as the projected maximum.

California’s grasslands have been dominated by naturalized exotic forage grasses for 250– 300 years, but nonetheless serve as quintessential examples of “working landscapes” that both support rich biodiversity and provide essential rangeland feed for California’s livestock industry. However, the more recent invasion of two noxious weedy grasses—Elymus caput-medusae and Aegilops triuncialis—into the naturalized forage grass-dominated community has reduced biodiversity, and harmed forage production and livestock welfare. Thus, both weed species are targets for control by for conservation professionals and ranchers alike. Critical information needed to support adaptive management includes information about how to optimize livestock grazing for weed control and how to assess the impacts of treatments over large areas and longer time. Remote sensing has offered promise as a tool for grassland assessments, but it has proven particularly challenging to distinguish among groups of similar grasses and some methods—such as use of hyperspectral imagery —are currently beyond the reach of most managers. To meet the pressing need for information to evaluate management strategies, we thus developed and tested a relatively simple phenologically-based remote sensing approach that used aerial color infrared imagery to detect and monitor weed cover dynamics in this system. To our knowledge, pot growing systems this is the first robust and cost-effective method to distinguish annual weed grasses within an annual grass community.

There is much potential application for this approach because it uses a relatively simple multi-temporal classification approach that is not species-specific, aerial imagery as its backbone, which is easily understood by land managers, and a methodology that is easily applied across a large range of remote sensing platforms, including emerging UAS/drone technologies. Moreover, the mapping scale is fine enough to provide early detection of new noxious weed infestations, which is critical for early eradication efforts that can prevent landscape-scale spread of invasives .We developed this mapping approach in partnership with private ranch managers and conservation professionals to quantify landscape-level effects of management actions on invasion in California grasslands. Previously, we developed means of using remote sensing to monitor field-by-field forage production values in order to support rancher decision-making about grazing regimes and to assess the effectiveness of restoration strategies. This partnership allowed us to select technologies and to develop mapping approaches with land managers in mind. For the base data, we chose aerial CIR imagery because of its long history of acquisition and use, and because it was already familiar to ranch managers. To allow the most effective comparisons, we standardized the imagery by using the red and infrared bands to calculate values of the Normalized Difference Vegetation Index , a classic index of plant greenness that is a good predictor of chlorophyll content in grasslands. Digitized imagery provided aspatial resolution that was fine enough to detect weed patches as small as 1 meter, which is appropriate for the clump size of this vegetation and essential for detecting patch edges and for tracking patch size and shape over time.

Even more importantly, aerial imagery could be acquired within precise windows of time at specific phenological transitions when spectral differences between the weed and forage grass groups were greatest. Moderate spatial resolution imagery from satellite platforms such as Landsat may be available for little or no charge, but the grain size is too coarse to detect the patch dynamics of interest and the schedule for image acquisition is usually too inflexible and infrequent. Finer-scale satellite imagery is now available from numerous commercial companies, but at present is too costly for many ranch managers; future market changes may increase use of such resources by the rangeland management community. The CIR-derived NDVI imagery well captured the biology of our study site, including the overall reduction in greenness during the 2009 drought. Consistent with our findings in 2004 when we first prototyped this approach, weed-dominated patches showed a distinctive signature of directional NDVI change from spring to summer that contrasted with forage-dominated patches in which change had the opposite sign . Thus, the distribution of cover dominated by either or both of two weedy annual grasses—Elymus caput-medusae and Aegilops triuncialis—was best identified with maximum likelihood supervised multi-temporal classifications that took advantage of the combined NDVI characteristics of the weed group in March and in May . Most importantly for studies of vegetation change, this mapping approach worked well in both normal and dry years. In semi-arid regions like California, drought is a frequent challenge managers must contend with, so weed assessment tools need to be robust to dry conditions. During both normal and low rainfall periods, kappa statistic values describing the accuracy of this classification were good to excellent .

The mean producer’s accuracy over time indicated that the maps missed only ~7% of weed-dominated cover. The mean user’s accuracy was a little lower , reflecting the primary emphasis on detecting weed cover , but still strong. It indicated that about 14% of the area identified as weed-dominated on the maps might instead be equal parts weeds and forage, or forage-dominated. Lastly, a key strength of the resulting maps was the new view they provided of weed patch dynamics over large areas at a fine-grained spatial scale. The mapping approach described here distinguished notably more patch detail than previously achieved elsewhere with multitemporal analysis . In use, we found the vegetation classification maps to be so consistent with the landscape that when we loaded maps onto our GPS units and walked through the grasslands monitoring our positions in real-time, the fine-grained map details nearly always matched our field observations. As a tool, these maps offer means of quantifying the persistence of, or fine-grained change in, vegetation types from year-to-year, which is extremely difficult without the assistance of remote sensing methods. The general approach we outline has potential to be applied to weed mapping efforts in many grassland systems.Using the multi-temporal classification described, we were able to produce annual distribution maps of cover dominated by goat grass and medusa head or by forage at the 1-m scale, evaluate change in cover type over time, and quantify vegetation cover differences among management units. For field use, the size and shape of patches made of numerous pixels aggregated together can be readily evaluated by eye. Researchers needing more quantitative measures of patch dimensions or their vector outlines could use additional landscape analysis tools , or object-oriented approaches . At our study site, the maps revealed clear effects of management on weed distribution, with fence-line contrasts across identical soils highlighting management influence. In management units with little to no grazing, weeds dominated a greater percentage of the area than in grazed units, the proportion of area covered by persistent weeds was 1.6–2.8 times greater, and the proportion of total weed cover that was stable from year-to-year was 1.3–1.4 times larger. In contrast, planting racks the area of beneficial forage-dominated cover was greater and more persistent ingrazed units. Although livestock may also spread weeds, these observations are consistent with indications that sheep-grazing can help control medusa head in some cases and with rancher observations that removal of thick weed thatch by livestock consumption or trampling is important in breaking up weed monocultures. Similarly, Mariotte and colleagues found by manipulating thatch quantities that increased thatch enhances the growth and/or seed production of invasive species in California grasslands, while harming the performance of natives. While our investigation does not delve into the specifics of grazing regime impact on these invasive weeds, our findings demonstrate the promise of this mapping tool in quantifying responses to management actions. In ecological terms, this vegetation analysis highlights the extent to which weed monocultures can form and persist in the absence of disturbance. For weeds such as goatgrass, aggregation in patches can reduce the negative impact of interspecific competition and increase seed output per individual.Fine-scale maps of vegetation cover provide essential data both for land managers and for biologists seeking to understand the complexity of plant communities. It is increasingly recognized that to understand community dynamics it is necessary to assess patterns and controllers across multiple scales.

The mapping methods developed here for aerial imagery can be extended to digital imagers on unmanned airborne systems  to increase the potential for rapid, inexpensive, and user-driven assessment of grassland condition. Ultimately, these remote sensing technologies will enable more data-driven optimization of grassland management, paralleling the application of precision agriculture in crop management. For example, GPS-tracking of animal movement could be combined with fine-scale patch data to gain more insight about how specific grazing regimes and range resources drive vegetation change. Alternatively, such landscape analyses could be used to evaluate broader state-and-transition or gradient models of land cover change. Finally, understanding of vegetation patch dynamics and configuration can illuminate a broad range of spatially-explicit ecological processes .Understanding the evolutionary genetics of adaptation to human-mediated practices like small and large-scale production agriculture is critical to address global challenges including the security of food, fuel, bio-product, and fiber production. A central challenge in agriculture is to harness the genetic variation controlling key traits in crops to produce stable populations that can be planted, managed, and harvested effectively. Evolutionary models frame and explain the domestication, continued improvement, and management of cultivated plants. Examining these processes sheds light on the roles of selection and demography on genetic interactions of populations and species during adaptation. During domestication and crop improvement, individuals are selected for predictable traits. The means and variances of these traits in breeding lines over generations depend upon the relative roles of genetics and the environment in shaping variation and the number of alleles at loci governing these phenotypes. Likewise,the additive genetic variance associated with a given domestication trait may control how easy it is to fix a population for a trait value, particularly for traits that are vastly different from wild or weedy close relatives. Domestication is a selection process for adaptation to agro-ecological niches favorable for human use, harvest, consumption, and management. Historical gene flow between wild progenitors and domesticated plant populations ensures that cultivated varieties vary in their composition of domestication versus wild traits. Domesticated lines and wild relatives that can interbreed are common among plants and animals. Genome-wide studies of these interbreeding complexes help us understand how genetic introgression modulates adaptation and the maintenance of species boundaries in the face of gene flow. Although weedy rice physiologically and phenotypically resembles cultivated rice, it differs in several important weedy traits, including seed shattering habit, seed dormancy, protracted emergence, and the presence of red pigmentation in the seed pericarp in many cases. Shattering furthers propagation of the weed because seeds scatter in the field before cultivated rice is harvested. Variation in gene sequence and expression has been shown in many genes related to seed shattering, including qSH1, sh4, and SHAT1. The shattering trait in weedy rice has been shown to re-evolve after fixation of the non-shattering sh4 allele in its domesticated ancestors. Additionally, QTL analysis indicates that shattering has reemerged independently and is controlled by different genetic locations in weedy rice. Variable seed dormancy makes control of weedy rice by crop rotation difficult due to the ability of weedy rice to remain dormant for extended periodsin the field. Protracted emergence patterns make control by chemical means difficult because late or early emerging individuals can escape herbicide applications. Prolonged and highly variable emergence also makes control by non-chemical means, such as cover crops, difficult. Finally, weedy rice is commonly referred to as red rice when characterized by a red-pigmented pericarp. Contamination of commercialrice with pigmented red rice seed significantly lowers its commercial value. Most traits distinguishing crop from weedy forms are determined by recessive alleles of major-effect loci. A subsequent focus on the molecular evolution of genes important in de-domestication can guide our understanding of the tempo and process of evolution in weedy and feralized crop populations, but first we must examine the evolutionary origins and morphologies that characterize emergent weed populations. This knowledge informs agricultural management strategies that account for how weeds evolve and mitigate infestation. Understanding the genetic interplay underlying these processes will predict their directionality, identify traits for crop improvement in the face of new or changing environmental constraints, and outline ecosystem management strategies for sustainability. Cultivated Asian rice and its progenitor O. rufipogon are both diploid AA genome species,which facilitates introgression and the maintenance of hybrid feral forms .

This distribution is to be optimized according to the size and shape of the area in which nutrients and light are provided to plant to obtain the maximum possible yield. These factors are controlled by the spacing between crop rows and the spacing of plants/seeds in a row. For many crops, row spacing is determined as much by the physical characteristics of agricultural machinery used to work in the field as by the specific biological spacing requirements of the crop. According to the crop and machinery used, the accuracy of planting by the precision transplanter/seeder to the desired square grid pattern must be adequate for the operation of agricultural machinery in both longitudinal and transverse crop directions. The current designs of vegetable crop transplanters and seeders utilize several uncoordinated planting modules mounted to a common transport frame. These systems use sub-optimal open-loop methods that neglect the dynamic and kinematic effects of the mobile transport frame and of plant motion relative to the frame and the soil. The current designs also neglect to employ complete mechanical control of the transplant during the entire planting process, producing an error in the final planting position, due to the increased uncertainty of plant location as a result of natural variations in plant size, plant mass, growing trays soil traction and soil compaction. Accurately locating the crop plant, in addition to allowing automatic control of weeds, allows individualized treatment of each plant .

Seeking to ensure minimum physical interaction with plants , different remote sensing techniques have been used for the precise localization of plants in fields. For these localization methods, some authors have decided to address automatic weed control by localizing crop plants with centimetre accuracy during seed drilling or transplanting using a global positioning system in real time . These studies, conducted at UC Davis, have shown differences between RTK-GNSS-based expected seed location versus actual plant position. The position uncertainly ranged from 3.0 to 3.8 cm for seeds, and tomato transplants, the mean system RMS was 2.67 cm in the along-track direction. Nakarmi and Tang used an image acquisition platform after planting to estimate the inter-plant distance along the crop rows. This system could measure inter-plant distance with a minimum error of ±30 cm and a maximum error of ±60 cm. Today, one of the biggest challenges to agricultural row crop production in industrialized countries is non-chemical control of intra-row weed plants. Systems such as those developed by Pérez-Ruiz et al. or the commercial platforms based on computer-controlled hoes developed by Dedousis et al. are relevant examples of innovative mechanical weeding systems. However, the current effectiveness of mechanical weed removal is constrained by plant spacing, the proximity of the weeds to the plant, the plant height and the operation timing. Other methods for non-chemical weed control, such as the robotic platform developed by Blasco et al. , the laser weeding system developed by Shah et al. or the cross-flaming weed control machine designed for the RHEA project by Frasconi et al., demonstrate that research to create a robust and efficient system is ongoing. A common feature of all these technological developments is the need for accurate measurement of the distance between plants. Spatial distribution and plant spacing are considered key parameters for characterizing a crop. The current trend is towards the use of optical sensors or image-based devices for measurements, despite the possible limitations of such systems under uncontrolled conditions such as those in agricultural fields.

These image-based tools aim to determine and accurately correlate several quantitative aspects of crops to enable plant phenotypes to be estimated. Dworak et al. categorized research studying inter-plant location measurements into two types: airborne and ground-based. Research on plant location and weed detection using airborne sensors has increased due to the increasing potential of unmanned aerial systems in agriculture, which have been used in multiple applications in recent years. For ground-based research, one of the most widely accepted techniques for plant location and classification is the use of Light Detection and Ranging sensor. These sensors provide distance measurements along a line scan at a very fast scanning rate and have been widely used for various applications in agriculture, including 3D tree representation for precise chemical applications or in-field plant location. This research continues the approach developed in [21], in which a combination of LiDAR + IR sensors mounted on a mobile platform was used for the detection and classification of tree stems in nurseries. Based on the premise that accurate localization of the plant is key for precision chemical or physical removal of weeds, we propose in this paper a new methodology to precisely estimate tomato plant spacing. In this work, non-invasive methods using optical sensors such as LiDAR, infrared light-beam sensors and RGB-D cameras have been employed. For this purpose, a platform was developed on which different sensor configurations have been tested in two scenarios: North America and Europe .IR light-beam sensors were used in two configurations: first as a light curtain and later a simpler setup, using only one pair of sensors , which simplifies the system while still allowing the objective to be attained. In the light curtain, light-beam sensors were placed transversely in the middle of the platform to detect and discriminate the plant stem in a cross configuration to prevent crossing signals between adjacent sensors. Due to the short range and focus required in laboratory tests, it was necessary to reduce the field of view and the strength of the light signal by masking the emitter and receiver lens with a 3D-printed conical element. In laboratory tests, the height of the first emitter and receiver pair above the platform was 4 cm, and the height of 3D plants was 13 cm. In the field tests, the sensor was placed 12 cm from the soil to avoid obstacles in the field . In both cases, the receiver was set to obtain a TTL output pulse each time the IR light-beam was blocked by any part of the plant. The signals generated by the sensors were collected and time-stamped by a microcontroller in real time and stored for off-line analysis. Technical features of the IR light-beam sensors are presented in Table 1. A LMS 111 LiDAR laser scanner , was used in the laboratory and field testing platforms to generate a high-density point cloud on which to perform the localization measurements. Its main characteristics are summarized in Table 2. The basic operating principle of the LiDAR sensor is the projection of an optical signal onto the surface of an object at a certain angle and range. Processing the corresponding reflected signal allows the sensor to determine the distance to the plant. The LiDAR sensor was interfaced with a computer through an RJ 45 Ethernet port for data recording. Data resolution was greatly affected by the speed of the platform’s movement; thus, maintenance of a constant speed was of key importance for accurate measurements.

To maximize the accuracy of the distance measurements obtained by the sensors, an experimental platform was designed to avoid the seasonal limitations of testing outdoors. Instead of working in a laboratory with real plants, grow tray the team designed and created model plants using a 3D printer . These plants were mounted on a conveyor chain at a predetermined distance. This conveyor chain system, similar to that of a bicycle, was driven by a small electric motor able to move the belt at a constant speed of 1.35 km·h −1 . For the odometry system, the shaft of an incremental optical encoder was mounted so that it was attached directly to the gear shaft and used to measure the distance travelled, thus serving as a localization reference system. Each channel in this encoder generates 256 pulses per revolution, providing a 3-mm resolution in the direction of travel. The data generated by the light-beam sensors and the cumulative odometer pulse count were collected using a low-cost open-hardware Arduino Leonardo microcontroller programmed in a simple integrated development environment . This device enabled recording of data that were stored in a text file for further computer analysis. Several repetitions of the tests were made on the platform to optimize the functions of both light-beam and LiDAR sensors. From the three possible LiDAR orientations, lateral scanning was selected for the field trials because it provided the best information on the structure of the plant, as concluded in [17]. In lab tests, two arrangements of light-beam sensors were assessed: one in a light curtain assembly with three sensor pairs at different heights and another using only one emitter-receiver pair.The initial tests, performed in Davis, CA , were used to assess the setup of the light-beam sensor system and detected only the stem of the plants rather than locating it within a local reference system. Once the tomato plants were placed in the field, tests were conducted at the Western Center for Agriculture Equipment at the University of California, Davis campus farm to evaluate the performance of the sensor platform for measuring row crop spacing. For this test, an implement was designed to house the sensors as follows. The same IR light-beam sensor and encoder, both described in Section 2.1, were used . The output signals of the sensors were connected to a bidirectional digital module , while the signal encoder was connected to a digital input module . Both modules were integrated into an NI cRIO-9004 , and all data were recorded using LabVIEW . In these early field trials, the team worked on three lines of a small plot of land 20 m in length, where the methodology for detecting the plants within a crop line was tested.To continue the study of plant localization in a different scenario, additional experiments were designed at the University of Seville, in which a refinement of the LiDAR sensors and data processing were performed. These tests were conducted on several lines of tomato plants manually transplanted from trays, with the plants placed with an approximate, though intentionally non-uniform, spacing of30 cm. Two of these lines were analysed further, one with 55 tomato plants and the other with 51, and a line of 19 wooden sticks was also placed to provide an initial calibration of the instruments. The methodology followed to analyse data obtained from the light-beam curtain was similar to that described in [21]. The algorithm outputs the moment that the beam was interrupted and associates the beam with an encoder pulse. Because the 3D plants had a wider shape at the top than the bottom , and therefore more interruptions were received, the algorithm had to be adapted to each sensor pair and each height for plant detection. To discriminate correctly between plants for the light curtain case, the developed algorithm implemented a distance range, measured in pulses from the encoder, that allowed the verification of the presence or absence of a plant after the detection of the stem, inferring that interruptions received from the sensors placed at the middle and top heights before and after the stem corresponded to the leaves and the rest of the plant structure, respectively. For the analysis of data obtained from the single pair of IR light-beam sensors, a Matlab routine was developed. System calibration was performed using 11 artificial plants in the laboratory test and 122 real tomato plants in the UC Davis field test.For the analysis of the data obtained from the LiDAR, it is important to mention the high complexity of its data, in both volume and format, compared with those data obtained by the light-beam. This is reflected in the following section, which explains the proposed methodology for obtaining both the aerial point clouds of the tomato rows referenced to the encoder sensor and the tomato plant identification. This is a prerequisite for tomato plant localization. For this purpose, it was necessary to pre-process the data, followed by a transformation and translation from the LiDAR sensor to the scanned point.Invasive weeds substantially reduce forage production and biodiversity in grasslands worldwide , eroding their value for grazing and conservation. Despite continuous effort and resource investment , control of invasive grassland weeds remains a persistent challenge in part because of the logistical demands of detecting and monitoring infestations. Critical steps in control include early detection and eradication of small infestations and prevention of spread to uninfested areas . Once an invading weed becomes established, management strategies such as prescribed burns or timed grazing are then needed to keep the weed from becoming dominant or to dissolve weed-dominant patches already formed.

As more effective herbicides have become available, the need for cultivation has decreased. Over the past two decades, many growers have successfully eliminated cultivation, reducing equipment and labor costs, moisture loss, root damage, and subsequent weed flushes. However, eliminating cultivation can lead to more dependence on herbicides, which may hasten the development of herbicide-resistant weeds.Field monitoring and early detection of glyphosate-resistant weeds are the first and most important steps for preventing and managing glyphosate resistance. Before assuming that weeds that survived glyphosate application are resistant, rule out other factors that might have affected herbicide performance, such as misapplication, unfavorable weather conditions, improper application timing, and weed flushes after application. If multiple weed species are present in the field after glyphosate application, then it is likely that other factors related to performance, and not resistance, are responsible. If a single weed species remains in the field, however, this may indicate possible glyphosate resistance. If a high level of resistance is occurring in a field, you will observe dead plants next to uninjured or nearly uninjured plants, with a few intermediate-responding plants. If a low level of resistance is occurring in the field, dead plants will be right next to plants that appear nearly normal, with the majority of plants having an intermediate response. In early development stages of glyphosate resistance, grow trays it is common to observe patches of single weed species scattered in no particular pattern throughout the field.To prevent weeds from growing and to keep glyphosate-resistant weeds under control, incorporate multiple IPM tools into your weed management plan.

The potential for developing glyphosate resistant weeds should receive serious and thoughtful attention when glyphosate-resistant crops are introduced. Resistance management must be an integral part of the production system. If selection pressure is maintained through the continuous use of the same herbicide, resistance is likely to soon render that herbicide ineffective. Glyphosate controls a wide range of grasses, sedges, and broad leaf weeds and can be highly effective on deep-rooted perennials. The question is whether we can have an effective weed control program that is comprised only of postemergence glyphosate applications. This program would certainly cut down on costs, and it may work initially under certain field conditions, but problems with weed species shifts and glyphosate resistance may develop rapidly. The success of total postemergence glyphosate programs depends on what weeds are present in a field, their severity, proper timing of the herbicide application, and crop competitiveness. Fields withlow weed pressure and few late-germinating weeds are most likely to have acceptable weed control with a total postemergence glyphosate program in the short term, but it is not sustainable and should be avoided. A combination of residual preemergence herbicides followed by postemergence tank mixtures of glyphosate with herbicides that have other modes of action is the ideal herbicide program. If glyphosate is used as a burn down treatment prior to tillage, consider a tank mix of glyphosate with an herbicide that has another mode of action, such as an auxin-type herbicide. If you have glyphosate-resistant weeds such as marestail or hairy fleabane, tank mixes of glyphosate and 2,4-D can give good control, but the treatment must be made when the weeds are small. However, when contact non-selective herbicides such as paraquat replace glyphosate as a base program, antagonism has been reported for some tank-mix partners. To maximize weed control, the timing of glyphosate application is vital in order to maximize weed control and minimize the chances of crop yield loss due to early-season weed competition.

Our research has shown that glyphosate-resistant weeds are less resistant to glyphosate when they are young . Glyphosate should be applied in combination with herbicides that have other modes of action when weeds are small , if glyphosate-resistant weeds are present. Controlling weeds at this stage reduces the chances for crop yield loss as well as the risk of weed control failures with larger weeds that may be under stressful conditions. Applying the appropriate glyphosate rate is another important consideration for consistent weed control. Proper glyphosate rates should be based on weed type, weed size, and spray volume. It is important to match the weed spectrum and size with the appropriate glyphosate rate to reduce chances of weed control failures under extreme conditions. In most cases, the appropriate rate to use for weed control in glyphosateresistant crops is 0.75 pounds acid equivalent per acre of glyphosate. However, larger and more difficult-to-control weeds may require higher rates . In addition, when glyphosate is tank mixed with other herbicides, select herbicide partners that are effective on weeds present in your field. For help in selecting proper herbicide partners, visit the University of California IPM Program’s website , consult the Pacific Northwest Weed Management Handbook , or contact your county extension office for local recommendations.Corn Under ideal conditions, preemergence programs may provide full-season weed control. This requires applying a combination of products at full label rates to extend control. Combining preemergence herbicides with postemergence herbicides or cultivation often provides more consistent weed control and protects crop yields better than one-pass preemergence or total postemergence programs. Application timing is critical to achieve the desired control with postemergence herbicides. Generally, weed tolerance to postemergence herbicides increases with plant height. Cotton The most effective weed management program includes combinations of soil-applied herbicides, cultivation, and postemergence herbicides. Glyphosate, however, is the primary herbicide used to control weeds in glyphosate-resistant cotton, and it provides excellent control in most situations. Two postemergence applications typically are required to achieve full-season control and protect yields. Using soil-applied herbicides in glyphosate-resistant cotton often improves the consistency of control and reduces early-season weed competition that can lead to crop yield loss. This practice also introduces herbicides with an alternative mode of action, reducing the selection pressure for glyphosate-resistant bio-types. Alfalfa Using glyphosate offers alfalfa growers an alternative weed management system that may enhance alfalfa growth and persistence by providing a wider spectrum of weed control. Glyphosate can be applied to glyphosate-resistant alfalfa at any stage of growth. Glyphosate-resistance management should always be implemented when using glyphosate-resistant alfalfa. Repeated applications of glyphosate may result in glyphosate-resistant weeds. Once glyphosate-resistant weeds become established within a field, glyphosate becomes ineffective against those weeds. Measures to reduce the potential of developing glyphosate-resistant weeds can include crop rotation, herbicide rotation, proper herbicide rate, proper herbicide timing, and alternative weed control methods. Canola Various options exist for preventing glyphosate resistance, depending on the weeds that are prevalent in a particular field. Rotate herbicides and use tank mixtures of glyphosate with herbicides that have other modes of action to reduce the chance of developing glyphosate resistance. Depending on the weed populations and pressure, growers should alter crop rotations and herbicide-tolerant canola system selections to ensure a diverse weed control program in the cropping system. Sugarbeet Using glyphosate for weed control is expected to make weed management simpler and more effective for sugarbeet growers. Conventional sugarbeet herbicides must contain many herbicidal ingredients in order to simultaneously manage all the different weed species, whereas glyphosate provides broad-spectrum weed control. Glyphosate may be used before sugarbeet is planted or as a postemergence herbicide after this crop is planted. It is critical to use a preemergence residual herbicide to achieve full-season control and protect yields. In addition, tank mix herbicides that have other modes of action with glyphosate to use prior to planting and during the growing season. Several postemergence herbicides available for sugarbeet have contact activity and may antagonize or reduce glyphosate activity, grow systems for weed especially when glyphosate is used at low rates.Most crops are sensitive to weed competition during the early seedling stage of growth, which can ultimately reduce crop yield and quality . Weeds can also host many pathogens and insects, which can infect crops if left uncontrolled . For example, an outbreak of wild mustard in a cabbage field can attract aphids. Large infestations of Johnson grass in a maize field can host maize dwarf mosaic virus, which allows it to overwinter in the weed rhizomes . Weeds can also compete with the crop by taking up plant nutrients and water by interfering with the crop roots by not allowing the crop plant grow to its desired size, leading to yield loss . High-density leafy green plantings can mitigate weed competition due to the extensive crop canopy cover, blocking sunlight from reaching the soil . In contrast, a lowdensity spinach crop can be severely infested by weeds because more sunlight can reach the soil . Greater light penetration to the soil can lead to more weeds germinating and becoming established creating complication and even difficulty in harvesting due to the weeds getting stuck on the harvester, resulting up to 20% crop loss . Identifying weeds in a field seedbank can help guide weed management plans by knowledge of weed population pressure, seasonal weed germination patterns, life span longevity, and survival to understand the competitive advantage weeds have on crop plants .

Uncontrolled weeds will reproduce and disperse their seed as much as they can before they die creating a seed bank . After seed maturity, seed dispersal occurs with the help of wind, water, animals, and humans . Eventually seeds end up in the soil, which allows the seeds to remain in a dormant state until favorable conditions allows seed germination. . Where annual weeds are allowed to grow and reproduce, they create a weed seed bank that varies by field and therefore can be difficult to control . Lati et al., observed that weeds could impede crop development in the first four weeks of production. A farmer should know the weed seed bank in fields because the critical period to weed control can be based on the timing of emergence and weed management strategies can vary by season to determine crop choice and herbicide selection . Weed management strategies can vary by crop but the ultimate goal is to reduce hand weeding and prevent loss of quality and weed contamination in salad mixes. For example, high value crops, such as lettuce and spinach are fast-growing; therefore, these crops can often be harvested before weed seeds can fully mature leaving little room of weeds to contaminate salad mixes . Weeds and weed seeds are very sensitive to their environment. In addition, fully mature weeds can detect short and long day lengths in order to set flowers and get ready to produce seed . For example, dandelion responds to short day lengths and can set large numbers of seed in the fall . Emergence of weeds like common purslane emergence peaks in July when it loses light sensitivity, whereas burning nettle comes up throughout the year. The time of emergence peaks can vary by species and is crucial to know in order to make an effective weed management plan and prepare for future crop plantings. With a few herbicides available for organic vegetable growers and few new herbicides being introduced for conventional crops, it is necessary to look for non-pesticide solutions. One area that appears promising is the non-chemical soil pasteurization method to manage weeds and pathogens . In the late nineteenth century, steam pasteurization was developed as a method of soil disinfestation. Steam is injected into the soil to disinfest the soil from pathogens, insects, and weed seeds by maintaining 70 °C for 15-20 minutes . Treating an entire field with steam is expensive but strategic steam placement in bands is a way to reduce costs . Band steaming can reduce the viability of weed seed in the bands that are treated where the crop plant will be planted. The effects of steam treatment can last for weeks or months. A study done by Runia showed that steam soil pasteurization can be part of an integrated program to manage some of the most troublesome pathogens like Pythium. spp. caused by pythium wilt and Sclerotinia minor caused by lettuce drop in lettuce production. Pythium wilt can be very destructive toward the end of the growing season affecting plant roots, and lettuce drop can cause total crop collapse affecting the lettuce crown. Soil disinfestation with steam appears to have potential, given the need for better control of pests in organic vegetable production in California. With continued research and further machinery improvement, it may be possible to reduce operating costs on a commercial scale by precision application of steam. This literature review will describe the latest advancements in steam to control pathogens and weeds for California vegetable production.