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The degradation of imazapyr, fiumetsulam and thifensulfuron applied at 500, 40 and . M) g active ingredient (a.i,) ha"', respectively, to si!t loam soil was sttidied under laboratory and field conditions. Herbicide residues were analysed by a lentil {Lem culinaris L.) bioassay. Results showed ihat temperature had a significant effect on herbicide degradation. Vk'hereas the impact of soil organic matter ami pH were less well defined. Half-lives for iniazapyr, flumetsulam and thifensulfuron in soil samples from the 0-5 cm layer (64% organic carbon) at 15 "C were 125, 88 and 5-4 days, respectively, and 69, 30 and .^-9 days at 30 °C. ]n soil sampled from the 15-20 cm layer (3-5% organic carbon) half-lives were 155, 70 and 6-4 days, respectively, at 15 "C and 77, 24 and 4-8 days at 30 °C. A field experiment investigated the degradation and teaching of each herbicide under two precipitation regimes (natural precipitation (208 mm), and nattjral precipitation pJus 75 mm irrigation (283 mm)] over 4 months to a soil depth of 25 cm. Thifensiitfuron degraded rapidiy, whereas residues of flumetsulam and imazapyr leached below 25 cm in both the low-and highprecipitasion treatments after 4 months. Significant itnai^apyr residues were still present in the soil to 25 cm depth after 3 months. A miilt:component model for herbicide dissipation was developed and evaluated using data from the laboratory and fieid experiments. lntrottuction

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