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 lo ngterm persistence and movement under different tillage and year-to-ye ar climatic differences have not been evaluates. We compared the persi stence and movement of alachlor [2-chloro-N-(2, 6-diethylphenyl)-N-(me thoxymethyl) acetamide] and cyanazine [2-{4-chloro-6-)ethylamino) 1,3, 5-triazin-2-yl]amino}-2-methylpropanenitrile] in soil under no-till an d conventional-till corn production plots that received equal amounts of herbicides from 1991 to 1994. Four large (two no-till and two conve ntional-till) field plots, established in 1986 to evaluate pesticide m ovement to groundwater, were used for this study. The tillage treatmen ts for the respective field plots were reversed before the corn planti ng in 1993. Thus, the plots were 5- and 6-year-old no-till and convent ional-till plots in 1991 and 1992, but only 1- and 2-year-old plots, r espectively, 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 week s after application. For both herbicides, time zero recovery was about 90% of the amount applied. Over the 4-year period, the amount of herb icide 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 we eks after application, the amount of alachlor and cyanazine on crop re sidue decreased by an average (over 4 years) of 83 and 75%, respective ly. Alachlor persisted in soil about 2 weeks longer than cyanazine, re gardless of tillage practice, and overall persistence was nearly two t imes longer for the conventional-till than for the no-till. For all ye ars, regardless of year-to-year rainfall differences, cyanazine leache d deeper in the soil profile than alachlor under no-till, whereas the reverse was true under conventional-till. Yearly comparison of the inf luence of rainfall patterns on herbicide movement in soil during the f irst 2 weeks after application showed that the presence of macropores and more movement of water through soil do not necessarily result in m ore herbicide leaching.