Alachlor and cyanazine persistence in soil under different tillage and rainfall regimes

Herbicide dissipation in soil has been reported extensively using soil cores/columns in the laboratory or in short-term held studies, but longterm persistence and movement under different tillage and year-to-year climatic differences have not been evaluates. We compared the persistence and movement of alachlor [2-chloro-N-(2, 6-diethylphenyl)-N-(methoxymethyl) acetamide] and cyanazine [2-{4-chloro-6-)ethylamino) 1,3,5-triazin-2-yl]amino}-2-methylpropanenitrile] in soil under no-till and conventional-till corn production plots that received equal amounts of herbicides from 1991 to 1994. Four large (two no-till and two conventional-till) field plots, established in 1986 to evaluate pesticide movement to groundwater, were used for this study. The tillage treatments for the respective field plots were reversed before the corn planting in 1993. Thus, the plots were 5- and 6-year-old no-till and conventional-till plots in 1991 and 1992, but only 1- and 2-year-old plots, respectively, in 1993 and 1994. Each year, after herbicide application, alachlor and cyanazine residues were determined at the soil surface, at time zero, and in the upper 50-cm soil profiles at 2, 4, and 8 weeks after application. For both herbicides, time zero recovery was about 90% of the amount applied. Over the 4-year period, the amount of herbicide intercepted by crop residue in the no-till plots ranged from 60 to 70% for alachlor and 43 to 55% for cyanazine. During the first 2 weeks after application, the amount of alachlor and cyanazine on crop residue decreased by an average (over 4 years) of 83 and 75%, respectively. Alachlor persisted in soil about 2 weeks longer than cyanazine, regardless of tillage practice, and overall persistence was nearly two times longer for the conventional-till than for the no-till. For all years, regardless of year-to-year rainfall differences, cyanazine leached deeper in the soil profile than alachlor under no-till, whereas the reverse was true under conventional-till. Yearly comparison of the influence of rainfall patterns on herbicide movement in soil during the first 2 weeks after application showed that the presence of macropores and more movement of water through soil do not necessarily result in more herbicide leaching.