Leaching and runoff losses of herbicides in a tilled and untilled field

Abstract

Transport of herbicide mass by leaching and runoff was evaluated over several years. Herbicides were applied at recommended rates to conventional tillage (CT) and no-tillage (NT) planted corn ( Zea mays L.) fields on Hagerstown silty clay loam (Typic Hapludalf). Pre-emergence herbicide treatments on each tillage system included simazine (6-chloro- N, N′-diethyl-1, 3,5-triazine-2,4-diamine), atrazine (6-chloro- N-ethyl- N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine), cyanazine (2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl]amino]-2-methylpropanenitrile) and metolachor (2-chloro- N-(2-ethyl-6-methylphenyl)- N-(2-methoxy-1-methylethyl) acetamide). Application rates (active ingredient basis) were 1.7 kg ha −1 (simazine, atrazine) and 2.2 kg ha −1 (cyanazine, metolachlor). Percent herbicide losses (1984–1988) in root zone leachates, sampled in pan lysimeters embedded 122 cm deep in the soil profile, were greater under NT than under CT corn production. The opposite was true for runoff losses of herbicides and the magnitude of loss was less for runoff than for leaching. Total percolate yield and spray date-leaching or runoff event intervals were critical factors controlling surface or subsurface herbicide “loadings” in water. Areal losses were directly related to percolate or runoff yield and inversely related to these time increments. Also, unusually dry conditions in 1988 increased the soil residence time for cyanazine, resulting in a late-season leaching pattern somewhat atypical for this low persistence herbicide. During the 5 years, average CT leaching losses calculated from herbicide concentrations and lysimeter percolate volumes, collected in several pits, ranged from less than 0.01 to 1.05% of sprayed amounts. The more persistent herbicides, atrazine and simazine, were generally at the upper level of this range. Under NT conditions, average leaching losses ranged from 0.05 to 6.16% of sprayed amounts. Cyanazine losses were equal to or greater than simazine and atrazine losses in several seasons. In general, metolachor was the least mobile herbicide. Runoff losses (3–5% slope) were greatest in 1985 and 1986 for simazine under CT (0.62 and 0.51% of the applied rate, respectively). Respective losses under NT were 0.18% and less than 0.01%.