The agronomic, environmental, and economic implications of the introduction of terbacil for weed control in Oregon commercial peppermint operations Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9z903234x

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  • The use of terbacil for weed control was adapted by a high percentage of Oregon peppermint growers soon after its introduction in 1967. To help clarify some of the implications of terbacil usage, three studies were undertaken to collect information concerning (a) the movement and build-up of terbacil in various soils, (b) possible contamination of "waste" and ground irrigation water, and (c) the economic and cultural practice changes which resulted from terbacil usage. Using soil samples taken from commercial fields, depth of terbacil penetration into the soil profile was greater in lighter soils than in heavier soils. Residues seemed to accumulate near the surface of the ridge in rill irrigated fields. No evidence of continued build-up of the chemical with repeated applications was seen in the soils tested. Analysis of water samples taken from the waste ditch of a rill irrigated field showed only a low concentration of terbacil, and this decreased with each irrigation. No residues were found in water samples taken from two shallow wells located in treated fields. Analysis of cash weed control costs of growers in the Willamette Valley and Jefferson County showed sizeable savings with the use of terbacil in the majority of cases. There was a corresponding savings in time spent on mint operations. This savings in time and money may have contributed to the increase in peppermint acreage noted among those growers who participated in the study. Because of fear of spreading Verticillium wilt (Verticillium albo-atrum or V. dahliae), many growers had quit annual plowing of their mint before terbacil was introduced, and many had experienced problems with weeds under this scheme. When terbacil was introduced, it appeared to solve this problem by giving very good weed control on most of the weeds. However, problems developed because most perennial weeds and a few annuals were not controlled. At present this is a major concern. Terbacil has a long residual life in the soil which can be an important advantage for control of weeds in peppermint since peppermint is a perennial crop and quite tolerant of terbacil. However, few other crops are tolerant. Over half of the growers interviewed who had taken a treated field out of production had experienced trouble in following with other crops. Unfortunately, there seems to be no pattern or set of circumstances which can help the grower predict whether or not he will have difficulty.
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