Effect of Herbicide Concentration on Atrazine, Pyroxasulfone, and Saflufenacil Dissipation Under Field and Lab Conditions

Widespread herbicide resistant weeds over the last several years have fueled a renewed interest in the utility of soil-applied residual herbicides. Residual activity of a herbicide is based on a myriad of factors within and among environments. A common assumption used in simulation models examining herbicide persistence is that herbicide degradation is independent of application rate. These studies compared herbicide concentration on dissipation with three commonly used residual herbicides under field and lab conditions. Atrazine, pyroxasulfone, and saflufenacil represent herbicides with a relative field persistence of medium, high, and low, respectively. Field studies were conducted over two years where herbicides and rates were assembled in a factorial arrangement of treatments and herbicides were applied at rates of 100, 1000, and 10000 g ai ha^-1. These application rates are quite divergent from labeled rates, and were used to elucidate the effect of herbicide dose on subsequent dissipation. Soil samples were collected on selected intervals over the course of 365 days to detect dissipation of the herbicides. The field study indicated that persistence is affected by herbicide concentration. Generally, higher herbicide concentrations exhibited slower dissipation rates while lower herbicide concentrations exhibited more rapid dissipation. Lab studies used a factorial arrangement of treatments where the three herbicides were combined with varying concentration rates based on water solubility. Soils were placed in an incubator set for ambient temperature conditions. Lab studies generally agreed with the field studies, and indicated higher herbicide concentrations showed slower herbicide dissipation compared with the lower herbicide concentrations. These studies suggest that herbicide concentration has an effect on the dissipation of atrazine and pyroxasulfone in field and lab conditions. This finding would be important for those using herbicide degradation rates in simulation modeling, since herbicide degradation is often assumed to be independent of rate applied.