FLUOMETURON WASH-OFF FROM COVER CROP RESIDUES AND FATE IN A LOESSIAL SOIL

Abstract

Cover crop residues on no-till soil will intercept a portion of applied herbicides. Thus, herbicide efficacy in no-till systems depends, in part, on rainfall to wash the herbicide onto the soil. Tillage and cover crop residue may also influence sorption and degradation of a herbicide in soil. This series of studies examined fluometuron [N,N-dimethyl-N'-[3-(trifluoromethyl)phenyl]urea] wash-off from native vegetation, hairy vetch (Vicia villosa), and wheat residue (Triticum aestivum), related wash-off to sorption on these residues, and compared fluometuron sorption and degradation in soil from long-term native, vetch, and wheat cover crop plots used with either conventional or no-till cotton (Gossypium hirsutum). A rainfall simulator was used to wash spray-applied fluometuron from plant material. Through-flow was analyzed for fluometuron by high-performance liquid chromatography. More fluometuron was washed off native vegetation than vetch or wheat residues, which retained fluometuron about equally. Fluometuron sorption on these residues was determined in a batch study. Sorption was least with native vegetation (K D = 11 L kg -1 ), and there was minimal difference between vetch and wheat in sorption (K D = 17 L kg -1 ). Fluometuron wash-off could be modeled from the batch sorption data. Sorption of fluometuron in surface soil from each tillage by cover crop combination was determined in a batch experiment. Sorption was adequately described by the Freundlich model with N ∼ 0.9 for all soils but K values of no-till soils (average 2 L kg -1 ) nearly twice that of corresponding conventional-till soils. Degradation of fluometuron in these surface soils was determined by incubating fortified samples for 6, 15, 30, and 60 days. Soil extracts showed that degradation was more rapid in any no-till soil than in its conventional-till analog. Within either tillage treatment, degradation was slowest in vetch soil. Half-lives ranged from 7 and 8 days (no-till native and wheat soils, respectively) to 51 days (conventional-till vetch soil). Fluometuron half-lives in conventional-till wheat and native (19 and 27 days, respectively) and no-till vetch (31 days) soils were intermediate. Microbial activity was higher in no-till soil, consistent with faster degradation.