Soil Metabolism of the Herbicide Isoxaben in Winter-wheat Crops

Winter wheat fields were treated wi th the herbicide isoxaben after sowing. Trials were made in 1990-1991 and 1991-1992. The main isoxaben soil metabolite was demethoxyisoxaben (N-[3-(1-ethyl-1-methylpropyl)isoxazol-5-yl]-2-hydroxy-6-methoxybenzamide), i.e., the monodemethoxylation product of isoxaben. 5-Isoxazolone(3-(1-ethyl-1-methylpropyl)isoxazolin-5-one) was the second main isoxaben metabolite. When 5-aminoisoxazole(5-amino-3-(1-ethyl-1-methylpropyl)isoxazole) was detected in soil, it always was at very low concentrations. It never accumulated in soil; 4 months before winter wheat harvest, it could not be detected in soil. Benzamides 2,6-dimethoxybenzamide and 2-hydroxy-6-methoxybenzamide and 2,6-dimethoxybenzoic acid also were detected in soil. Organic fertilizer treatments increased isoxaben soil persistence. At the crop's end, their effects, however, progressively disappeared, the soil residues of isoxaben and of its metabolites becoming very low and similar in the organic fertilizer treated and untreated plots. 5-Aminoisoxazole was not detected. This work thus indicated that isoxaben was soil-metabolized into nontoxic products, unable to generate toxic ones, during the wheat crops whose soil had been treated or not treated with organic fertilizers.