SOIL DISSIPATION OF THE HERBICIDE ISOXABEN AFTER USE IN CEREALS

The soil dissipation of the herbicide isoxaben was studied in field trials in Belgium in winter cereals treated with green manure, cow manure or pig slurry before sowing. Wheat was sown and the soil sprayed with 125 g isoxaben ha-1 in October. Soil samples were analysed by GLC. Isoxaben dissipated with first-order kinetics over the first 6 months with half-lives of 2.9, 4.0, 4.8 and 6.6 months in unamended plots, and plots treated with green manure, cow manure and pig slurry respectively. The organic fertilizer treatments thus increased the isoxaben soil persistence. After 6 months, the organic fertilizer effect was less pronounced, leading to isoxaben soil concentrations in all treatments becoming similarly low. Isoxaben at 75 g ha-1 applied to winter wheat or barley in the spring gave dissipation half-lives of 2.1 and 2.4 months, leading to isoxaben soil residues at the beginning of August of 15-21 p.p.b., similar to levels resulting from the autumn application. The isoxaben soil residues remaining after harvest would be too low to damage a rotational crop. The rates of isoxaben soil dissipation were greater in the spring and summer seasons than during the winter. The dissipation rates of autumn-applied isoxaben were also studied in spring-sown sugar beet and the effects on a variety of spring-sown rotational crops were investigated.