S -Metolachlor Persistence in Soil as Influenced by Within-Season and Inter-Annual Herbicide Use

The ability to predict fate of pesticides in the environment is crucial for sustainable management. This process starts with a proper characterization of the dissipation of a parent compound under different sets of conditions. There is widespread use of S-metolachlor across multiple common cropping systems, modes of action among commercially available herbicides are limited, and concern is rising of increase in resistance to S-metolachlor in problematic weeds, including species such as Palmer amaranth (Amaranthus palmeri). Therefore, S-metolachlor dissipation was investigated in fortified soil collected from a field plot experiment in order to determine the influence of single or repeated within-season herbicide applications to accelerate dissipation. S-metolachlor dissipation in fortified soils was then investigated in samples collected from paired, commercial fields, all of silt loam soil texture. Producer fields from five Arkansas counties were categorized as low-use (0 to 2 years of application) or high-use (5 to 6 years of application within the past six years). S-metolachlor concentrations were extracted from soil during a 56-day incubation and S-metolachlor degradation was modeled to calculate half-life. Two within-season applications of S-metolachlor increased persistence in fortified soils, and dissipation was faster in soils collected 1 and 24 days compared to 59 days after planting soybean [Glycine max (L.) Merr.].The half-life of S-metolachlor did not decrease in soil from fields with high-use (5 to 6 years) compared to soil collected from fields with low-use (0 to 2 years) of herbicide application history. Dissipation of S-metolachlor in soil was not enhanced by repeated short-term intra-annual or longer-term inter-annual applications. Results highlight the importance of management practices and soil properties in assessing the fate of S-metolachlor.