Effect of initial herbicide concentration of atrazine, pyroxasulfone, and saflufenacil dissipation under field and laboratory conditions

Abstract Saflufenacil, atrazine, and pyroxasulfone represent herbicides with a relative field persistence of low, medium, and high, respectively. Field studies were conducted over 2 yr when herbicides and rates were assembled in a factorial arrangement of treatments, and herbicides were applied at rates of 100, 1,000, and 10,000 g ai ha −1 . Soil samples were collected over the course of 365 d and analyzed to detect dissipation of the herbicides. Regression analysis was used to quantify the dissipation of each herbicide. The initial herbicide concentration had no effect on the observed dissipation rates of atrazine or saflufenacil; however, pyroxasulfone dissipation was slower at the highest field dosage in both years. Soils from Georgia, Illinois, and Tennessee were fortified with known concentrations of the three herbicides dissolved in water and incubated at 22 C for 154 d. Laboratory studies generally demonstrated slower dissipation compared to field studies, which is plausible because the important loss mechanisms of volatilization or photodegradation do not occur in the laboratory test system. Pyroxasulfone and saflufenacil exhibited no effect of half-life from various initial concentrations, but atrazine exhibited slower degradation occurring at lower initial concentrations. Findings from these studies suggest that initial herbicide concentration has a limited effect on the dissipation of some herbicides: pyroxasulfone in the field and atrazine in the laboratory. This finding is important for researchers who use herbicide degradation rates in simulation modeling because herbicide degradation is often assumed to be independent of the rate applied. Another aspect of this research was the application of each herbicide alone and in combination with the others. Under field and laboratory conditions, there was no change in dissipation if the herbicides were applied alone or in combination.