Site-specific herbicide applications based on weed maps provide effective control

Abstract

More-effective weed control in agricultural fields can be achieved by utilizing information about the spatial distribution of the previous year's mature weeds. In our study, variable-rate herbicide applications based on weed infestation maps developed just before the previous year's harvest provided effective weed control. The results showed that when information about the spatial distribution of the previous year's weed seedlings or mature weeds was used, weed control was comparable to uniform, one-rate, herbicide applications, while the total amount of herbicide applied decreased. Herbicide use was reduced an estimated 39% for the seedling map and 24% for the mature map approach. However, incorporating the weed-seed redistribution from harvest to application time into the treatment maps could further improve weed control. S ite-specific weed control matches site-specific conditions (such as soil properties and weed infestation densities) 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 her-bicide costs, time savings because of fewer stops to refill, and less nontarget spraying, which reduces potential environmental risks (Felton 1995). 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% (Gerhards et al. 1997). In an evaluation of site-specific, postemergence weed control of broadleaf and grass weeds in corn, Williams et al. (2000) showed a 51% reduction in rimsulfuron and an 11.5% reduction in bromoxynil plus terbuthyla-zine use, compared with conventional herbicide spraying. In a preliminary trial of postemergence weed-patch spraying in spring barley, a nonsignifi-cant yield increase was observed when weeds were controlled in patches, but 41% less herbicide was used compared with whole-field spraying (Heisel et al. 1997). 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, geostatis-tics 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. Field test on sunflower 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 sunflower 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 postemergent herbicides as well. Processing-tomato seeds were planted from May 4 to 8, 1999. A pre-emergent herbicide, napropaminde (Devrinol), 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 layby postemergent herbicide, Weed maps were developed to guide variable-rate herbicide treatments in sunflower. When herbicides were applied based on the weed-seedling and mature-weed maps, 15% and 19% of the respective plots did not receive any treatment. Above, the nightshade density was low in parts of the sunflower field where no herbicide was used, indicating a good relationship with the previous year's weed population.