Runoff of two sulfonylurea herbicides in relation to tillage system and rainfall intensity

The use of sulfonylurea herbicides is increasing because they are used at very low rates and offer greater weed management flexibility in pre- and postemergence programs. Information is needed on the off-site movement of these compounds under different tillage systems. Our objective was to evaluate the runoff potential of chlorimuron ethyl [(2-[[[(4-chloro-6-methoxy-pyrimidin-2-yl) amino]carbonyl]-amino]sulfonyl]benzoic acid)], nicosulfuron [(2[[(4,6-dimethoxypyrimidin-2-yl) aminocarbonyl]aminosulfonyl]-N, N-dimethyl-3-pyridinecarboxamide)], and bromide in conventional (CT) and no-tillage (NT) production systems under simulated 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 Kanhapludult) 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 applied 24 h after herbicide and bromide application. One week later, the high rainfall rate was repeated on all plots. Consistently greater runoff occurred with CT compared to NT. Runoff differences between tillage systems were more pronounced for the second rainfall simulation and also with the Piedmont soil. The average herbicide losses from the initial high rainfall simulation were 1.2 and 2.2% of the total application (0.014 kg ha-1) for CT and NT, respectively, at the Coastal Plain location. Similar results were obtained at the Piedmont location. Mean herbicide losses during the second rainfall simulation represented 0.2% of the applied rate. There were no detectable concentrations of either herbicide found in the sediment phase of runoff. On the basis of these results, it is not expected that chlorimuron or nicosulfuron pose a significant threat to the environment via transport in runoff