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This study was designed to compare rates of herbicide dissipation and leaching in side-by-side microplots that have been under no-till and plow-till practices for various time periods. Microplots were established within eight field plots (0.1 to 0.25 ha) that had been in no-till for 1 or 4 years, resulting in 1-year and 4-year no-till and 1-year and 4-year plow-till treatments. Before application of atrazine, alachlor, and cyanazine, surface crop residues were removed from the no-till treatments to ensure comparable and uniform applications to soil surfaces. Soil samples were collected at 8, 14, 21, and 32 days after application at depth increments of 0 to 1.5, 1.5 to 3, 3 to 5, 5 to 10, and 10 to 20 cm (weed root zone). The leaching rate was slower in the no-till than in the plow-till treatment. Atrazine and cyanazine levels in the top 1.5 cm of soil, relative to the remaining soil profile, were nearly 50% higher in the 4-year no-till than in the 4-year plow-till microplots. The leaching trend of the 4-year plow-till was similar to that of the 1 year for both no-till and plow-till soils. Regardless of the tillage age differences, the concentrations of atrazine and cyanazine were higher in the no-till than in the plow-till microplots for the 8-day sampling, contrary to observation in previous studies where crop residues were present on the no-till system at the time of herbicide application. Dissipation rates for all three herbicides were approximately linear and in the order of cyanazine > atrazine > alachlor. Also, results indicate distribution of residues within the soil profile were different for atrazine and cyanazine than for alachlor. This observation is most likely caused by the differences in herbicide formulations.

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