Impact of application time on soil residual herbicide dissipation in early-planted soybean

Abstract In the Midwest United States, early soybean planting is becoming more common, but the implications for soil residual herbicide dissipation and optimal application time remain unclear. Earlier planting extends the interval between soil-residual herbicide application at planting and the onset and peak of weed emergence, potentially reducing efficacy through an extended window for dissipation. This study aimed to evaluate the dissipation and weed control efficacy of soil residual herbicides applied at different timings in early planted soybean crops under varying soil conditions. At Arlington, Wisconsin, which has a silt loam soil, herbicide concentrations when soil was sampled, 21 d after the fourth and final application of herbicides, which followed a series of treatments from planting to the V1 growth stage of soybean, were similar across application times, whereas at Brooklyn, Wisconsin, which has a sandy loam soil, herbicide concentrations were usually higher when herbicides were applied later. Despite these differences, weed density at postemergence was similar across application times within each site. However, an additional late postemergence herbicide application was necessary at Brooklyn following the earliest application times in 2022, indicating more rapid herbicide dissipation. Herbicide dissipation and efficacy varied by soil texture. In sandy soils, early applications may lead to reduced control of late-emerging weeds due to rapid dissipation. In contrast, finer-textured soils may allow for more flexible application timing. These insights support site-specific residual herbicide application strategies in early planted soybean systems.