Influence of straw management, nitrogen fertilization and dosage rates on the dissipation of five sulfonylureas in soil

The dissipation behaviour of metsulfuron-methyl, tribenuron-methyl, thifens ulfuron-methyl, triasulfuron and amidosulfuron were studied in soil followi ng post-emergence spring applications to cereal crops. Incorporation or rem oval of straw and different fertilizer applications had no influence on the disappearance time (DT50) Of the herbicides. However, in laboratory trials dissipation of metsulfuron-methyl, triasulfuron and amidosulfuron at highe r application rates was accelerated after incorporation of straw into the s oil. The addition of straw decreases soil pH thereby causing faster hydroly sis of the compounds. Addition of nitrogen fertilizer increased the half-li ves (t(1/2)) of the herbicides. Combination of both straw and N fertilizer, however, had no effect. Neither straw nor N influenced the degradation of tribenuron-methyl and thifensulfuron-methyl. The DT50 of all herbicides investigated varied from 6 to 17 days in the fie ld trials. Except for amidosulfuron, herbicides could not be detected 38-68 days after application. In the laboratory, t1/2 values were higher than th ose in the field at 75 days (amidosulfuron), 63 days (metsulfuron-methyl), 35 days (triasulfuron) and 13 days (tribenuron-methyl). Only the t(1/2) of thifensulfuron-methyl was at 29 h faster under laboratory conditions. T-1/2 values of an herbicides were influenced by their initial concentration. Th e t(1/2) values of amidosulfuron and tribenuron-methyl at 1 mg a.i. kg(-1) soil were 2-2.5 times higher than at the recommended held application rate of 42 mug a.i, kg(-1) soil.