Effects of Soil Water Content on Pendimethalin Dissipation

Abstract

Abstract Laboratory and field studies were conducted to evaluate pendimethalin [ N ‐(1‐ethylpropyl)‐3,4‐dimethyl‐2,6‐dinitroaniline] dissipation over time in a Crowley silt loam (Typic Albaqualfs). In the laboratory, dissipation rates approximately followed pseudo first‐order kinetics over a 56‐d period, except in air‐dried soil that had no significant loss of pendimethalin detected after 56 d. Half‐lives for 30 kPa, continuous flood, and alternately flooded and dried treatments averaged 59, 63, and 30 d, respectively, under laboratory conditions. Application rates of 0.5, 1.0, and 2.0 µ g/g of soil did not have a significant influence on the half‐lives. The ratio of pendimethalin residues in laboratory systems detected by a root bioassay with grain sorghum [ Sorghum bicolor (L.) Moench.] to those detected with gas‐liquid chromatography (GLC) declined by 17% from 0 to 56 d after treatment. Dissipation in the field in each of 2 y was studied with lowland rice ( Oryza sativa L.) (flush irrigated then flooded 2–3 weeks after application), upland rice (flush irrigated throughout the season), and soybean [ Glycine max (L.) Merr.] (furrow irrigated as needed) management systems. Soil water content had a strong influence on the amount of pendimethalin that dissipated, especially for about the first 2 weeks after herbicide application. Half‐lives in the field were much shorter during the initial 2 weeks than after 2 weeks, with >50% of the applied herbicide having disappeared in 1 week for all treatments except in soybeans the first year. Soil persistence of pendimethalin was greater under soybean culture (soil‐incorporated herbicide, low irrigation frequency) than under rice culture (surface‐applied herbicide, high irrigation frequency, and/or flooded conditions).