Pendimethalin Wash‐Off from Cover Crop Residues and Degradation in a Loessial Soil

Abstract

Tillage and cover crops affect soil biological, chemical and physical properties that control the fate of herbicides in soil. Effects of conventional tillage (CT) and no tillage (NT) and either native winter annual vegetation, hairy vetch (Vicia villosa) or wheat (Triticum aestivum) on degradation of pendimethalin [N‐(1‐ethylpropyl)‐3,4‐dimethyl‐2,6‐dinitrobenzenamine] were investigated. Effect of pendimethalin sorption on residues of these cover crops on subsequent washoff by simulated rain was also examined. Surface (0 to 3 cm) Gigger soil (fine‐silty, mixed thermic Typic Fragiudalfs) from triplicate plots of each tillage by cover crop combination under long‐term cotton (Gossypium hirsutum) were fortified with pendimethalin (10 nmole g− 1), incubated (25 ± 1°C) for 7, 21, 42, and 63 d, extracted and analyzed for pendimethalin by HPLC. Degradation was generally faster in NT than CT soil. Within tillage treatments, degradation was faster under native vegetation and wheat cover than under vetch. First‐order degradation, half‐lives ranged from 47 d (CT vetch) to 17 d (NT native), however, the data were better described by a nonlinear degradation model. Sorption KDs (= µmole kg− 1/µmole L− 1) for pendimethalin retention on residues were determined in a batch study (1:500, residue:0.80 µM pendimethalin, shaken for 24 h, and analyzed by HPLC). Measured KDs were large, ranging from ~3000 L kg− 1 for native vegetation and wheat to ∼ 6000 L kg− 1 for vetch. When pendimethalin was spray‐applied at 1.12 and 2.24 kg ha− 1 to triplicate 4‐g samples of residues and desorbed by three 2‐cm simulated rains, < 1% was recovered. Low recovery was consistent with high sorption and predicted using KDs but indicates that benefits of pendimethalin for weed control in NT systems with dense cover crop residues warrants further investigation.