Soil persistence and metabolism of iodosulfuron in winter wheat crops

The sulfonylurea herbicide iodosulfuron 1 has been applied post-emergence at the dose of 10 g a.i. ha-1 on winter wheat crops grown on sandy-loam (Melle) or on clay soils (Leke, Gistel and Zevekote). The dissipation of iodosulfuron in soil followed a first order kinetics. After the application of iodosulfuron at Melle at the beginning of April 2000, the soil half-life of iodosulfuron in the 0-10 cm surface soil layer was 60 days. At the end of September 2000, i.e. one month after the winter wheat harvest, only 8% of the applied dose of iodosulfuron remained in soil as iodosulfuron itself. At the mid of November, iodosulfuron 1 was no more detected in the 0-10 cm surface soil layer. From the beginning of May till the end of July 2000, low concentrations of iodosulfonamide 2 and of iodosaccharin 3 were observed in the 0-10 cm surface soil layer, their maximum concentrations being 0.7 and 1.5 micrograms of equivalents of iodosulfuron 1 kg-1 dry soil, respectively. At the end of September, the metabolites 2 and 3 were no more detected in soil. At Leke, Gistel and Zevekote, iodosulfuron was applied at the beginning of May 2001, and its half-life in the 0-10 cm surface soil layer was 44, 30 and 35 days, respectively. The later application of iodosulfuron and the higher soil pH (about 8) at Leke, Gistel and Zevekote should explain the soil half-lives lower than at Melle (soil pH 6.2). In all the trials and since the treatment till the mid of November, iodosulfuron 1 and its metabolites 2 and 3 were not detected in the 10-15 and 15-20 cm surface soil layers.