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The water-seeded production system in California is a common method to suppress weedy grasses and non-aquatic weed species. In California, pregerminated rice seed is air-seeded onto fields with a 10- to 15-cm standing flood and the fields are typically maintained continuously flooded throughout the growing season . The California rice cropping system is again unique because of its presence near growing urban communities and a variety of neighboring high value crops. Surface water used for rice production is mainly derived from reservoirs that capture water in the Cascade Mountain Range and Sierra Nevada from the Sacramento River and the Feather River, respectively . There is potential for contamination of drinking water and water for wildlife by herbicide use in California rice fields, which has historically been documented with the rice herbicides thiobencarb and molinate . Production lands further away from the water sources will also use drainage water downstream as irrigation . Many neighboring crops can be susceptible to pesticide residues at low concentrations and this can be of concern if herbicide residues are present in the irrigation water . Historically, regulatory agencies and the California rice industry have collaborated to implement successful programs to manage and reduce off-target pesticide effects by mandating report of pesticide use, monitoring water quality, indoor grow table and water-holding periods after chemical applications . Pesticide use reporting and monitoring encourage stewardship of chemical use among agencies and applicators .

Water-holding periods prevent the pesticide active ingredient from becoming runoff in the tail water and contaminating non-target areas and organisms. The water-holding period can differ among pesticides based on their physico-chemical properties and degradation pathways . Therefore, it is important to understand the behavior of herbicide active ingredients in the water-seeded system to successfully characterize them in support of sustainable stewardship and efficacious use of chemicals. Herbicide products can be developed in various formulations to assist with weed control, for instance, to achieve longer soil residual activity, reduce crop injury, affect dissipation or forapplicator safety . Formulation is also suggested to influence the potential of the active ingredient to contaminate surface waters . Pendimethalin is a mitotic inhibiting herbicide from the dinitroaniline chemistry, it is a selective pre-emergent that ceases seedling growth shortly after germination of susceptible plants . Physico-chemical properties of pendimethalin are presented in Table 1. Pendimethalin has been proposed for use in water-seeded rice, since it controlled herbicide-resistant grass populations and if labeled would provide an additional tool for management over herbicide-resistant grasses in California rice. However, there has been no work characterizing pendimethalin’s behavior in water from a water-seeded rice field. It is hypothesized, based on the physico-chemical properties, that pendimethalin will not persist in surface water, however, product formulation could affect dissipation in water. Therefore, the objectives of this study were to evaluate the dissipation behavior of pendimethalin across three formulations in rice flood water after an application in a water-seeded rice field.

A field study was carried out at the Rice Experiment Station in Biggs, CA . Because of scrupulous quality assurance for each experimental unit to meet regulatory standards, which led to extensive costs associated with the analysis and labor, the study was only conducted in 2021 with three replications. Individual plots were arranged in a randomized complete block design across the field. Soils at the site are characterized as EsquonNeerdobe , silty clay, made up of 27% sand, 39% silt, and 34% clay, with a pH of 5.1, and 2.8% organic matter. Irrigation waters at the research site on average have a pH of 7.81 and electrical conductivity of 0.12 ds/m. Individual 3- m wide by 6-m long plots surrounded by 2.2-m wide shared levees were made to prevent contamination from adjacent treatments. Water temperature, when delivered from the irrigation canal, can average as low as 13°C, and in the field, it is recommended for the water to not be below 18°C for appropriate rice growth and development . Irrigation water was first delivered on June 2, 2021 into a warming field basin, where it circulated before traveling to the field basin with the plots. To move water inside each individual plot, 5-cm diameter by 1.5-cm length single bend aluminum siphon irrigation tubes were placed over the 2.2-m wide levees. The plots were flooded to 4-inch by June 4, 2021 and maintained at that depth for the duration of the study. ‘M-206’ rice was air-seeded at a rate of 170 kg ha-1 onto the field with a standing flood on June 5, 2021.Rice flood water was sampled at 1, 3, 5, 10 and 15 days after treatment application for each plot and replication separately. At each individual plot, a composite water sample was collected with a glass beaker from four areas in each plot near the center and quickly homogenized in a ~1-L plastic container . Then, 3 oz were poured in a 4-oz tight seal jar and placed in storage at 0°C immediately until delivered inside the lab within four hours. For each individual plot, new containers were used to sample each time. In the lab, water samples were cleaned and 50 mL were allocated from the filtered sample and placed in storage at -20°C until analysis. Daily temperature, relative humidity and solar radiation data were obtained from the California Irrigation Management Information System , Biggs, CA weather station number 244 .

Liquid-liquid extraction methods were modified from USEPA . High pressure liquid chromatography tandem mass spectrometry was employed to analyze for residue in water samples. A standard for pendimethalin, were obtained as a reference to quantify residue in samples. The recovery in water samples was on average 79%. See supplementary material for details on method. Data analysis were performed using R v4.1.2 . Linear regression analysis and analysis of variance was used to determine associations on the concentrations across formulations, rates and sampling time with LMERTEST R package . Means separation with Tukey’s honestly significant difference at α=0.05 was then used where appropriate with EMMEANS R package . The data was log transformed to fulfill homogeneityand linearity requirements for a linear regression .There were differences in concentrations recovered from water samples across rates , sampling time , and formulation by sampling time . At 1 DAT sampling, the EC had the highest concentrations at 73.0 parts per billion  averaged over rates . The CS and EC formulations maintained similar concentrations throughout sampling times after the 1 DAT . The GR maintained the greatest concentrations at 10 and 15 DAT compared to the CS and EC . The differences in dissipation across formulations could be attributed to the formulation properties. The EC is constructed of an oil-water-emulsion with organic solvents, drying rack cannabis while the CS encapsulates the active ingredient in layers of water-soluble polymers . As an oilbased formulation, the EC would make pendimethalin persist in suspension on the water at higher concentrations early on because of the inactive carriers being not water soluble. The encapsulating polymers in the CS would allow the compound to be water soluble and extend the amount of time the compound is suspended in water . These characteristics can explain the higher concentrations early on from the EC formulation compared to the other two formulations. GR herbicide formulations tend to have the active ingredient adsorbed to inert material, allowing slow and continuous release of the active ingredient . This characteristic of the GR formulation may help explain the increases of concentration in water three days after the application of the 3.4 kg ha-1 rate . The delayed increase in concentration was rate dependent, however. Similarly, Ngim and Crosby observed formulation affected dissipation of the insecticide fipronil in water-seeded rice, with the granule formulation being most persistent. A GR pendimethalin application onto a water-seeded rice field may need a longer waterholding period than the liquid formulations. Dissipation generally followed first-order kinetics . The GR demonstrated halflives up to 6.9 days. The CS had half-lives three to four days less than GR and the EC had halflives nearly seven days less . The average daily temperature for the duration of the study was 25°C with a low of 16°C and high of 34°C. Daily solar radiation averaged 346 Watts m2 with a low of 341 Watts m2 and high of 366 Watts m2 .

Relative humidity averaged at 50% with a low of 30% and high of 80%. These are the typical conditions during the early rice growing season in California and are important to note as factors that can affect the pendimethalin degradation. Half-lives of pendimethalin in water were reduced in this study probably due to greater degradation occurring in a field environment stimulated by microorganisms, photolysis degradation and partitioning onto organic sediments from the soil . Pendimethalin residue half-lives in water have been previously reported at 12.7 and 13.7 days afteran application of an EC pendimethalin formulation at 0.5 parts per million and 1.0 ppm , respectively, onto irrigation canal water . Degradation pathways can be inferred based on the physico-chemical properties of pendimethalin. The pendimethalin molecule is not high water soluble, non-ionizable and not hydrolyzed in water and possesses a high affinity for organic matter ; therefore, sediment partition is most likely the significant degradation pathway. Partitioning of pendimethalin onto sediment in water/sediment investigations in dark demonstrated to be within 0.4 to 1.6 days for 50% allocation onto sediments . Pendimethalin is moderately volatile and volatilization is an important dissipation pathway in dry and moist soil, however, as soil moisture increases over soil field capacity, volatilization decreases due to lower movement of the vapor phase in wetter soils . Solar radiation was high in the study area and can be a significant degradation pathway. Both photolysis and sediment partitioning are most likely the important pathways of pendimethalin degradation. While this study negates the pendimethalin metabolites, it is important to note there are three metabolites that can form in water . Nevertheless, the pendimethalin residues in the water indicate the importance of holding flood water in the field after an application to allow the herbicide molecule to settle on the soil surface when applied onto a flooded rice field.The US EPA has recorded an observed maximum level of pendimethalin in surface water at 17.6 ppb, probably contaminated by spray drift, and expressed the risk of pendimethalin contaminating surface waters to be less than 2% . While there is no water quality criteria level for pendimethalin, residues of pendimethalin have been observed in surface water tributaries near agricultural regions with concentrations up to 0.02 ppb . Additionally, pendimethalin residues as low as 30.0 ppb in soil have shown to cause injury on tomato , a common crop grown near California rice fields . Despite observed concentrations above these levels from the EC and CS formulations early on, pendimethalin dissipated quickly below levels of concern . Apart from preventing potential herbicide runoff, water-holding periods can be useful for increasing herbicide efficacy. Some pesticides currently used need the water for activation or to evenly distribute in the field and holding water in the field is common practice for California growers when using granule pesticides in rice . The concentrations observed from this study also suggest pendimethalin could benefit from a water-holding period to increase the efficacy when applied onto the flood. However, an increase in efficacy can also develop greater rice crop injury and should be balanced through application rates and timings. The rates used in this study were the typical use rates in dry-seeded rice, which are known to provide adequate weed control. This study did not focus on weed control but ongoing work is examining this aspect to enable efficacious and safe use of pendimethalin for water-seeded rice. Pendimethalin did not persist to levels of concern in the surface-water of a water-seeded rice field and was detected at very low concentrations, in general. The results from this study can assist regulatory agencies and registrants in articulating a water-holding period for pendimethalin in water-seeded rice, which can help prevent potential contamination to municipal drinking waters, prevent damage to downstream high value crops and ensure efficacious use, therefore, promoting responsible stewardship of chemical use in California rice.In the lab, water samples were cleaned from debris by periodically pouring the 90 mL sample through a funnel with filter paper of 11 µm Whatman 1 of 90 mm diameter outlining the inside the funnel’s wall.

Additionally, downstream water quality affected by use of herbicidesin water-seeded rice is of paramount concern because of the proximity to other high value crops which may use the water for irrigation and proximity to urban settlements which may use the water for consumption . The US Environmental Protection Agency recorded pendimethalin risk of contaminating surface waters in agricultural use to be less than 2% . There are no water quality criteria for pendimethalin; however, pendimethalin residues in surface water tributaries near agricultural regions have been documented up to 0.02 parts per billion. The US EPA documented 17.6 ppb to be the maximum level of observed pendimethalin residue in surface water, most likely contaminated by spray drift . Pendimethalin metabolites are formed in various soil, water and plant environments In a water-seeded system, rice is pre-germinated in water for 24-36 hours and air-seeded onto flooded fields with 7-12 cm of standing water, creating an anaerobic environment. The excessive soil moisture immediately after a pendimethalin application in dry-seeded rice sowing significantly increases rice injury . Since water-seeded rice systems have a high-water saturation, then this makes rice seedlings prone to injury from pendimethalin. In drill-seeded rice, clone trays rice is seeded to a depth of about 3.2 cm into the soil and the application of pendimethalin occurs on the soil surface about 1 to 3 days after seeding.

The depth of the seed allows for germination and early growth of the seedling to occur before it comes into contact with the herbicide on the soil surface . Pendimethalin has low volatility, low solubility and strongly attaches to the soil, and will only stay on the top surface layer of the soil . The placement of the seed provides crop safety to pendimethalin in a drill-seeded system . Conversely, in water-seeded rice, rice seed is placed on the soil surface and the initial seedling roots can have direct contact with the herbicide applied on the soil surface. Therefore, an application of pendimethalin later in the season after the seedling is developed and more deeply-rooted may reduce the risk of rice injury. The herbicide formulation may also influence the risk of injury in water-seeded rice. Hatzinikolaou et al. demonstrated that greater injury to oat roots from pendimethalin occurred from the emulsifiable concentrate formulation compared to the granular and the capsule suspension formulations. Hatzinikolaou et al. suggest that the granular and the capsule suspension formulations influence the release rate of the active ingredient resulting in lower risk of crop injury. If increased soil moisture results in greater pendimethalin effectiveness and the emulsifiable concentrate pendimethalin formulation causes greater injury to grass species, then,the use of a slow-release formulation of pendimethalin will reduce rice injury and result in greater crop safety on water-seeded rice. There is no previous research evaluating the partitioning behavior of pendimethalin in water-seeded rice.

The knowledge of pendimethalin behavior in water-seeded rice will help establish proper use of the herbicide to increase herbicide efficacy and decrease off-target contamination potential. Therefore, the objectives of this research discussed in the following dissertation chapters were to evaluate pendimethalin use in water-seeded rice, optimize pendimethalin use for the water-seeded rice system and characterize pendimethalin behavior in flood water of a water-seeded rice field.Rice is an important staple food in many countries and produced worldwide . Water-seeded rice is a common production system in California , Europe, Australia and some Asian countries . The water seeded system is useful for managing grasses, weedy rice, and other non-aquatic weeds . In California water-seeded rice, pregerminated rice seed are air-seeded onto fields with a standing flood of 7-cm to 10-cm, the field will typically be continuously flooded throughout the growing season . Weeds are a major management challenge encountered in rice production . Weedy grasses in the California water-seeded rice agroecosystem include barnyardgrass [Echinochloa crus-galli Beauv], early watergrass , late watergrass [E. phyllopogon Koss], and bearded sprangletop [Leptochloa fusca Kunth ssp. fascicularis N. Snow]. There is potential for up to 70% rice yield loss from season-long barnyardgrass competition and up to 36% rice yield loss from competition with bearded sprangletop . Therefore, weedy grasses are the most economically important weeds in rice production . In California, herbicides continue to be an important tool for weed management in waterseeded rice, but herbicide-resistant weeds have exacerbated the issue leading to poor weed control. A high observed incidence of resistant weed populations is common . The prevalence of resistance has developed due to the limited number of effective herbicide sites of action available and continuous rice production year after year . Multiple herbicide resistance in Echinochloa spp. has made control in rice production a significant challenge. Therefore, there is need for new tools to help implement herbicide resistance management like herbicide mode of action mixtures and herbicide mode of action rotations .

Pendimethalin is a mitotic inhibiting herbicide from the dinitroaniline chemistry, its use is a selective pre-emergent that ceases the seedling growth shortly after germination . Pendimethalin has activity on Echinochloa spp. and bearded sprangletop . Currently, there is no recorded resistance to pendimethalin in California rice; therefore, it has potential to be a new herbicide for waterseeded rice . Pendimethalin is registered for use in drill-seeded rice as a preemergence or as an early post-emergence , however, it is not available in water-seeded rice because of significant crop injury potential . In drill-seeded rice, pendimethalin application is suggested to be at three to seven days after planting and rice should be seeded at depths of 3.2 cm or greater to reduce injury . A deeper planting depth allows the seedlings to grow before contacting pendimethalin on the soil surface . In water-seeded rice, rice seed is sown on the surface of the soil in high moisture levels, therefore, a post-emergence application may reduce injury by allowing seedlings to establish before a pendimethalin application. The 1.1 kg ha-1 rate is the typical label rate used in drill-seeded rice for watergrass control . Pendimethalin degrades faster in anaerobic conditions than in aerobic conditions . Using higher rates may still provide adequate activity in an anaerobic condition. Therefore, the 2X and 3X of the labeled rate were selected to evaluate for rice response and weed control. Herbicide formulation and application timing can be significant factors to reduce the rice injury to acceptable levels in a water-seeded system. Hatzinikolaou et al. recorded theemulsifiable concentrate of pendimethalin had greater soil activity, cannabis drying room but the water dispersible granule and capsule suspension formulation remained active in the soil longer, producing an extended soil residual activity. Hatzinikolaou et al. observed that the EC formulation resulted in a greater reduction in root length than GR and CS formulations, however, the GR and CS formulations also resulted in root length reduction in various plant species tested. Tolerance to herbicides can also vary among rice cultivars. Koger et al. observed differential response to pendimethalin among three long grain rice cultivars, with the ‘Wells’ cultivar demonstrated greater susceptibility to pendimethalin when compared to ‘Cocodrie’ and ‘Lemont’ cultivars in a conventional tillage, dry-seeded system at different seeding depths. Bond et al. observed no differences with minimal to no rice injury, among the same three long grain cultivars in a stale seedbed dry-seeded field study. Because of differences in cultivars and production practices, it is important to examine the response from common California rice cultivars to pendimethalin to understand the practicability and limitations of its use in the waterseeded system. Field and greenhouse studies were conducted to examine the response of water-seeded rice to a pendimethalin application. In the field study, we evaluate rice plant response to three pendimethalin formulations, GR, EC and CS, at three different application timings and three pendimethalin rates. The greenhouse study evaluated the response of five common California rice cultivars after a GR and CS pendimethalin application in a simulated water-seeded condition. The objectives of these studies were to characterize the response of water-seeded rice after a pendimethalin application and evaluate its potential use for water-seeded rice.The field study was conducted in 2020 and 2021 at the Rice Experiment Station in Biggs, CA. Soils at the study site are characterized as Esquon-Neerdobe , silty clay, made up of 27% sand, 39% silt, and 34% clay, with a pH of 5.1, and 2.8% organic matter. Following rice cultivation during the off-season winter months, the field was flooded to 10 cm above the soil after a pass with a single offset stubble disc and then drained in early spring of the following year. Field preparation in spring consisted of one pass with a chisel plow and two passes with a single offset disc, followed by a land plane to smooth the soil surface. A corrugated roller was used to pack the soil and eliminate large clods on the soil surface prior to planting. A granule fertilizer starter mixture of ammonium sulfate and potassium sulfate was applied by plane at 336 kg ha-1 prior to the corrugated roller pass. Seeds of the rice cultivar ‘M-206’ were pregerminated in steel bins filled with water until all the seeds were completely covered. For disease control, a 5% sodium hypochlorite solution was added in the water for the first hour, then drained and refilled with only water for the remaining 24 h.

The seed was then drained until dry for 12 h, and seeded by aircraft at 140 kg ha- 1 seeding rate in 2020 and 170 kg ha-1 seeding rate in 2021 onto the field with a 10-cm standing flood. Individual 3-m wide by 6-m long plots surrounded by 2.2-m wide shared levees were made to prevent contamination from adjacent treatments in a replication. The flood was maintained the whole season and other than being temporarily lowered for application of foliar herbicides for sedge and broadleaf control. Standard agronomic and pest management practiceswere followed based on the University of California rice production guidelines . Seeding dates were May 23, 2020 and June 5, 2021. The study design was in a factorial arrangement of the treatments under a randomized complete block design with four replications. The treatment factors were three formulations, three application timings, and three application rates. The pendimethalin EC formulation was BAS 455 39H with 0.4 kg L-1 of active ingredient, the CS formulation was BAS 455 48H with 0.5 kg L-1 of active ingredient, and the GR was BAS 455 20H with 2% of active ingredient per weight. Application timings were 5, 10, and 15 days after seeding , corresponding to 1-, 2- to 3- and 3- to 4-leaf stage rice, respectively. The application rates were 1.1, 2.3 and 3.4 kg ai ha-1 . A non-treated control plot with no pendimethalin applied was randomly placed within each replication to serve as a reference for the assessments. The CS and EC formulations were applied with a CO2 pressurized backpack sprayer calibrated at 206 kPa to deliver 187 L ha-1 . The sprayer boom was 3-m wide equipped with six flat-fan 8003VS tips traveling at 4.8 km h-1 and spraying onto the water surface. The GR formulation was spread by hand in each respective plot. Additional herbicides were applied for control of emerged grasses in 2020 and for control of other weed species not controlled by pendimethalin both years. Due to a high population of grasses surviving the pendimethalin treatment in 2020, an additional post-emergence rescue treatment cyhalofop-butyl at 0.3 kg ai ha-1 and propanil at 1.7 kg ai ha-1 were applied at 21 DAS was applied which likely influenced the yield and weed control data to some degree. Copper sulfate crystals were applied byplane at 17 kg ha-1 three DAS for control of algae. In 2020, a mixture of carfentrazone-ethyl at 0.1 kg ai ha-1 and triclopyr at 0.3 kg ai ha-1 was also applied at 52 DAS for sedge and broadleaf control. In 2021, only carfentrazone-ethyl at 0.1 kg ai ha-1 and triclopyr at 0.3 kg ai ha-1 were applied for sedge and broadleaf control at 32 DAS. Visual weed control of the of Echinochloa spp. and bearded sprangletop were recorded on 14 and 56 days after pendimethalin treatment , on a scale of 0 to 100, where 0=no control and 100=complete control. Echinochloa spp. counts in the non-treated were conducted 30 DAS by sampling twice in the plots within a 30-cm by 30-cm quadrat.

Similar to the no or little effect of cover crop treatments on insect pest population densities, my research findings cannot confirm that off-season cover cropping reduces crop damage in the subsequent vegetable crop. In contrast, many researchers observed that crop damages can be minimized in a mixed cover and main crop interplanting. These researchers argue that cover crops in a mixed stand interfere with host locating capability and oviposition of insects, masking the main host crop or obstructing the odor profiles , hence reducing pest pressure and damage on the main crop. However, Finch and Collier ; Finch et al. argue that there is little support for general masking theory, except when visual cues are restricted .Cover crops in my trial were used as off-season cropping rotation consequently a major masking effect would not be expected. The most positive effect of the off-season cover crops was enhancement of parasitoids and insect pest parasitization levels, although Furlong et al. state that the real role of cover crops in manipulating population dynamics of insects and natural enemies still remains unclear. There were greater pest parasitization levels on the cover crop treatments for all years compared to the fallow system. The most likely reason for greater parasitization on cover crop treatments was greater pest population densities on these treatments and that the parasitoids were responding strongly to host population density. It is also possible that natural enemies could be more abundant in diverse vegetation systems because of the continuous variety of microhabitats or food resources . A long season cropping period may allow naturally occurring biological control agents to sustain higher population levels on alternate hosts or prey and to persist in agricultural environment throughout the year .

Within the individual insect pests, vertical grow system parasitization was significant for cabbage loopers and diamondback moths, but not the cabbage worms. However, since insect population density and broccoli leaf damage occurred regardless of the patterns of pest parasitization, such natural pest-parasite relations did not off-set and stabilize broccoli insect pests or its leaf damages. Never the less, tachnid flies, chalcid wasps and braconid wasps are some of the major parasitoids against broccoli insect pests.Regardless of the occasional greater pest population densities and higher vegetable crop damages on the cover crop treatments, higher broccoli marketable yield was obtained from plots that received summer cover crop treatments than crops from the summer fallow plots . Therefore, growers should consider the holistic contributions from cover crops than their effects on insect pest population density. This study determined that off-season cover crops suppress weeds , enhanced beneficial saprophytic nematode populations , and enhanced the soil environment . Cover crops may provide many benefits; however, they are not do-it-all wonder crops . Growers need to make proper selections of cover crops considering many factors that may include benefits to ecosystem biodiversity, contribution to the productivity of agricultural systems and compatibility with the main vegetable crop.Modern agricultural systems involving monocropping have become productive, but only because of their high dependence on external chemical inputs . Questions are being raised about the growing dependence of modern farming on chemicals and other non-renewable resources .

There is also an increasing consumer demand for safe agricultural products and hence, a need for services that may help producers, processors, and distributors adapt to changing consumer preference . Such practices require adoption of alternative management practices or enhance functional biodiversity and sustainable production . The use of cover crops is a step towards a sound practice that may accommodate the changing needs of consumers and increase confidence in the quality of agricultural produce. Accordingly, there has been a growing interest in using short-season annual legumes and others as cover crops in vegetable production systems . Cover crops in farming systems improve soil health, reduce environmental pollution, and improve crop yields Walp have been identified as the best candidate for summer cover-crop rotation with winter vegetable crops and improve soil fertility and crop yield . Although several studies have been conducted on cover cropping systems, their use in vegetable crops have rarely been studied or the research has been mainly on winter annual cover crops with very little research on summer cover crops. However, summer cover crops can produce biomass, contribute nitrogen to cropping systems, increase soil organic matter, and suppress weeds , and they are compatible with both organic and conventional farming practices whether incorporated or used as surface mulches . Although cover crops are important components of a sustainable crop production system, their beneficial effects depend on the selection of appropriate cover crops and their management . This research is aimed at evaluating the effect of summer cover cropping on the subsequent vegetable crop. It is hypothesized that incorporation of cover crop plant material provides a valuable source of N and enhances crop growth and yield. Summer cover crops are used between spring and fall vegetable crops . I hypothesized that cover crops would increase soil nutrition, with subsequent improvement of broccoli yield.

Two types of summer cover crops, a legume and a nonlegume, were compared with the standard practice of summer fallow in a Mediterranean type climate.A three-year field study was conducted from 2007-2009 at the University of California South Coast Research and Extension Center in Irvine, CA on a loamy-sandy soil. The field site was loamy sand with a history of root-knot nematode infestation. Three summer cropping treatments were employed: 1) French marigold , 2) cowpea , seeded at 56 kg/ha, and 3) a summer dry fallow as the untreated control. Cowpea was chosen because it is a drought hardy legume, resistant to weeds and enhances some beneficial organisms . Marigold was chosen because it is known to control nematodes . Each treatment plot was 12 m long x 10.7 m wide and laid out into 14 planting rows. The cover crops were direct-seeded in the last week of June in the center of the planting rows of each plot, watered through drip-tubing and grown for three months. The fallow control plots did not receive water during the summer. Each cover crop treatment plot was planted with the same cover crop in each of the three years of study. Plots were separated from each other with a 3 m wide buffer bare ground. The three treatments were replicated four times in a completely randomized design. At the end of the summer cropping period , the cover crops were mowed at the soil line, chopped, and the residues left on the ground. Concurrently, industrial grow alternate rows of each of the cover crop treatments were incorporated into the soil at about 0.4 m intervals using a hand-pushed rotary tiller.The fallow plots were not tilled. Plots for cover crop and broccoli planting are shown in Figure 1a. At the beginning of the subsequent cropping season , broccoli seedlings were transplanted in double rows into the tilled strips of the summer cover crop and fallow plots at an inter and intra-row spacing of 13 and 35 cm, respectively . Broccoli transplants were drip irrigated and fertilized with emulsified fish meal at 5 gallons/acre rate. Broccoli was chosen because it is a high-value vegetable crop that is sensitive to weeds, insect pests, nematodes , and requires high soil nutrients . All plot treatments were maintained in the same location for all three years of study in order to assess a cumulative effect of cover crops over time.Soil nutrient concentrations oscillated between sampling periods and years. During the second year , only soil Ca and Na concentrations and cation exchange capacities were slightly higher for the cover crop treatments at ACCP sampling. However, none of the soil nutrition was different between the cropping treatments at ACCI sampling of the same year . Soil potassium , Na, and CEC was higher for cover crop treatments at ABH sampling for 2008, relative to the fallow treatment . In 2009, the only time soil nutrient contents were visibly different for the cropping treatments was at the ACCI sampling and all soil nutrients were at the same level for all cropping treatments at other sampling periods of 2009. The pH of the soil for both study years ranged from 7.9 to 8.2 and was not different among the cropping treatments. In addition to the above nutrient types, soil NO3 showed unique responses based on cropping treatments . Soil NO3 was consistently higher for the cover crop treatments relative to the summer fallow, but not until after cover crop incorporation of2008. Soil NO3 level declined and was not different among the cropping treatments at ABH sampling of 2009 . In relative comparisons soil NO3 levels were higher in 2009 than in 2008. Soil SO4, and percent cation saturations were higher for the cover crop treatments, compared to the fallow, but not until 2009. Mn and B were higher in the fallow than in the cover cropped plots at ACCI.

Soil nutrient concentrations, particularly NO3 and SO4 were generally greater when the cover crop was a cowpea than marigold . Some, but not all of the soil nutrient enrichment from cover cropping is reflected in the nutrient uptake of the vegetable crop . As for the plant nutrients, higher N, S and K were detected in the shoots of broccoli grown on the summer cowpea plots compared to the fallow treatments. However, these nutrient increases were only in the 2009 crops, but not in 2008 . Other nutrients such as Mg, Ca, and Na showed higher levels in broccoli shoots that were grown on the summer cowpea plots as early as 2008 and also in the shoots of the 2009 crops. Al and B were also higher in broccoli shoots from the summer cowpea treatments, compared to the marigold and fallow treatments . In general broccoli benefitted from higher nutrient uptakes from treatments that had a summer cowpea than marigold and least when broccoli was grown on summer fallow plots. As with the soil and vegetable crop nutrient conditions, broccoli growth differed depending on the summer cropping systems. Broccoli grown on the summer cover crop plots were taller and had more vigorous growth than those on the summer fallow plots. During almost all sampling weeks, broccolis crops were consistently taller for the two cover crops than the fallow treatments. Consistent with nutrient status, crop height growth was highest for those from cowpea, followed by marigold and least for crops grown on the summer fallow . The increase in height of broccoli grown on the cover crops is more prominent after the third week of sampling for all study years and more pronounced for the 2008 crops. Broccoli canopy spread was similar to the crop‘s height responses in that broccoli on the summer cover crop treatments for all years had relatively broader canopy, but were most significant for the 2008 cropping year . The mean number of leaves per individual plant was also variable. Once again these crop growth parameters differed based on sampling years and cropping treatments . During all trial years, broccoli grown on the summer cowpea fields had relatively higher mean leaf numbers per plant than any other cropping treatments, particularly at about 8 weeks after broccoli transplant. Mean leaf number production and variation between cropping treatments were clearly visible for the 2008 and 2009 cropping seasons than for the 2007 broccoli . Broccoli shoot biomass determination from destructive crop sampling at harvest time showed that there was no significant broccoli shoot biomass gain from cover cropping for the first year rotation, although broccoli grown on the summer cowpea were relatively heavier than the other two cropping treatments . Vegetable crops of heavier shoot biomass from cover cropping were observed for 2008 and 2009. During those latter two years, broccoli shoot biomass was heavier for those from cowpea followed by marigold and least for crops grown on the summer fallow . The 2008 broccoli shoot biomass for crops from summer cowpea and marigold were about 43% and 23% higher, respectively than these grown following a summer fallow treatment. Although broccoli shoot biomass for 2009 was generally lighter than the 2008 crops, similar trends as for the 2008 crops was observed for treatment effects. Accordingly, broccoli grown on cowpea, followed by those on marigold had heavier biomass than crops grown on the summer fallow field.Finally, all effects of cropping treatments is expected to show the benefits of cover cropping with the evaluation of the marketable yields of the vegetable crop.

Lower population densities of common purslane and all weeds combined were observed at harvest time for cover crop treatments in 2009 compared to summer fallow . Among the cover crops, vegetable crops that had cowpea as a summer cover crop had fewer population of weeds than the marigold . Compared to the previous years, the lowest weed population densities were observed at any sampling date in 2009. There were no significant differences in the population densities of common purslane at the harvest time sampling of 2009 . Cowpea as a cover crop showed stronger weed suppression capabilities than marigold.Biomass accumulation of individual weeds was related to the specific weed population densities. During all years, common purslane attained the highest dry biomass accumulation depending on the cropping treatments. At early sampling of 2007, common purslane attained 100 and 36 times higher dry mass in the fallow , relative to the marigold or cowpea cover crops, respectively . Dry mass accumulation from all weeds combined for the early sampling were also reduced by 37 times when the summer cropping was either a marigold or a cowpea compared to the fallow treatment. Similar to the weed population densities, reduction in weed biomass was stronger for the broadleaves than on grass weeds . Weed biomass accumulation during the mid and harvest time samplings did not vary among the cropping treatments, drying rack for weed attributing the cover crops and initial hand weeding that might have already depleted weed seed banks and new weed germination.

Cover crop suppression of purslane biomass accumulation and all weed biomass accumulation was also observed for 2008. Cowpea and marigold as cover crops reduced purslane biomass by 6 and 20 times, respectively at the mid sampling compared to the same time sampling on a fallow treatment . Similarly, dry mass of all weeds at the mid and harvest time sampling were significantly lower for the cover crop compared to the Fallow treatment . Stronger suppression of on biomass accumulation of broad leaves than grasses is consistent for this year as well. The only exception from the first year trial was the suppression of the dry biomass of a grass weed, Eragrostis barrelieri, under the cover crop treatments at the second year harvest time sampling . Weed biomass was generally lower for 2009 than either 2007 or 2008, suggesting that repeated years of cover cropping rotations and hand weeding may provide increased weed suppression in the subsequent vegetable crop. Greater biomass accumulation in all weeds combined was observed in the fallow plots at early , mid and harvest time samplings compared to plots that had summer cover crops with stronger suppression on broad leaves than grasses weeds.The greenhouse weed seed germination tests showed poor responses for all three years. Even the most dominant weed, Portulaca oleracea germinated poorly or failed to germinate at all. Among these germinated, none or only very few weeds showed variation among cropping treatments for both 2007 and 2008 . Therefore, this portion of the research finding is inconclusive and has been omitted.The time required for the initial weeding in 2007 was not different among the cropping treatments , probably due that the cover crops did not provide efficient weed suppression at this initial stage of crop rotation.

Differences among cropping treatments on supplemental hand weeding duration appeared at the second hand weeding in 2007 and all other weeding periods of the subsequent years. Longer weeding hours were required for the initial weeding on the fallow plots for 2008 and 2009 compared to the cover cropped plots. The combined initial and second round time spent on hand weeding was higher in the fallow plots of 2007 , 2008 , and 2009 . At all hand weeding periods of all years, longer time was spent weeding in the marigold than in the cowpea treatment, showing stronger weed suppression of cowpea as cover crop than marigold. The relative total weeding time in the fallow plot to the time required in a cowpea plot was about twice in 2007, 2.5 times in 2008, and 2.8 times for 2009, indicating stronger reduction in labor with increasing years of cover cropping rotations.The three consecutive experimentation years revealed that common purslane was the most prevalent weed both in population density and biomass accumulation. Adler and Chase states that common purslane is a prolific seed producer that can rapidly colonize warm, moist sites. Either cowpea or marigold used as summer cover crop suppressed common purslane. While individual weeds of the other species were not responsive to the cover cropping treatments, the combined population density and biomass of all weeds was significanly reduced under the summer cover cropping treatments compared to the summer fallow. Hutchinson and McGiffen also observed sufficient levels of weed suppression with cover crop mulches in desert pepper production.Weed suppression with off-season cover cropping treatment was more robust when coupled with supplemental hand weeding, because post-weeding population and biomass accumulation of weeds were lower under the cover cropping treatments than the fallow.

Therefore summer cover cropping can provide long-term weed suppression, even after the establishment of the vegetable crop. The lower weed population and weed biomass during the subsequent sampling periods of all years relative to the early sampling stages reveal that supplemetal hand weeding is critical and reduces early stage weed pressure on vegetable crops. It also shows the importance of integrating cover cropping rotations with supplemental hand weeding for more efficient weed management. Weed suppression during the early growth of a vegetable crop is desirable as most crops suffer serious weed competition during their early growth stages. Eliminating or minimizing early stage crop-weed competition may help a crop to make vigorous growth, develop dense canopy faster and suppress emergence and growth of weeds in the subsequent crop growth season. A long-term weed supression and nitrogen contribution from foxtail millet [Setaria italica Beauv.] and cowpea [Vigna unguiculata ] cover cropping has been observed to produce greater total marketable yield of bulb onion . Consistent and prominent cover crop weed suppresion was observed against broadleaf than grass weeds, suggesting that cover cropping is more beneficial in agricultural fields dominated by broadleaf weeds. In a similar experiment, Wang et al. observed a higher suppression of broadleaf weeds than grasses using sun hemp mulches. While cover cropping consistently suppressed weeds during all years, pipp mobile storage systems the more cover cropping rotation , the stronger was the weed suppression. These findings suggest that cover cropping has incremental effects with increasing years of cover crop rotations. Among the cover crops used, cowpea, if used as a summer cover crop, could provide more weed suppression than if the cover crop was marigold. Many researchers showed that cowpea provided excellent suppression of weeds when used as an intercrop or organic mulch. The stronger weed suppression and hence less supplemental hand weeding time in cowpea cover crop is probably attributed to the nitrogen fixing ability and more nutrient supply potential, and enhancement of the subsequent vegetable crop growth and facilitating the vegetable crop to suppress weeds on its own at its subsequent growth stages. Enhanced soil quality is one of the reasons contributing to the suppression of weeds within the subsequent vegetable crops . The off-season cover cropping that leaves crop residues on soil surfaces increase soil N level and supress weeds in vegetable crops . Weed seed greenhouse germination was generally very poor and showed mixed responses among the different weed species. Some weed species germinated more in soil samples collected from the cover crop plots while others were relatively higher in the fallow treatment. Therefore, the effects of cover cropping for the portion of soil weed seed population densities are inconclusive and the ―potentially higher‖ weed seed bank hypothesis within a fallow summer cannot be confirmed.

The amount of soil used for germination tests may have not been sufficient enough or our greenhouse conditions might have not provided optimum condition for weed seed dormancy breaking. A further study is recommended to verify the potentials of cover cropping in reducing soil weed seed banks. The study on effects of cover cropping on supplemental hand weeding duration revealed that cover cropping reduce the time it may take for supplemental hand weeding. The reduction of supplemental hand weeding time within the cover crop treatments compared to the fallow treatment was consistent for all three years. The total amount of time needed during the whole crop growing season was almost double in the summer fallow plots compared to the summer cover cropped fields. Reducing supplemental weeding needs decreases production costs and provides higher economic return from vegetable crop production. Although Kumar et al. suggest that there is no clear mechanism by which cover crop residues may suppress weeds, there are many possible mechanisms listed for weed suppression using off-season by summer cover crops and their residues. Adler and Chase suggest that cover crop residues suppress weeds through its modification of soilmicroclimate and physical impedance of weed seed germination or serving as a physical barrier and inhibiting light penetration . Others suggest that the exclusion of light is an indirect mechanism by which cover crop residues may suppress weeds and that the actual mechanism is through the reduction of soil temperature fluctuations useful in breaking weed seed dormancy and germination. For example, in the absence of fluctuating soil temperature, Portulaca oleracea , Chenopodium album and Amaranthus retroflexus failed to germinate. Cover crop residues may also suppress weeds through stimulating or suppressing of soil microbial populations which deplete soil weed seed banks . The off-season cover crop stand may utilize water and nutrients that would otherwise be used by weed species and hence provides a mechanism by which off season cover crops could suppress weeds in the subsequent crops. Some cover crops inhibit weeds through allelopathy . If there were allelopathic effects from the cover crops used in this trial, the allelopathic compounds must have been more specific to the broadleaved weeds, because the cover crops used were more suppressants to broadleaves. Selective phytotoxicity of allelopathic compounds to broadleaf weeds has been discussed by Ercoli et al. 2007; Jelonkiewicz and Borowy 2005; Hill et al. 2007; Adler and Chase 2007. Benzoxazinoid compounds specific to rye cover crop mulches suppressed different sets of weed species . A proper and detailed understanding of allelopathic cover crops may aid scientist to develop effective biological weed control and reduce future reliance on synthetic herbicides . Although both marigold and cowpea significantly suppressed weed pressure when used as cover crops in a subsequent vegetable crop relative to the fallow summer, none of the cover crops provided a complete control of weeds without supplemental weed management options. Therefore cover crops may not be considered as the sole control of weeds, but as an integrated component and holistic approach of weed management options. The greater effectiveness of cover cropping as component of integrated weed management strategies was emphasized by Dıaz-Pe´rez et al. . Within the cover crops, the species that possesses rapid growth and large biomass production characteristics may provide more weed suppression. Finally, weed management using cover crops is ecologically friendly and if coupled with some traditional weed control methods could eliminate or reduce reliance on chemical weed control. In this respect, cover crops may be particularly appealing and useful for organic crop production systems where chemical weed management is not an option. Since the off-season cover crops are not grown simultaneously with the major crop, there would not be a resource competition threat as when the cover crops are inter-cropped with the main crop. To make efficient use of cover crops, growers must also identify the adaptability of cover crops to their local farm condition, the weed species and the economic considerations of agricultural systems. One must also confirm that the specific cover crops and residues could suppress diverse weed species with no or little interfere with the major crop. The USDA–AMS emphasizes that the screening of cover crops along various crop productions may fulfill the National Organic Regulations and Guidelines that require preventive measures, safe crop production practices and use of competitive crops as a first line of defense against weeds.Plant-parasitic nematodes cause severe damage to vegetable crops and cause an estimated annual yield loss of $77 billion to $125 billion worldwide .

The small grain crop amendments used in these tests were air-dried and milled straw residues of mature wheat , barley , oats and triticale plants which were collected from recently harvested fields in the Fresno, California area. All amendments were uniformly incorporated into soil at a concentration of 1.9% , the approximate quantity of stubble residues which would be incorporated into the plow layer of field soil at the end of a cropping cycle in commercial production. Effects of treatments on M. incognita were estimated after 7 days’ incubation using a bioassay procedure, in which treated soil was aired in open plastic bags for 24 h following incubation in bioreactors, then placed in two 10-cm-diam pots per replication. A single plant of a susceptible tomato cultivar was transplanted into each pot and the pots were maintained in a glasshouse at 30°C maximum and 21°C minimum. After 6 weeks’ growth, root systems were excised, washed and an arbitrary gall rating was made by visual examination . Sensitivity of S. rolfsii to treatments was determined by retrieving and surface-disinfesting the 30 sclerotia from each container, then incubating them on potato dextrose agar plates to determine germinability. Effects on P. ultimum were determined by sampling soil from containers, then air-drying and spreading aliquots on a selective agar medium, as described previously . Fungal colonies were identified and enumerated after incubation.Three field studies were conducted at the University of California, Kearney Research and Extension Center, ca 12.5 km southeast of Fresno, California .

The soil type was Hanford fine sandy loam , drying marijuana and experiments were done in conjunction with vegetable crop transplant experiments, according to methodology described previously. In Experiment 1 , raised planting beds, 102 cm between centers, were formed and pre-planting fertilizer was incorporated to a depth of 15 cm. Six rows of sudex were planted on each bed on 6 August, at the rate of 13.6 kg ha−1 . Two drip irrigation lines were placed on the surface of each planting bed and water was applied to field capacity. Following seedling stand establishment, liquid fertilizer was added through the drip system. The green sudex plants were shredded when they reached a height of ca 1.4 m on 24 September, using a tractor-drawn mower. Plots were sprayed 10 days later with a 2% solution of glyphosate, using a CO2-powered backpack sprayer, to preclude plant regrowth. The shredded sudex plants formed a dense mulch layer over the surface of the planting beds. Four replications were prepared for each of the following treatments: plants shredded and sprayed with glyphosate, then left on the soil surface; plants shredded, sprayed, then incorporated in soil with a rototiller; and fallow control . Each plot was 1 m long. All plots were regularly drip irrigated and fertilized every 2 weeks. Vegetable plants were transplanted into the beds as described by Summers et al. . Following the vegetable crop harvests, all weeds from a 0.093 m2 area were harvested on 29 November, dried and weighed. Methodology for Experiments 2 and 3 was similar to that of Experiment 1, but used 152-cm-wide planting beds. Sudex seed was planted on 1 May and 26 July . The plants from Experiment 2 were shredded as before, on 27 June, when green plants were ca 1.8 m tall; and those from Experiment 3 were shredded on 5 September, when plants were ca 2 m tall. Plant stubble regrowth was sprayed 10 days later with 2% glyphosate herbicide, as described above.

Plots in both Experiments 2 and 3 were arranged in randomized complete block design, with six replications each of the following treatments: plants shredded, sprayed with glyphosate, and shoot residues left on the soil surface; shredded plants raked off and placed on a fallow bed that had not previously been seeded with sudex plants shredded, sprayed, then shredded stems manually removed from plots, leaving only the roots plus 3–5 cm of surface stubble ; plants shredded, sprayed, then shoots and roots incorporated into soil with a tractor mounted rototiller, 14 days after shredding; and fallow control . Each plot was 4.5 m long by 1.5 m wide. Prior to planting, two drip tape lines were placed on the surface of each planting bed. Irrigation water and liquid fertilizer were applied weekly through the drip system as before. Forty-three days after sudex shredding , the total weed biomass in Experiment 2 was determined, following tomato harvest, by removing the weeds from 1 m2 of soil surface, selected at random in each plot. Weeds were placed in paper bags, dried for 5 days at 70°C and then weighed. These procedures were repeated 50 days and 57 days after sudex shredding. Similarly, the weed biomass from Experiment 3 was collected on 25 October and 20 December for the first planting, and on 21 December for the second planting, then dried and weighed as before.The laboratory bioreactor experiments conducted in this study showed that amendment of phytopathogen infested field soil, with certain poaceous crop residues at a constant temperature of 23°C, provided mostly significant levels of deleterious activity against M. incognita, P. ultimum and S. rolfsii. Of the amendments tested at 23°C, ‘Yolo’ wheat provided the most consistent activity against M. incognita and S. rolfsii, and triticale the least. When incubated at the higher temperature regimen of 38o /27°C , all amendments demonstrated consistently increased deleterious activity that was statistically indistinguishable, except that triticale residues had the least biocidal activity against P. ultimum. As with most bioactive chemicals, including synthetic pesticides and brassicaceous and alliaceous plant residues , deleterious activity of the tested poaceous amendments increased with increasing soil temperature.

These very consistent results across the various plant taxa tested indicate that, as expected, the volatility and concentration of bioactive chemicals released during plant residue decomposition in soil increases with increasing temperature . Also, given the statistically significant interactions of the [amendment] and [temperature] factorial effects tested with S. rolfsii and P. ultimum , the targeted phytopathogens were shown to incur more harm from simultaneous application of the dissimilar stress sources, i.e., chemical and temperature, than from either stress source alone. These results confirmed the utility of combining plant residue soil amendments with soil heating techniques for improved soil disinfestation. It is commonly assumed that in vitro, bench-top experiments, such as those conducted in bioreactors inthis study, often give more dramatic results than those obtained under similar conditions in a natural environment. Therefore, the field experiments conducted with the sorghum-sudangrass cover crop plants and residues provided strong support for our laboratory study. Over the course of three experiments conducted at different times during the year, the sudex plant residues, particularly the shoot portions, clearly gave a dramatic and long-lasting reduction of both summer and winter annual weed species, regardless of seasonal climate. The deleterious effects were apparent on both broadleaved weeds and grasses, and were similar to those on vegetable transplants grown in the same plots . The consistent and significant inhibition of targeted organisms by certain cultivated grasses demonstrated in these experiments is not surprising, given many previous reports of lethal or inhibitory effects against various plant pests . A portion of the below-ground, inhibitory activity of grass family members results from production of toxic, decomposition compounds . However, the effects of allelochemicals in growing plants can be potent and long-lasting . It is generally accepted that allelopathy results from the release of specific chemicals that influence such factors as seed germination, radicle and hypocotyl elongation, and seedling growth and development . The effect of such chemicals gradually diminishes as they are leached below the root zone by irrigation or rainfall , or microbially degraded following tissue disruption and/or burial in soil . This phenomenon of enhanced degradation was clearly demonstrated by the comparatively milder, and less persistent, deleterious activity of sudex residues when shredded and/or soil-incorporated in the present study. The broad-spectrum, cannabis drying rack biocidal/biostatic activity demonstrated by these agronomically important, poaceous plants presents a challenge to those wishing to maximize their promising pest control potential, without having to worry about subsequent crop phytotoxicity. Clearly, there is a range of allelopathic or biotoxic activity in poaceous plants, and presumably across cultivars of specific taxa as well. Phytotoxicity to subsequent crops may not always occur, or be noticeable in the field if it does. In crop rotations with long fallow periods, or with satisfactory leaching, even rotations into highly bioactive varieties, such as sudex, may present no problems for subsequent crops. In the case of a planned fallow, it may be advantageous to begin the crop-free period with a bioactive, poaceous crop to discourage weed growth and/or reduce populations of soilborne nematodes or fungal propagules. Future efforts to enhance agricultural sustainability will include development of strategies for crop multitasking, i.e., maximizing uses for both harvested and non-harvested portions . Biological and physical alternatives to synthetic chemical soil disinfestation can be important components of crop multi-tasking. However, alternatives that will be attractive to growers for implementation must provide predictable and relatively rapid reductions of pathogen/pest inocula, at reasonable cost, and without harming subsequent crops or soil quality. Development of guidelines for the pesticidal use of cultivated grasses, such as those tested here, as well as other members of the Poaceae, can contribute to these goals.Since the 1930s, the growth of road transportation has exploded in the United States, and about 83% of the land in the United States is within one kilometer of any type of road . Road infrastructure facilitates economic growth and human-social interaction; however, it is also well-known that roads facilitate invasive and weedy plant dispersal .

In most cases, roads act as barriers and filters to block most wildlife species’ movement because of the frequent and intense disturbances; however, roads also act as habitats and conduits for invasive and weedy species adapted to the disturbances . Plants along the roads usually raise safety concerns, such as blocking traffic and affecting drivers’ vision. Additionally, massive road networks have now become a part of the ecosystem. Since roads connect both natural landscapes and agricultural fields, roadside vegetation management is important to minimize the impacts of invasive and weedy species on those ecosystems. In 2021, a study demonstrated that invasive species, including plants and animals, cost North America $1.26 trillion from 1960 to 2017 and $26 billion annually in the 2010s . In 2008, according to the survey by California Invasive Species Council, the economic impact of invasive plant species was estimated at $82 million annually . In addition to the economic impact and safety concerns, invasive and weedy plants can cause ecological damages to the ecosystem, including reduction of species biodiversity, changes inwildfire regime, and water pollution . A changing fire regime can affect local species and human society to a large extent, so managing roadside invasive species is inevitable and necessary. For example, Bromus tectorum and Andropogon gayanu can increase fire intensity and frequency by adding more fuel load to the ecosystem . In general, a successful plant species establishment consists of several factors. Reichard & Hamilton suggested that weedy traits, especially reproductive traits, are the most dominant factors in determining a successful invasion of an ecosystem. In contrast, Parendes & Jones argued that environmental factors or human interferences also partially explained the invasive species’ distribution and dispersal. In the case of roadside habitats, human activities and the environment are also as prominent as weedy traits. The frequent disturbances along the road, including road maintenance and building, create long-lasting bare soil for species colonization. Parendes & Jones reported that locations with intensive disturbances and adequate resources have higher frequencies of exotic plant species. Mills et al. measured and examined the soil properties in two segments from two different highways in Nebraska. The data indicated that roadside soil contained high sodium concentration and high soil compaction, which inhibited the growth of native vegetation . A meta analysis demonstrated that invasive species have higher plasticity than co-occurring native species under many environmental stresses . Besides disturbances, water resources are another factor that increases the invasibility of roadside environments. Roads are considered water collectors and rainfall storage. The surface of roads is designed to have a few degrees of incline to drain excess water during rainfall, and there will be a slide slope with more incline and a ditch to collect the water . A study measured the soil moisture of the ditch along forest roads in 36 samples, averaging 53.5% .

As of December 2020, U.S. consumers can choose from more than 50 light-duty EV models that span multiple vehicle classes, markets, and a wide MSRP price range from $27,500 to more than $100,000. Projections for continued EV growth through the present “second decade” of mass deployment are varied, but many suggest a sustained exponential growth as evidenced by Figure 1-1, which shows future market share as a fraction of new vehicle sales. EVs are increasingly seen as a win-win solution by many policymakers, in that they can provide benefits to consumers, automakers, and utilities, while also reducing environmental impacts associated with tailpipe emissions. Furthermore, while much public attention is focused on Light Duty vehicle markets, additional opportunities exist in Medium Duty and certain Heavy Duty applications. Despite substantial progress, widespread optimism, and proactive policy support, new and nontrivial barriers remain. These barriers may simultaneously threaten both broader adoption and certain beneficial outcomes of EV growth. Among the most critical and poorly understood, is the need to ensure environmental benefits live up to their promise, in particular under deep deployment scenarios where EVs comprise more than 10% of the future fleet by 2030, bud curing and demand commensurate new supplies of electric power in both time and space. It is desirable to assess the criticality of environmental impacts, so as to quantify the levels of decarbonization enabled by EV penetration.

However, empirical methods are presently insufficient for near term projections, due to uncertainties related to related to charging levels, charging times and the spatial temporal impact of different electricity generation mixes. Further compounding this challenge are insufficient data on EV growth and uncertain adoption rates. What happens, for instance, when EV load growth will require 20% more power demand than is currently forecasted in existing integrated resource plans, which already must also provision for an approximate 15% peak reserve margin? What happens when this average increase in power demand is considered on an hourly or seasonal basis, spiking to much greater shares of reserve . Assuming that utilities embrace an opportunity to sell more kWh to meet new market demands, what assurances are in place to protect the environmental footprint of new load growth?This research requires the synthesis of three independent models developed uniquely by the research team in the areas of vehicle propulsion to satisfy prescribed trip/travel demands for a range of vehicle technologies, EV charging profiles to reflect typical approaches for light duty vehicle use cases, and grid generation dispatch with commensurate consideration of emissions intensities for CO2 and major criteria pollutants. The team has an established track record of developing high-fidelity sub-system models and applying them to both generalizable and regional scenarios.

The team has leveraged more than three years of prior efforts, during which time we acquired and conditioned open-source data and amassed specifications for five representative alternative vehicle architectures, customized datasets for regional electric power dispatch , and numerous travel route pathways. The scope of this project is to update and develop new, more accurate sub-system models and datasets that are relevant, representative, and granular. As described in the original proposal, the team has leveraged these data and iterated upon prior sub-system models with the express purpose of devoting focused attention to integration, simulation, and assessment of results and implications. The end result, therefore, is an integrated model that pulls high-fidelity data from real-world use cases to generate a range of simulations. The simulations will be primarily used to draw comparisons, understand the impact of fundamental assumptions around charging behavior and grid emissions, and develop initial guidance around the relative merits of EVs under representative use cases.The first step in the analysis is the refinement of physics-based vehicle energy consumption models that permit comparison of a range of vehicle architectures that utilize energy from disparate primary sources . A parallel task is to impose upon the vehicle propulsion model a range of driving cycles that can best approximate typical characteristics of representative use cases. Our methodology affords access to established data and extends prior vehicle propulsion energy and emissions analyses.

As a parallel input, the team has utilized individual EPA dynamometer schedules, replicated the 5-cycle fuel economy label weighting protocol, and also consulted independently derived travel demands from representative use cases . A detailed discussion of the theory, model development, source data, and initial applications can be found in [13]. Minoradjustments have been made to vehicle modeling to accommodate key vehicle classifications of interest , and to ensure appropriate reasoning to walk from prescribed EPA dyno schedules to the 5 cycle weighted means, and further to practical interpretations of household travel for representative use cases.As discussed above, we adapt the physics-based power train models developed in [13] to accommodate target vehicle technologies of interest. This includes baseline vehicles , as well as electrified power trains . For the purposes of this study, only pure battery electric vehicles have been evaluated. However, all relevant LDV vehicle technology models have been developed and coded, meaning PHEV analysis is readily available and may be of interest. Owing to their unique architecture which operate as both HEVs and EVs, depending on the battery state of charge, the environmental impacts of PHEVs can be estimated as a weighted mix of the individual impacts of HEVs and EVs respectively. To facilitate a direct comparison among vehicles using dissimilar energy sources, we identify vehicle specifications for a given light-duty vehicle classification and hold constant key parameters such as vehicle power output, vehicle footprint, passenger and cargo capacity, and so forth. Table 2-1 below depicts some of these operative specs.

Note that some differences are inherent in other categories, such as vehicle curb weight. But these have been left as specified by the OEM, under the argument that mass-production specs are reflective of the current state of the art and therefore an excellent proxy for the inherent tradeoffs or interactions to deliver vehicles of similar performance.The five distinct driving cycles that comprise the EPA test and labeling protocol are well documented and widely used for comparative analyses. The three 23°C tests include a derivative of the Urban Dynamometer Driving Schedule known as the Federal Test Protocol , the high-acceleration aggressive driving schedule identified as the Supplemental FTP , and the Highway Fuel Economy Driving Schedule . The 35°C drive cycle is the Air Conditioning Supplemental FTP driving schedule referred to as SC03. The -7°C cold weather test schedule repeats the original FTP at the reduced temperature. As mentioned, curing weed the study has adopted the EPA “5-cycle” protocol and created an approach whereby a weighted mix of driving schedules is obtained to approximate major modes . Please see Appendix A for more details about the weighting of the constituent driving cycles, and the governing formulae. With the original development of the vehicle architecture models, and assumptions around the weighted driving cycle protocols, the team’s next step was to develop a MATLAB/Simulink code that generated a series of energy consumption values based on inputs of vehicle type and driving cycle. These intermediate outputs were then combined to generate effective fuel economy values, analogous to the EPA 5-cycle approach, for the stipulated categories . This was done and a set of energy consumption outputs were generated. These outputs are depicted in Table 2-2.Regarding EV charging behavior, we consider about four primary sources of data to establish representative EV charging profiles. Two are explicitly for residential charging, one is explicitly for workplace charging, and the fourth speaks with survey data collected for both and other categories . The authors acknowledge that there is a growing body of literature on the subject of charging behavior by numerous transportation research centers of note . The authors further suggest that the approach taken herein is appropriate for the purposes of these comparative analyses. It is of note that this research study draws from a combination of analytical and empirical sources of information and data to develop its charging profiles and use cases. Included in this, as detailed below, are first hand studies by researchers involved in the study, utility rate structures that are specific to EV users in the target region, and real-world observed EV charging behavior for a selected network in downtown Atlanta. None of these is unique, and similar approaches are used elsewhere. This, this approach is intended to demonstrate the types of sources of data that this methodology may leverage, and to showcase how they may be applied in a representative set of simulations and outputs. As a first step, we refer to synthetic data generated by a separate research team from Georgia Tech that is evaluating the benefits and challenges associated with smart charging algorithms. Second, we consult the Georgia Power Electric Vehicle Rate scheme, which provides customers with EVs at a deeply discounted rate during off-peak times. In exchange, the rate is tiered, with a relatively expensive energy rate during summer afternoons, and then a fairly nominal price during all other times of the day/year. Third, we refer to data from a Charge Point dashboard portal and database that has been aggregated for workplace charging on the Georgia Tech campus since about 2015. An example of some of this data for a sample month is presented below. It is noteworthy that typical workplace charging occurs in two waves: morning and immediately following the noon hour. A part of the explanation for this has to do with policy: the GT Parking administrator provides a much lower rate for the first 4 hours and then adjusts this to several fold higher to incentivize the EV owner to vacate the parking space and permit additional EV owners an opportunity to charge. The dashboard data is extremely valuable in providing statistically significant information , that can inform real, not perceived or stated, preferences.The team has developed a system-of-systems model that enables the integration of the three sub-system models described in this section. Doing so enables comprehensive and quantitatives imulations of EV deployment for multiple driving cases, under varying EV charging and grid scenarios. The produced MATLAB/Simulink model consists of two user-loaded look-up tables for the selected grid emissions profile and EV charging profile that are imported using the initialization code. The look-up tables take the form of a time series with 1440 distinct time stamps, equal to the number of minutes in a day. The emissions profiles available to the team consisted of hourly emissions rates. The emissions rate during a given hour was assumed to be the same for each minute in that hour. In this manner, each hour of emissions was dissected into 60 periods to achieve 1440 rows of data. By creating minute-by-minute lookup tables, the model is able to stop accumulating grid emissions the same minute the vehicle’s battery is recharged, minimizing returns of surplus charge. Besides loading look-up tables, the initialization code also provides an opportunity for the user to calibrate the energy target . For this study, the energy targets were calculated for each simulated use case using our Vehicle Energy Model described in the previous section. The initialization code and input parameters utilized in this study can be found in Appendix C. Once the initialization code is run, the Simulink component of the model references the loaded look-up tables and parameters, integrating the sub-system models using a series of logical arguments. The completed simulation provides an aggregated output that describes the cumulative grid emissions attributed to the simulated recharge event. These emissions totals are easily converted to a per-unit distance rate. The architecture of the Simulink model can be seen in Figure 2-9.When comparing resulting emissions rates under each grid assumption, it is immediately clear that accepting a monthly or annual average grid emissions rate fails to capture the significant variance that occurs throughout a given day. At higher temporal resolutions, daily grid emissions profiles begin to emerge that have important implications for finding the true environmental benefits of EVs and how those benefits vary depending on the timing of charging events. On average, CO2 emissions per kilometer for an EV charged under the Residential Overnight or Residential Evening charging profiles were found to be less than an EV charged under the Workplace Morning or Workplace Afternoon profiles, especially in the summer and shoulder months. For example, an EV performing the Short Commute trip and charging with the Residential Overnight charging profile in August was found to emit over 3% less CO2 per kilometer when using hourly grid emissions profiles compared to annual averages and nearly 7% less CO2 per kilometer compared to monthly averages.

Cover crop mixtures were designed to fulfill different ecological goals, and we evaluated their impact on weed population density and species communities across a wide geographical area in central California. We hypothesize that both functionally uniform and diverse cover crops can effectively provide orchard ground cover that displaces weeds, but that functionally diverse cover crops will emerge and compete for resources more consistently across growing seasons and locations and are therefore better able to impact weed community composition. To evaluate these hypotheses, we examined indicators of cover crop function: 1) elimination of bare ground compared to ground cover provided by weedy resident vegetation, 2) relative incidence of cover crops and weeds, 3) stability of cover crop incidence over space and time, and 4) downstream impacts on weed communities.Experimental design and soil cover treatments. We compared the effects of several ground cover treatments in replicated large-plot experiments in commercial almond orchards in Tehama, Merced, and Kern Counties in California. These locations span nearly 600 km in the Central Valley of California, including a range of environmental variables, cannabis racks especially rainfall. This region produces virtually all of the almonds in the United States .

The experiment used a randomized complete block design with four replications of three or four soil cover treatments at each site, and practices were implemented for two years on the same plots, beginning in the fall of 2017 and ending in the late summer of 2019. Plots were about 25.5 m wide at each site, encompassing four orchard alleys , and the entire length of the orchard management units . Two different winter cover crop mixes were planted in orchard alleyways. The “uniform” mix consisted of five functionally similar species that are designed to provide diverse floral resources for honeybees to support almond tree pollination. This mix is used commercially in California and distributed as ‘PAm Mustard Mix’ by the Project Apis m. Seeds For Bees program. The mix was comprised of 35% canola , 15% ‘Bracco’ white mustard , 15% ‘Nemfix’ yellow mustard Czern 20% daikon radish , and 15% common yellow mustard . The uniform mix was planted at 9 kg per planted ha. The “diverse” mix consisted of five species from the grass, brassica, and legume groups that are commonly used together in functionally-diverse cover crop mixes . This mix was comprised of 10% ‘Bracco’ white mustard, 10% daikon radish, 30% ‘Merced’ rye , 20% ‘PK’ berseem clover , and 30% common vetch . The diverse mix was planted at 56 kg per planted ha. Two control treatments were also implemented which reflected mainstream orchard management practices of winter vegetation. The “resident” vegetation treatment involved winter vegetation management with mowing and seasonal herbicide applications which allowed resident vegetation growth. The “bare” treatment involved multiple herbicide applications, as determined by grower cooperators, to eliminate winter vegetation.

The Tehama site included only the resident treatment to better reflect standard practices in this region of California which has more abundant winter rainfall. The Merced and Kern sites featured both the resident and bare treatments to better reflect high intensity production systems in these regions. Sites and horticultural management. The study was designed to use commercially relevant spatial and temporal scales, and orchard management was determined by grower cooperators for agronomic relevance. All orchards were equipped with microsprinkler irrigation, and irrigation schedules were determined based on almond evapotranspiration models in accordance with local weather conditions and recommendations. Conventional irrigation, insecticide, fungicide, and fertilizer treatments and rates were determined by each grower and applied to the tree rows only. Tree rows were maintained with conventional herbicide programs to create vegetation-free zones at the base of trees. Each of the sites was subjected to regular traffic from machinery and farmworkers to complete these orchard management operations throughout the cover crop growing season. The Tehama County orchard was located in the northern Sacramento Valley on Kimball loam soils . Average precipitation at the site is 645 mm annually. The site was planted in 2016 with almond varieties ‘Nonpareil’ and ‘Monterey’ in alternating rows. Cover crops were drill seeded in a 3.6 m wide swath down the alleyways on November 6, 2017 and November 9, 2018 and mowed for termination on March 30, 2018 and May 25, 2019. The young trees were pruned in February 2018, and every other alley was subsequently mowed to mulch tree prunings. No data described in this paper were collected from those mowed alleys.

The whole orchard was mowed on January 29, 2019 to destroy unharvested nuts for navel orange worm sanitation; data were collected from cover crop regrowth after this mowing event. Frost during almond bloom was a concern at this site, and irrigation was applied in 12-hour long sets to mitigate forecasted frosts in February or March of each year, which is outside of typical almond irrigation timings. The Merced County orchard, planted in 2008, was located in the northern San Joaquin Valley on Alamo clay soils . Average precipitation at the site is 325 mm annually. The site had 50% ‘Nonpareil’ and 12.5% each ‘Monterey’, ‘Fritz’, ‘Carmel’, and ‘Wood Colony’ almond varieties, with ‘Nonpareil’ in every other row and the remaining varieties mixed evenly in the alternate rows. Cover crops were direct seeded on November 2, 2017 in a 3.6 m wide swath with a seed drill and mowed for termination on April 9, 2018. In year two, cover crops were broadcast planted on December 21, 2018 with a rotary spreader and mowed on March 19, 2019 for navel orange worm sanitation following data collection and again on April 12, 2019 for final cover crop termination. The first replicate of the uniform mix was not planted at this site in 2017, and data from that plot was not included in the analysis.The Kern County orchard, planted in 2006, was located in the southern San Joaquin Valley on primarily Hesperia sandy loam . Average precipitation at the site is 180 mm annually. The site had 50% ‘Nonpareil’ and 25% each ‘Monterey’ and ‘Fritz’ almond varieties, with ‘Nonpareil’ in alternate rows and the other two varieties evenly mixed in every other row. A 4.8 m wide swath was planted down the center of each orchard alley. Cover crops were direct seeded on October 30, 2017 and mowed for termination on April 2, 2018. In year two, cover crops were planted on November 1, 2018 and mowed on April 5, 2019. Immediately prior to both planting dates, indoor grow rack alleyways across the whole orchard were disked for seedbed preparation and ground leveling. Supplemental irrigation was applied across the orchard in 20-hour long sets throughout the winter of 2017-2018 to support the cover crop a45hapiro45gate frost concerns. At this site, the bare ground cover treatment only involved a deep ripping tillage operation to address soil compaction. Data collection. Orchard alley plant communities were evaluated with point-intercept transects. Each plot was surveyed with a single 50 m long transect with points observed evenly at each meter along the transect. Each transect was placed beginning 75 m from the end of the second tree row over from the edge of each plot. The transect extended diagonally across a single orchard alley, starting and ending on opposite edges of the planted swath. Plant incidence was observed for the top layer of vegetation, with occurrence of one actively growing plant or bare ground recorded at each point along the transect. Therefore, incidence is a relative measure of how much ground cover is associated with each vegetation type. Plants were identified to species visually, except in the case of the white and yellow mustards in the uniform mix which were identified as one operational taxonomic unit due to morphological similarities. Transects were surveyed on March 29, 2018 and March 22, 2019 at the Tehama site, March 30, 2018 and March 15, 2019 at the Merced site, and March 27, 2018 and March 16, 2019 at the Kern site. These timings coincide with cover crop flowering for most species as well as winter weed flowering for many endemic species in the study area. Statistical analysis. Analyses were performed in R 4.0.3 . Comparisons of bare ground among treatments were made with ANOVA. ANOVA assumptions were inspected visually with qqPlot from the car package , and subsequently the response variable was arcsine square root transformed to deal with a heavy tailed distribution.

One outlier was identified with the Bonferroni outlier test using outlierTest. This outlier value was excluded from further analyses because it was collected in the same plot at the Merced site that had been previously excluded because it had not been planted in 2017 . Finally, normality of the transformed, outlier-free model was formally assessed with a Shapiro-Wilk test usi46hapiroiro.test. Models with combinations of possible predictor variables , modeled as fixed effects due to the number of sites and years in this study) were compared with Aikake information criterion using the aictab function from the AICcmodavg package . The best model included treatment, site, and their 2-way interactions as predictors, and neither year nor block were included in the final model. The resulting ANOVA analysis was performed with Anova from the car package, and contrasts were made with least-squares means using the emmeans package . Associations between cover crop and weed incidence were analyzed using linear models. Linear models were created with the lm function in base R. Weed incidence was the responsevariable, and we created models with cover crop incidence and cover crop treatment , both with and without their interaction terms, as well as a model which only included cover crop incidence. Linear models were compared with the anova function from base R. Linear regression including only cover crop incidence as a predictor for weed incidence was statistically similar to linear regression that additionally used cover crop mix or cover crop mix and the two-way interaction as predictors. Therefore, we considered the most parsimonious model with only cover crop incidence as a predictor of weed incidence. Cover crop stability was assessed by comparing coefficients of variation for incidence of each cover crop mix as pooled across sites and years in this study. Pooled coefficients of variation were compared with the modified signed-likelihood ratio test as implemented in the cvequality package . Weed communities in the different cover crop treatments were analyzed with non-metric multidimensional scaling . NMDS was based on Bray-Curtis dissimilarity and was calculated using the metaMDS function in the vegan package . Cover crops influenced weed communities but to different extents depending on the site and year. Throughout the springtime evaluations in this study, we observed five weed species at the Kern site, six weed species at the Merced site, and 22 weed species at the Tehama site. The Kern site primarily included annual bluegrass and common chickweed Vill with lesser populations of little mallow , shepherd’s purse Medik and Italian ryegrass Husnot. The Merced site also had large populations of annual bluegrass and common chickweed, as well as little mallow, whitestem filaree L’Hér. California burclover , and wild oat . The Tehama site had significant populations of annual bluegrass, common chickweed, shepherd’s purse, whitestem filaree, buckhorn plantain , chicory , annual sowthistle , field bindweed , and bermudagrass Pers. The remainder of the species at the Tehama site were primarily dicotyledonous, winter annual species, with lesser populations of some grasses and summer annual or perennial dicotyledonous species. Weed communities clustered by cover crop treatments , though sites also predicted weed communities and effect sizes were generally small . While no fixed factors significantly explained weed communities in Merced, year was a significant factor in Tehama and cover crop treatment was a significant grouping factor for weed communities in Kern .Orchard cover crop mixes, as implemented in this study, were effective at establishing, reducing bare soil, and suppressing weeds. However, these effects were highly variable, and there is little evidence that the cover crop mixes we used had fixed impacts on the composition of orchard weed communities. Differences in management and climate at each site year, especially as related to cover crop planting, spring mowing, and weather conditions during cover crop establishment, likely contributed to this variability.

Fertile spikelets are 0.15 to 0.25 inch long, and the smaller sterile spikelets may have bent and twisted awns up to 0.6 inch long. Rhizomes are branching, scaly, and fibrous, 0.5 inch or more in diameter, and reach up to 6 feet long. Rhizomes are usually cream or tan, may have reddish brown streaks, and form a dense sod by the end of the season .Effective management of established infestations can be extremely difficult. Johnsongrass readily colonizes disturbed areas, so field margins, canals, ditches, rights-of-way, and roadsides must be monitored closely if an infestation is suspected. Care must be taken in pasture sown with johnsongrass as forage to prevent its escape. Livestock that may have eaten johnsongrass seed should not be taken to a johnsongrass-free pasture for at least a week, to prevent the spread of seed through feces. Cleaning of equipment, especially shared or rented equipment, prior to relocation is an essential preventative measure against its spread into new areas.Hand-weeding is effective only when the plants are young and when the soil is softened or loose enough to remove all roots and rhizomes without fragmenting them. Caution must be used when pulling or hoeing, as any rhizome fragments left in the soil can sprout new growth. The entire plant should be removed if possible. This is more easily done with smaller seedlings, as their rhizome system is rudimentary. However, sprouts may be attached to a large sod of rhizomes and roots, vertical growing weed in which case removal of the entire plant may be impossible by hand.

For these reasons, weed pullers are generally discouraged.Johnsongrass is known to be unable to tolerate repeated mowing. Mowing should begin when the plants are about 8 to 12 inches tall, before panicle initiation–boot stage, and continue at intervals of 2 to 4 weeks. Mowing can kill seedlings but may not kill all rhizomes, so when used alone it will only suppress established infestations, not control them.Tillage can aid carbohydrate depletion, but it must be used judiciously and only as a part of an integrated management program. Tillage is most effective when the soil is dry, as dried rhizomes are less likely to sprout, and any sprouts will be less vigorous. Summer fallowing can help maximize the effects of tillage, but only where the water table is not too shallow. Johnsongrass rhizomes have buds along their length, but only buds at the tips are active. When fragmented by discing, plowing, or other means, the dormant buds can awaken and send up shoots the current year or the next, depending on the timing of the field operations. A single tillage in spring will most likely result in a more-pronounced infestation. Therefore, early season tillage must be followed by repeated tillage, cultivation, mowing, or herbicide applications every 3 to 4 weeks to control new sprouts. Discing followed by mowing or discing followed by spraying can be effective in reducing an infestation year after year. Use caution when employing deep tillage or plowing, as fragmented rhizomes can send up sprouts from as deep as 12 inches below the soil surface.

Postharvest tillage and fallowing can also be an effective management method, as desiccation can weaken or kill rhizomes. If the harvest is early, desiccation above 85°F for 5 or more days can kill rhizome buds. Late-season tillage can expose some rhizome fragments to killing frost and desiccation, and surviving fragments should produce less-vigorous sprouts the following spring.Propane flaming has been used effectively, but it is usually more expensive than spraying or mowing. It is effective if used biweekly or monthly to manage seedlings and weaken rhizomes. Solarization immediately following harvest may be attempted, as rhizomes do not tolerate high temperatures. Three to five days at above 130°F should kill most rhizomes, but high temperatures will occur in about the first 2 inches of soil depth.Grazing can be used in lieu of mowing to manage seedlings and sprouts, but it will not harm rhizomes. Geese can also be used as a management aid in orchards and broad leaf crops, as they preferentially eat grass seedlings. Food-safety guidelines and best practices must be followed when using livestock as a pest control agent in crops. Grazing and weeder geese can also be used in non-crop areas. In all livestock-management schemes, ingested seed may pass through the animals undamaged, so care should be used when moving animals after grazing. Additionally, many herbicides restrict or forbid grazing of treated areas of certain crops; refer to the herbicide labels for specific details. Caution must be observed when johnsongrass is used as forage or when using grazing as a management method. When stressed by drought, frost, or injury, johnsongrass leaves can build up toxic amounts of hydrocyanic acid , which can be toxic or lethal to livestock. For this reason, grazing is not recommended until plants are 15 to 18 inches tall.

Additionally, johnsongrass can accumulate toxic levels of nitrates when stressed or in soils with high nitrate concentration. Dried johnsongrass cuttings that contain cyanide or high nitrate levels are still unsafe, but well-cured hay should be safe for livestock.Many herbicides are available for controlling johnsongrass, but not every herbicide is equally effective against it, and many can damage desired vegetation. Selection of the correct herbicide for an infested area is critically important. Herbicides that select for broad leaves, e.g., 2,4-D and dicamba, have little to no effect on johnsongrass, and some broad-spectrum and grass-selective herbicides may not be as effective on perennials like johnsongrass as they would be on annual grasses, even at maximum rates.Since the rhizome system of johnsongrass is so large and spreads so rapidly, killing the below-ground tissues or depleting carbohydrate stores is usually the primary goal with herbicide use. Many preemergent and preplant-incorporated herbicides can control seedlings and prevent reseeding, but they are unlikely to be effective against established plants and rhizomes. As such, the use of PRE herbicides may not be a good strategy in early-stage growth unless the target is to control only seedlings and sprouts. However, PRE herbicides such as EPTC, benefin, napropamide, trifluralin, and norflurazon can provide limited suppression of established plants. Contact herbicides, growing rack all postemergent applied to growing plants can prevent the development of seed and new rhizome growth but do not kill existing rhizomes, so new sprouts can be expected in the same season. In order to kill rhizomes, systemic POST herbicides such as glyphosate, fluazifop, clethodim, and sethoxydim are recommended. When applied early in the season these herbicides can prevent flowering but will only have a limited effect on rhizomes. Therefore, systemic POST herbicides are most effective against rhizomes when applied after flowering, when rhizomes are growing vigorously. Grass-selective herbicides like fluazifop, sethoxydim, and clethodim provide their most effective control when plants are 8 to 18 inches tall, though repeat applications may be needed. Glyphosate is most effective on actively growing plants that are 12 to 24 inches tall. Always consult the herbicide’s label for a list of common weeds controlled, as well as the legal limits on application rates and registered crops. Managing johnsongrass in the summer may be most effective in fallow fields, especially if left fallow for multiple summers. During a fallow season, multiple tillage and herbicide operations can maximize carbohydrate depletion and rhizome death. For fields with annual crops, a strategy of early preplant tillage to fragment rhizomes followed by light irrigation to encourage sprouting, mowing to deplete rhizomes, and application of a POST herbicide on actively growing plants can significantly reduce the competitiveness of johnsongrass. In this strategy, sprouts appearing after planting should also be expected and can be handled via cultivation andspot or band herbicide applications. In broadleaf crops, grass herbicides like fluazifop, clethodim, and sethoxydim are good options for controlling johnsongrass without harming crop plants. Perennial field crops will likely need between-row cultivation and spot application of herbicides to minimize damage to crops. In orchards, johnsongrass should be less invasive when canopies are closed and will likely be found only along field margins, so effective management of new infestations may be accomplished with mowing and a spot application of a POST herbicide. Established infestations in smaller tree orchards, such as prunes, may require cultivation between rows to about an 8-inch depth to fragment rhizomes and later a spot application of herbicide to kill new sprouts.

In non-crop areas, herbicides used in conjunction with mowing can be effective in managing or even eradicating johnsongrass, but care must be taken when selecting herbicides, as the desired vegetation may consist of species with different susceptibilities to the chemicals.spot or band herbicide applications. In broadleaf crops, grass herbicides like fluazifop, clethodim, and sethoxydim are good options for controlling johnsongrass without harming crop plants. Perennial field crops will likely need between-row cultivation and spot application of herbicides to minimize damage to crops. In orchards, johnsongrass should be less invasive when canopies are closed and will likely be found only along field margins, so effective management of new infestations may be accomplished with mowing and a spot application of a POST herbicide. Established infestations in smaller tree orchards, such as prunes, may require cultivation between rows to about an 8-inch depth to fragment rhizomes and later a spot application of herbicide to kill new sprouts. In non-crop areas, herbicides used in conjunction with mowing can be effective in managing or even eradicating johnsongrass, but care must be taken when selecting herbicides, as the desired vegetation may consist of species with different susceptibilities to the chemicals.Many herbicides are effective in controlling johnsongrass, but overreliance on one type of herbicide can impose selection pressure on johnsongrass populations to develop herbicide resistance. Herbicide-resistant populations of johnsongrass have been reported in several states, however to date there has been no reported herbicide resistance in California populations of johnsongrass. In general, the most common types of herbicide resistance have been to a more effective and sustainable johnsongrass management strategy should incorporate a variety of herbicide modes of action in order to decrease the possibility of herbicide resistance. Some johnsongrass populations may simply be tolerant to an herbicide, requiring a dosage far higher than usual to kill, while others may be totally resistant, in which case the plant will survive the treatment even if the rate of herbicide is significantly increased . Table 1 summarizes pesticide recommendations from product labels at the time of publication. Always refer to the current product label for complete instructions and restrictions. In addition to the federally required label, many herbicides used for johnsongrass management may have a supplemental Special Local Need label for specific instructions or limitations for handling and use. In addition, some California counties may restrict or forbid the use of certain herbicides. Application rates may must be adjusted based on soil textures and pH, and many herbicides restrict or forbid use on soils that are frozen, compacted, or that have high levels of organic matter. In addition, many herbicides have requirements or restrictions for irrigation before or after application, or when applied to plants under stressed conditions. Management of johnsongrass may be diminished or crop injury may occur if herbicides are applied under nonspecified conditions. Furthermore, many herbicides have postapplication restrictions on reentry, minimum preharvest intervals, crop rotation, replanting, or grazing. Foliar herbicides usually require adjuvants such as nonionic surfactants, crop oil concentrates, or methylated seed oils in the final mixture.In cooler coastal production areas you can plants beans from April through August, and even into September, depending on the potential for early frost. Because production costs are low , growers can risk early or late plantings to have beans in the market when supply is lower and prices are higher. Plant beans grown for harvest as dry beans by mid May on California’s Central Coast. In warmer inland valleys it is best to plant early while daytime temperatures are relatively moderate. Avoid June and July plantings, since fresh market bean quality and quantity will be significantly reduced when daytime temperatures are much above 95ºF . Beans can then be planted again in the late summer to allow for good production until the first frost. On soils with relatively good water holding capacity, large areas can be bedded, pre-irrigated, and then worked with a rolling cultivator following weed seed emergence.

Exotic-induced changes to soil have been proposed as a limiting factor in native restoration success, with short-term greenhouse studies demonstrating that native perennial grasses perform worse in soil conditioned by exotic annuals . Exotic annual grass roots concentrate in the top 30 cm of soil while native perennial roots extend over 1 m and so invasion reduces nutrient cycling, soil organic matter and water holding capacity in deeper soils . The exotic and native grasses have also been found to cultivate different soil food webs and microbial communities , which may have many direct and indirect effects on plant performance . Further, exotic annual seedlings outcompete native grass seedlings for soil moisture and light and so exotic vs native competition needs to be included to properly address the importance of PSFs in California grasslands. In this experiment, mixes of native perennial or exotic annual grasses conditioned soil for 11 years before a 2-year feedback phase where the native and exotic mixes were seeded and measured for multiple traits. The native and exotic mixes were seeded alone to test for general feedbacks, as well as together to address the role of competitor identity in the feedback. Both the conditioning and feedback phases of the experiment experienced extreme variability in precipitation. As with most PSF experiments, our experimental design focuses on the netfeedback effect of all plant-induced changes to the soil, and we can only make inferences about which mechanisms may drive these feedbacks. We hypothesize that 1) the exotic and native grasses differ in their effects on the physical and chemical properties of the soil and cultivate distinct microbial communities. We expect these differences to be particularly strong in the sub-surface soil, commerical grow racks where native roots are abundant, and exotic roots are sparse.

We also hypothesize that 2) the soil changes will lead to feedbacks in plant performance, and whether the feedback is positive or negative will vary across life stages. 2a) One possible outcome could be that both species groups perform best on their own soil, mediated by changes in carbon and nitrogen cycling, indicating that feedbacks are playing a role in the exotic invasion . Alternatively: 2b) despite the chemical changes to soil discussed above, plant performance for both native and exotic species will be dominated by negative plant-soil feedbacks due to the build-up of localized pathogens . Further, we hypothesize 3) that competition that occurs from growing the natives and exotics together will eclipse the effects of any feedbacks, as exotic annuals generally outcompete native seedlings .Measures in the conditioning phase address our first hypothesis, that native vs. exotic grassland communities differ in their effects on soil water holding capacity, soil organic matter, total C and N content, soil nitrogen cycling , and microbial community composition . Soil cores were collected in August 2018, after 11 growing seasons of soil conditioning. Soil samples were taken at four depths to capture effects due to differences in rooting depths between the exotic annuals and native perennials. Soil was sieved within 36 hours and stored at 4℃ from the time of collection until analysis. Time sensitive analysis of N cycling extractions and incubations were performed within 48 hours. Subsamples were stored at -20°C for microbial DNA sequencing for 26 months. Soil chemical and physical properties Water holding capacity was measured by saturating soil and determining the % soil moisture the soil retained after draining under gravity for 24 hours at 100% humidity . Soil organic matter was determined by combustion in a muffle furnace at 550°C for 4 hours . Total carbon and nitrogen content of dried, ground soil was analyzed with an elemental analyzer interfaced with a mass spectrophotometer by the UC Davis Stable Isotope Facility.

To grind the soil, air-dried samples were placed in a scintillation vial with grinding bars and then set on a roller mill for four days. To determine net mineralization and nitrification rates, we measured inorganic N concentrations in 5g of firesh soil as well as in 5 g of soil that was incubated for a week in the dark at room temperature. These soils were extracted with 25 mL of 2 M KCl , shaken on a mechanical shaker for an hour, filtered using pre-leached Whatman No. 1 filter paper, and stored at -20℃ until analysis on the spectrophotometer. From these extracts we quantified nitrate and ammonium concentrations following methods developed by Forster . To determine gravimetric soil moisture, within 24 hours of soil sampling, soil was dried at 105°C until reaching constant mass. Microbial DNA extraction, amplification, and sequencing DNA was extracted from the 88 soil samples using the Power SoilTM kit . The 16S rRNA and ITS2 region were amplified in a two-step PCR procedure, with the final pooled library quantified via qPCR and sequenced using 300-bp paired-end method with an Illumina MiSeq instrument in the Genome Center DNA Technologies Core, University of California, Davis. DNA extractions and library preparation were performed by the UC Davis Host Microbe Systems Biology Core Facility. Detailed experimental procedures and primers are found in Supplementary Methods and Tables S1.1 and S1.2.The sequencing data for both bacteria and fungi were analyzed as Amplicon sequence variants using the “dada2” package following the dada2 pipeline workflows . The SILVA and UNITE databases were used to taxonomically classify the bacteria and fungi sequences. Data were further processed using the “phyloseq” package for downstream analysis and raw sequence reads were normalized using the “metagenomeSeq” package , as rarefying reads has statistical concerns . Further ASV processing details are found in Supplementary Methods. To assess functional differences in the fungal community, functional guilds were assigned to the already taxonomically classified fungal ASV dataset using the FUNGuild database . Only ASVs with guild assignations of ‘probable’ or ‘highly probable’ were used for analysis on functional guilds, representing 43% of the taxonomically classified dataset. We simplified the guilds to the following: arbuscular mycorrhizae, ectomycorrhiza, plant pathogen, endophyte, and saprotroph, as well as plant pathogen – saprotroph, endophyte – saprotroph, and plant pathogen – endophyte – saprotroph which exhibit traits of multiple guilds. We excluded non-plant pathotrophs, orchid and ericoid mycorrhizae, and plant pathogen – endophytes due to low presence.To address our second hypothesis and determine whether exotic and native communities were structured by negative or positive feedbacks, we compared both native and exotic performance on soils that were cultivated by either native or exotic plants. Each group was grown alone and in competition with the other group to determine whether competition influences the strength of the feedback. Seeding treatments were applied to a subset of the Phase 1 plots, resulting in 8 native-soil and 8 exotic-soil plots. Phase 1 plots not used in Phase 2 were excluded due to the 2018 percent cover levels not meeting the original plot selection criteria . The 8 exotic-soil plots included 7 of the original 11 Phase 1 plots with another exotic-soil plot that met the selection criteria for Phase 1 but whose soil was not sampled.

To initiate Phase 2, all above ground vegetation and litter were removed from the plots shortly after the soil sampling of Phase 1. In December of 2018, after rains induced germination of the seedbank, the seedlings were killed with RoundUp ProMax . After waiting 10 days for complete herbicide disintegration in the soil, and directly preceding another germinating rain, vertical grow weed the original 1.5-m x1.5-m plot was split into three subplots in a design that gave equal area and perimeter to each subplot. The locations of seedling treatments were fixed , and each subplot was seeded with the dominant grasses found in the community treatments used in the conditioning phase: native community, exotic community, and native + exotic mix . In the 2nd growing season, two native-conditioned, native community subplots were excluded due to ground squirrel or flooding disturbance. We assessed a variety of responses encompassing the life span of the plants for two growing seasons to test how soil conditioning influences plant performance. Learning which life stages are most affected by plant-soil feedbacks will help determine the overall strength of the feedback and the ultimate impact on plant fitness . While the entire plot was seeded, only the inner 1.2-m x 1.2-m square was used for measurements, resulting in each subplot having an area of 0.48 m2 .Height, which provides a useful proxy for growth and avoids destructive sampling, was measured throughout both growing seasons to assess how soil conditioning affects timing of growth. In each subplot, eight individuals of each species were haphazardly chosen and measured. Each season, natives were measured at three monthly time points once they could be identified from the exotic seedlings. The exotics were measured only once each season, as they reach full height by the time they are identifiable by seed heads. Above ground biomass of the exotic seeded subplots was taken twice each growing season because phenology varies, and no one time point can capture peak biomass for all species. The native subplot was assessed only in the late season to minimize destructive sampling of the young perennials. Samples were clipped within a haphazardly tossed 10-cm diameter ring, oven dried at 50℃, and weighed. Below ground biomass was sampled within the above ground biomass ring, the same day as the second above ground sampling, with a 5-cm diameter core at 3 depths . Roots were washed from the soil, dried, and weighed. We visually measured percent cover of each species with the Daubenmire method at three monthly time points during each growing season, ending once peak flowering of all species was captured.To test our first hypothesis and determine whether the exotic and natives differ in their effects on water holding capacity, total %C and %N, C:N, soil organic matter, and net rates of mineralization and nitrification at different depths throughout the soil profile, we fit linear mixed effect models for each soil property . No response variable required transformation to meet assumptions of normality. The fixed effects for each model included soil conditioning , depth , and their interaction, while the random effects were plot and block . Analyses were performed with the “lme4” and “emmeans” R packages. To further test our first hypothesis and determine whether potential feedbacks may be driven by the microbial community rather than solely soil chemical or physical properties, we looked at the net impact of soil conditioning on fungal and bacterial community composition. Analyses were performed on the entire soil profile as well as each depth separately, as the variation attributed to depth may overshadow differences from soil conditioning, for not only do rooting depths differ but physical soil conditions vary across depths and influence composition independently of plant conditioning . Community composition was examined with permutational multivariate analysis of variance using the Bray-Curtis dissimilarity matrix on normalized reads, with soil texture and soil conditioning as fixed effects. Soil texture was included additively , but not in interaction with soil conditioning due to the unbalanced sample sizes. Community dissimilarity was tested for homogeneity of dispersion with the betadisper function from the “vegan” package and visualized with non-metric multidimensional scaling using Bray-Curtis dissimilarity matrix on 3 dimensions. Because previous studies in California grasslands found that native and exotic-dominated grasslands differ in fungal guilds and nitrifying bacteria , we focused on these key taxa in addition to the overall fungal and bacterial communities. We compared total relative abundance of each fungal guild and nitrifying family with Kruskal-Wallis tests on the normalized reads but assessed composition only for the nitrifying community, as fungal guilds had too few ASVs for adequate analysis.To evaluate whether exotic vs native conditioning of the soil results in a plant-soil feedback , and whether the feedback is influenced by the competitive environment , we fit models for each performance variable according to the type and distribution of the data. Exotic and native grass groups were analyzed individually, since we were interested in how each group’s performance varied with soil conditioning. While other model terms such as year and depth may be significant on their own, we only addressed them further if they significantly interacted with the soil conditioning and community treatments. The traditional feedback design involves pairing up individual plants in ‘home’ and ‘away’ soil and then calculating a feedback effect variable for each pair . However, due to the complexity of our experimental design, which involves multiple time points and depths, and an interaction with the competitive environment, we tested for potential feedback due to soil conditioning by model fitting with the original data .

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 .