Impact of burn-down herbicides on atrazine washoff from vegetation

Crop residue and living vegetation in no-till fields can intercept large amounts of the pesticides applied at the time of planting. Previous studies have shown that the type of plant tissue intercepting the pesticide can affect the amount washed off this report compares the washoff characteristics of two cover crops with dead crop residue before and after treatment with burn-down herbicides. Laboratory studies were conducted to determine the effect of burn-down herbicides, paraquat (1,1′-dimethyl-4,4′-bipyridylium dichloride) and glyphosate (N-(phosphonomethyl)glycine) on washoff of atrazine (2-chloro-4-(ethylamio)-6-(isopropylamino)-s-triazine) from ryegrass ( L.) and hairy vetch ( Roth.). Ryegrass and hairy vetch were treated with paraquat and glyphosate and one or five days later 14C-atrazine was applied. Ryegrass, hairy vetch and dead crop residue not treated with paraquat and glyphosate were included as controls. One day after application of atrazine, all treatments were subjected to 4.5 to 5 cm of simulated rainfall at 9 mm h−1, leachate was collected and analyzed for atrazine. Atrazine washoff from hairy vetch, ryegrass and crop residue not treated with glyphosate or paraquat ranged from 29–37%, 43–49% and 70–75%, respectively, of the amount applied. Paraquat was more effective than glyphosate in increasing the amount of atrazine washoff in both ryegrass and hairy vetch. Washoff was increased when the time between application of the burn-down herbicides and atrazine was increased from one to five days, especially for the ryegrass treatments. Results indicate that availability of herbicides applied to no-till cropping systems may be significantly affected by type of vegetation and bum-down herbicide treatment.