DISSIPATION OF IMAZAPYR, FLUMETSULAM AND THIFENSULFURON IN SOIL

The degradation of imazapyr, flumetsulam and thifensulfuron applied at 500, 40 and 30 g active ingredient (a.i.) ha(-1), respectively, to si lt loam soil was studied under laboratory and field conditions. Herbic ide residues were analysed by a lentil (Lens culinaris L.) bioassay. R esults showed that temperature had a significant effect on herbicide d egradation, whereas the impact of soil organic matter and pH were less well defined. Half-lives for imazapyr, flumetsulam and thifensulfuron in soil samples from the 0-5 cm layer (6.4% organic carbon) at 15 deg rees C were 125, 88 and 5.4 days, respectively, and 69, 30 and 3.9 day s at 30 degrees C. In soil sampled from the 1520 cm layer (3.5% organi c carbon) half-lives were 155, 70 and 6.4 days, respectively, at 15 de grees C and 77, 24 and 4.8 days at 30 degrees C. A field experiment in vestigated the degradation and leaching of each herbicide under two pr ecipitation regimes [natural precipitation (208 mm), and natural preci pitation plus 75mm irrigation (283 mm)] over 4 months to a soil depth of 25 cm. Thifensulfuron degraded rapidly, whereas residues of flumets ulam and imazapyr leached below 25 cm in both the low-and high-precipi tation treatments after 4 months. Significant imazapyr residues were s till present in the soil to 25 cm depth after 3 months. A multicompone nt model for herbicide dissipation was developed and evaluated using d ata from the laboratory and field experiments.