Dissipation of imazapyr, flumetsulam and thifensulfuron in soil

Abstract

The degradation of imazapyr, flumetsulam and thifensulfuron applied at 500, 40 and 30 g active ingredient (a.i.)ha-1, respectively, to silt loam soil was studied under laboratory and field conditions. Herbicide residues were analysed by a lentil (Lens culinaris L.) bioassay. Results showed that temperature had a significant effect on herbicide degradation, 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 °C were 125, 88 and 5-4 days, respectively, and 69, 30 and 3.9 days at 30 °C. In 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 leaching of each herbicide under two precipitation regimes [natural precipitation (208 mm), and natural precipitation plus 75 mm irrigation (283 mm)] over 4 months to a soil depth of 25 cm. Thifensulfuron degraded rapidly, whereas residues of flumetsulam and imazapyr leached below 25 cm in both the low- and high-precipitation treatments after 4 months. Significant imazapyr residues were still present in the soil to 25 cm depth after 3 months. A multicomponent model for herbicide dissipation was developed and evaluated using data from the laboratory and field experiments.