Herbicide Leaching and Distribution in Tilled and Untilled Soil

Leaching and distribution of several pesticides, applied at recommended rates, were evaluated in a Hagerstown silty clay loam (Typic Hapludalf) by analyzing soil fractions and pan lysimeter percolates (PLP) collected at a soil depth of 122 cm under conventional tillage (CT) and no-tillage (NT) corn (Zea mays L.) management. In general, maximum herbicide concentrations detected and mean total amounts in PLP were higher in 1985 than 1984 for both CT and NT, and were greater under NT conditions where more leachate was collected each year than in CT. Concentrations and amounts of simazine (6-chloro-N, N′-diethyl-1, 3,5-triazine-2, 4-diamine) and atrazine (6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine) were generally more substantial in PLP than cyanazine (2-[[4-chloro-6-(ethylamino)-1, 3,5-triazin-2yl] amino]-2-methylpropanenitrile), and metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide, particularly under NT conditions. In 1984, mean areal leaching losses (NT) were 0.6% (simazine, atrazine) and 0.15% (cyanazine, metolachlor) of applied rates (1.7 and 2.2 kg ha⁻¹, respectively). In 1985, mean areal losses (NT) were 3.4% (simazine, atrazine) and 1.6% (cyanazine, metolachlor). Respective CT losses were <0.1% (1984) and ranged from 0.4 to 1.0% (1985). Where worst-case conditions were considered (1985), mean leaching losses (simazine, atrazine) were 0.15 kg ha⁻¹, therefore, at least 91% (1.55 kg ha⁻¹) was retained and/or degraded within the soil profile or in the corn tissue after absorption. Soil core analysis corroborated PLP results revealing that simazine and atrazine were the most persistent and mobile herbicides, and were consistently detected at all soil depths to 122 cm. Differences in the yearly extent and magnitude of leaching losses were related strongly to rainfall distribution and number of leaching events proximal to the herbicide spray date. Paper no. 7913 in the Journal Series of the Pennsylvania Agric. Exp. Stn., University Park, PA 16802. This research was supported by funds from the Northeast Agric. Pesticide Impact Assessment Program (NAPIAP). USDA-CSRS no. 82-CSRS-2-2057.