Fate of the herbicide isoxaflutole in the soil of corn fields

The herbicide isoxaflutole 1 (5-cyclopropyl-4-isoxazolyl)[2- (methylsulfonyl)-4-(trifluoro-methyl)phenyl]-methanone) was applied pre-emergence at the rate of 125 g ha-1 on corn fields located in three sites different as to their soil texture and composition. In the 0-10 cm surface soil layer, the isoxaflutole soil half-life (soil dissipation kinetics of second order) was 9 days in sandy loam (Melle), 15 days in clay loam (Zevekote) and 18 days in loamy sand (Zingem) soil. The sum of the concentrations of isoxaflutole 1 and of its herbicide active metabolite diketonitrile 2 (2-cyano-3-cyclopropyl-1-(2-methylsulfonyl-4- trifluoromethylphenyl)propane-1,3-dione) had a soil half-life (dissipation kinetics of first order) of 45 days in sandy loam, and 63 days in the clay loam and loam sand soils. The soil metabolism of isoxaflutole thus generated, in the soil of field corn crops, a metabolite, the diketonitrile 2, which had an herbicide activity as high as the one of the parent isoxaflutole, and which much extended the herbicide protection given by isoxaflutole. At the crop harvest, isoxaflutole, the diketonitrile 2 and the acid 3 (2-methylsulfonyl-4-trifluoromethylbenzoic acid) were no more detected in soil. During the corn crops, isoxaflutole, and its metabolites diketonitrile 2 and acid 3 were never detected in the 10-15 et 15-20 cm surface soil layers, indicating the very low mobility of these compounds in soil.