Weedy rice emerging in cultivated rice populations could be explained by the quick loss of a few domestication alleles in regions of the genome that experienced relaxation of selective pressure. Based on an evolutionary dynamic described in [75], the evolutionary genetic dynamics we observe here could be those of deep divergence of non-endemic wild species at neutral loci but allele-sharing at key wild-like loci, which necessitates a closer look at candidate genes. An interesting further avenue of research will be to sequence known genetic regions that confer weedy characteristics and whose patterns have been identified in both weedy and cultivated rice to have a better understanding of the evolution of weediness as manifested in the California ecotype . The Rc locus would be a good candidate because most rice cultivated in California would carry the loss of function mutation, resulting in a white pericarp. We can determine if California weedy rice shares polymorphisms associated with the loss of function allele , but has re-evolved pigmented pericarp either by reversion or by changes at another locus. We could also test if California weedy rice has captured the ancestral functional allele through gene flow from another source such as cultivated rice with a red pigmented pericarp or wild or weedy relatives present outside of the US. In closing, weed dry rack we present compelling evidence of rapid independent origin of weedy races of rice from cultivated relatives and contribute to our understanding of adaptive evolution under domestication.
The use of divergence population genetics to track crop and weed interactions, as done in this study, is useful in understanding how weeds evolve and what approaches can be used to best control their spread. However, there is still much to learn about the extent to which contemporary populations diverge in genetic and morpho-physiological background from their non-wild progenitors during de-domestication.Site-specific weed control matches site-specific conditions with the proper herbicide and application rate. Spatially variable herbicide-rate applications can achieve the most effective application, because each part of the field receives a precise amount of herbicide based on its need. The benefits of this technology include a reduction in spray volume and consequently lower herbicide costs, time savings because of fewer stops to refill, and less non-target spraying, which reduces potential environmental risks . Reductions in herbicide use achieved with site-specific applications depend on the level of weeds in the field, but can be as high as 40% to 50% . In an evaluation of site specific, post emergence weed control of broad leaf and grass weeds in corn, Williams et al. showed a 51% reduction in rimsulfuron and an 11.5% reduction in bromoxynil plus terbuthylazine use, compared with conventional herbicide spraying. In a preliminary trial of post emergence weed-patch spraying in spring barley, a non-significant yield increase was observed when weeds were controlled in patches, but 41% less herbicide was used compared with whole-field spraying . We tested the hypothesis that weed patches present in specific locations of a field before the previous year’s harvest indicate where weeds will be present during the following growing season. Mapping these weed patches indicates where herbicides should be applied, and conversely, the absence of weeds indicates where little or no herbicide is required.
Although sampling is often performed on a larger grid than the grid used for pesticide application, geostatistics allows the estimation of weed populations between sample points, and thus the application map can be made to correspond with the width of the sprayer. Our objective was to evaluate site-specific herbicide applications of a pre-emergent herbicide using two types of weed maps developed from weed counts made the previous year, and to calculate the herbicide savings.We conducted a variable-rate experiment on an 11-acre portion of a 79-acre field located in Yolo County. The crops were processing tomato in 1999 and sun- flower in 2000. We developed weed maps from the tomato crop and used them to develop variable-rate applications the following year to sunflower. In sunflower, a pre-emergent herbicide is applied either before planting and mechanically incorporated, or after planting but before crop or weed emergence and incorporated mechanically or by irrigation. We studied the effectiveness of variable-rate application of a pre-emergent herbicide, although this technology can be used for post emergent herbicides as well. Processing-tomato seeds were planted from May 4 to 8, 1999. A preemergent herbicide, napropaminde , was applied in an 8-inch band, centered on the crop row before tomato planting. The field was hand weeded on May 26 and cultivated on June 3. A lay by postemergent herbicide,trifluralin , was applied on the sides of the bed and in furrows on June 20. Another hand-weeding followed on June 27. Furrows and sides of beds were again cultivated on July 26. The crop was harvested from Sept. 10 to 14, 1999. Using weed maps developed from the 1999 tomato crop, we developed variable-rate application maps for the following year. In 2000, sunflower was planted on March 4 and March 23 to 25 . Ethalfluralin was applied post plant, pre-emergent on March 28, followed by two cultivations . Sunflower male plants were destroyed on July 15, and female plants were harvested on July 21 to 22. Both crops were furrow-irrigated.Weed distribution was mapped in the tomato crop in 1999. The density of the weed population was assessed in two ways: by cumulative weed-seedling counts throughout the crop season or by mature-weed counts at the time of crop harvest .
Weed densities were estimated using a grid 165 feet wide and 185 feet long . The measurement unit was a 20-inchby-20-inch quadrat for seedling counts, and a 15-feet-by-17-feet grid cell for mature-plant counts. All data points were assigned north and east coordinates to allow the weed maps to be spatially analyzed in a geographic information system . Weed population densities estimated by the different methods were used to create continuous weed-density maps, utilizing an interpolation method to estimate weed densities between the sampled locations. The interpolated weed-density maps were used to create treatment maps based on weed infestation levels. The field map was divided into a matrix of cells, and the average weed infestation level was estimated for each cell. Infestation levels were defined as weed-free , medium or high . Levels were arbitrarily set to cover the range of observed densities. Herbicide treatment maps were created by assigning varying herbicide rates to each location according to infestation levels, and dividing the field into zones receiving the same herbicide rate. Zones were marked with colored flags. The three herbicide rates were 0, 0.75 and 1.50 pounds active ingredient per acre of ethal- fluralin . A portion of the field with the steepest gradient in weed infestation was selected for a split-plot experiment. The main effect was the treatment map source , and the secondary effect was ethalfluralin rate at three levels . The main plots were 15 feet wide and 2,508 feet long, and replicated four times. Each plot was split into 38 subplots of 15 feet wide and 66 feet long. One of the three herbicide rates was applied to each subplot based on the weed map. Each replication included a three-bed strip, drying rack weed which received a constant, full herbicide rate . One bed strip did not receive any herbicide application and served as an untreated control. All plots except the control were treated on March 28, 2000. Ethalfluralin was applied to the soil surface and cultivated immediately after application to incorporate the herbicide and remove any emerged weeds. The entire study area was cultivated at that time, including plots where no herbicides were applied. The variable-rate herbicide application was evaluated by density measurements of weeds that survived the treatment. Weed density measurements were made 2, 4 and 6 weeks after the herbicide application or cultivation. Measurements consisted of visual estimates of total weed cover for each subplot and counts of weed seedlings in 100-square-inch quadrats placed randomly 10 times per herbicide level in each replication. A prototype variable-rate applicator developed by the UC Davis Department of Biological and Agricultural Engineering was used in the experiment . Zones corresponding to the same treatment were marked with colored flags and rate changes were done manually. The VRA changes the application rate in about 0.1 second. The VRA traveled at a speed of 5 miles per hour , resulting in 1 to 2 feet of travel before the desired application rate was reached. A 3-foot buffer area around each change in herbicide rate was delineated and excluded from measurement after the variable-rate herbicide application.The cumulative weed-seedling density throughout the growing season was 35.6 plants per square yard, and average mature-plant density was 1.2 plants per square yard.
Eighty-five percent of all weeds were from the Solanaceae family and therefore, these were used for the subsequent development of weed density maps. The dominance of a few weed species in arable fields is characteristic of different cropping systems . Since tomato was the crop in 1999, it was reasonable to expect that weeds escaping control would be from the same family . Members of the same family of plants have similar physiology, which would make them less susceptible to herbicides used in that crop. The combination of hand-weeding, cultivation and herbicide treatment reduced the number of weeds reaching maturity. As a result, mature-weed density in the 1999 tomato field was much less than the seedling density. Two weed-density maps were used for the variable-rate herbicide experiment: one created from seedling counts and another based on mature-weed counts. For example, three field surveys were conducted during the 1999 season. Weed densities and distribution measured from the three surveys had a high degree of spatial correlation, indicating that highly infested areas of the field had high densities of nightshade weeds throughout the season. For the tomato crop, it was observed that the weed-seedling density was highest in June, 1.5 to 2 months after planting . Conditions were ideal for nightshade emergence in June given the warm temperatures and a tomato canopy that was still open enough for light to reach the soil surface. Cultivation occurred after the May seedling counts and may have moved seed into the ideal position for germination and subsequent irrigation.Where no herbicides were applied, weed cover was significantly less when using the mature-weed map, because it better-estimated weed cover the following year . There was no significant difference between map source for the 0.75 and 1.5 lb. a.i./acre herbicide rates. Based on seedling emergence 2 weeks after application, overall weed control was significantly better when the treatment maps were based on mature weeds with 58 weeds per square yard than on seedlings, which had 142 weeds per square yard. Weed cover was significantly less at 2 and 4 weeks after herbicide application when treatments were based on mature-weed maps compared with seedling maps, but did not differ at 6 weeks after treatment . Weed cover when no herbicide was applied ranged from 15% to 55%, 2 weeks after the experiment was initiated. High weed cover on some noherbicide plots points to a major pitfall of map-based, variable-rate applications of pre-emergent herbicides: Locations where no weeds were predicted to grow received no herbicide. This prediction was based on the presence or absence of weeds the previous year. The treatment map shows the no-herbicide plots in the middle of the field surrounded by plots receiving medium and high rates. Although this location was predicted to have weeds below the treatment threshold, weed seedlings emerged here the following year. Redistribution of seeds during harvest is probably the main reason for poor estimates in the no-herbicide areas, although other factors, such as seed dormancy and movement of seed by water or animals may also be a factor.There was a significant difference in weed control among herbicide rates. The no-herbicide plots had the highest number of seedlings, averaging 86 plants per square yard, 2 weeks after the experiment was initiated. The average number of surviving seedlings in the medium-rate plots was significantly lower, nine plants per square yard. The plot with ethalfluralin at the full rate had the least number of weeds , but there was no statistically significant difference between the half rate and full rate. All plots receiving the medium or full herbicide rate had weed cover below 10%, 2 and 4 weeks after application. Black nightshade and hairy nightshade were the only weed species surviving the high-rate treatment in relatively higher numbers.