Submitted to: Environmental Science and Technology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/15/2000
Publication Date: N/A
Citation: Interpretive Summary: Isoxaflutole is a new herbicide marketed for broadleaf and grass weed control in corn, but little information has been published on the soil behavior and environmental fate of the compound. The herbicide exhibits an unusual behavior in which it is reactivated by rainfall events, providing control of small weeds that have emerged. Isoxaflutole degrades quickly in water, thus our knowledge of the compound's soil sorption properties are limited, because of extensive degradation that occurs during sorption measurements. An experiment was performed to characterize the sorption of isoxaflutole during rapid hydrolysis to its bioactive product. The studies were conducted with both isoxaflutole and the degradation product using four Illinois soils of the Drummer, Flanagan, Catlin, and Cisne series ranging about 2.5-fold in organic matter content. Sorption of isoxaflutole was 6 to 12-fold greater than diketonitrile, with the greatest difference in the soils low in organic matter. Results suggest that the combined effects of desorption (release of the compound from the soil) and degradation promote reactivation of the herbicide after rainfall, and contribute to the efficacy of the compound by resupplying the soil solution with a bioactive product. In addition to explaining this herbicide's unusual "rechargeable activity", these findings provide useful insight into the differences in tendency for isoxaflutole versus its degradation product to move in the environment. Such information is used in the development of predictive models that guide management practices in order to minimize herbicide inputs and reduce migration of chemicals off the field. The overall impact of this work is to reduce negative effects of agriculture on the environment.
Technical Abstract: Isoxaflutole is a new herbicide marketed for broadleaf and grass weed control in corn, but little information has been published on the environmental properties of the compound. the herbicide exhibits an unusual behavior in which it is reactivated by rainfall events, providing control of small weeds. Isoxaflutole is labile in aqueous solution, thus measuring equilibrium sorption is challenging. An experiment was conducted to characterize the sorption of isoxaflutole during rapid hydrolysis to its bioactive diketonitrile derivative. The transformation was measured over time aqueous solution with or without soil. At 25 C, 83% of the parent compound remained in solution at 24 h in the soil free system, but only 15% remained in the solution in the presence of soil. The sorbed phase consisted mainly of isoxaflutole, although a small percentage of diketonitrile was also detected in increasing concentrations as the study progressed. Hydrolysis prevented the attainment of sorption equilibrium, thus the apparent Kd of isoxaflutole increased over time, while that of diketonitrile remained close to zero at both 5 and 25 C. isotherms were conducted with both isoxaflutole and diketonitrile using four soils ranging in organic carbon (OC) from 1.0 to 2.5 %. Freundlich Kd values were 6 to 12-fold greater for isoxaflutole than diketonitrile, with the greatest difference in the lower organic carbon soils. Based on soil OC content, Koc values of 134 and 17 mL g-1 were calculated for isoxaflutole and diketonitrile, respectively. Results suggest that desorption coupled to hydrolysis promotes reactivation of the herbicide's function after rainfall, and contributes to the efficacy of the compound by resupplying the soil solution with a bioactive product.