RUNOFF OF 2 SULFONYLUREA HERBICIDES IN RELATION TO TILLAGE SYSTEM ANDRAINFALL INTENSITY

The use of sulfonylurea herbicides is increasing because they are used at very low rates and offer greater weed management flexibility in pr e-and postemergence programs, Information is needed on the off-site mo vement of these compounds under different tillage systems. Our objecti ve was to evaluate the runoff potential of chlorimuron ethyl {(2-[[[[( 4-chloro-6-methoxy-pyrimidin-2-yl) amino]carbonyl]-amino]sulfonyl] ben zoic acid)}, nicosulfuron {(2[[(4,6-dimethoxypyrimidin-2-yl) aminocarb onyl]aminosulfonyl]-N, N-dimethyl-3-pyridinecarboxamide)}, and bromide in conventional (CT) and no-tillage (NT) production systems under sim ulated rainfall. The soil types were a Norfolk sandy loam (fine-loamy, siliceous, thermic Typic Paleudult) at a Coastal Plain location and a Pacolet sandy clay loam (clayey, kaolinitic. thermic Typic Kanhapludu lt) at a Piedmont location. The first simulated rainfall event (30 min ) consisted of a low (1.27 cm h(-1)) or high (5.08 cm h(-1)) rate appl ied 24 h after herbicide and bromide application. One week later, the high rainfall rate was repeated on all plots. Consistently greater run off occurred with CT compared to NT, Runoff differences between tillag e systems were more pronounced for the second rainfall simulation and also with the Piedmont soil. The average herbicide losses from the ini tial high rainfall simulation were 1.2 and 2.2% of the total applicati on (0.014 kg ha(-1)) for CT and NT, respectively, at the Coastal Plain location, Similar results were obtained at the Piedmont location, Mea n herbicide losses during the second rainfall simulation represented < 0.2% of the applied rate. There were no detectable concentrations of e ither herbicide found in the sediment phase of runoff. On the basis of these results, it is not expected that chlorimuron or nicosulfuron po se a significant threat to the environment via transport in runoff.