SOIL DISSIPATION OF THE HERBICIDE ISOXABEN AFTER USE IN CEREALS

The soil dissipation of the herbicide isoxaben was studied in field tr ials in Belgium in winter cereals treated with green manure, cow manur e or pig slurry before sowing. Wheat was sown and the soil sprayed wit h 125 g isoxaben ha-1 in October. Soil samples were analysed by GLC. I soxaben dissipated with first-order kinetics over the first 6 months w ith 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 respective ly. The organic fertilizer treatments thus increased the isoxaben soil persistence. After 6 months, the organic fertilizer effect was less p ronounced, leading to isoxaben soil concentrations in all treatments b ecoming similarly low. Isoxaben at 75 g ha-1 applied to winter wheat o r barley in the spring gave dissipation half-lives of 2.1 and 2.4 mont hs, leading to isoxaben soil residues at the beginning of August of 15 -21 p.p.b., similar to levels resulting from the autumn application. T he 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 d issipation rates of autumn-applied isoxaben were also studied in sprin g-sown sugar beet and the effects on a variety of spring-sown rotation al crops were investigated.