HERBICIDE LEACHING UNDER TILLED AND NO-TILLAGE FIELDS

The effect of no-tillage practices on water quality exiting the root zone of deep, well-drained fields is largely unknown. This project was initiated to determine herbicide leaching characteristics as influenced by tillage practice and herbicide formulation. The research site consisted of four adjacent (0.25-ha) fields, two fields each dedicated to either tilled or no-tillage management. One field in each tillage regime received a controlled-release formulation of atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine and alachlor [2-chloro-N-(2,6-diethylphenyl)-N- (methoxymethyl)-acetamide, starch encapsulated], while the others received standard herbicide formulations of atrazine and alachlor. Both herbicide formulations were annually applied at the same rate: 1.7 kg ha(-1) for atrazine and 2.8 kg ha(-1) for alachlor. Atrazine, deethylatrazine [DEAT; 6-chloro-N-(1-methylethyl)-1,3 ,5-triazine-2,4-diamine), alachlor, and Br- concentrations were monitored with 12 suction lysimeters (six each at 1.5 and 1.8-m depths) in each field. Alachlor was detected in <3% of all samples collected, regardless of tillage practice or herbicide formulation, while atrazine was detected in >41% of the samples. Under no-tillage, atrazine was detected in <28% of the samples with <13% exceeding the U.S. Environmental Protection Agency Health Advisory level of 3 mu g L(-1) atrazine. Under tilled conditions, 53% of the samples contained atrazine, with 35% exceeding 3 mu g L(-1) atrazine. Averaged atrazine metabolite concentration of DEAT under no-tillage was 0.52 mu g L(-1) vs. 0.39 mu g L(-1) for tilled fields. Similar Br- transport between tillage practices and reduced atrazine levels under no-tillage fields suggest that no-tillage management, on deep well-drained soils, can have a positive impact on groundwater quality.