Degradation of the herbicide EPTC in three Willamette Valley soils Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/m326m608d

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  • EPIC (S-ethyl dipropylthiocarbamate) is a selective herbicide which controls some annual grasses, annual broadleaves, and perennial grasses when incorporated into the soil before planting. In the last few years, reports have been received of certain fields in the Willamette Valley of Oregon in which EPIC was no longer effective. Preliminary investigations had indicated that this was not due to development of resistant weed strains but was more likely due to an abnormally rapid degradation of EPIC following application. Field and greenhouse studies were conducted to determine the rate of degradation of EPIC and the mechanism of loss of EPIC in soil from three Willamette Valley fields. Two "problem" soils, from fields where EPIC effectiveness had declined over the past 5 years, and one "non-problem" soil were used. In field studies, an extender treatment, R-33865, was used at three rates with EPTC and with vernolate (S-propyl dipropylthiocarbamate) at one rate. A safener, R-25788 (N,N-dially1- 2,2-dichloroacetamide), was added to all treatments to reduce corn injury. Oats were planted at weekly intervals to monitor herbicide disappearance. In the problem soils, oats survived when planted 3 weeks or more after herbicide treatment; in the non-problem soil, none survived until 4 weeks had elapsed. Addition of the extender at all three rates increased the persistence of EPTC in the soil by approximately 1 week. Addition of the extender had no effect on grain yield or growth of corn foliage on either the problem soils or the non-problem soil, but caused severe corn ear malformation from EPIC on the non-problem soil. Results from two greenhouse experiments, which included addition of the extender and two sterilization techniques, autoclaving and potassium azide (KN₃) treatment, were inconsistent. The extender and the KN₃ treatments were the most consistent in extending the life of EPIC in all three soils, but the effect was considerably less than expected. Autoclaving did not increase EPTC persistence.
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