Mapping risk areas for resistance to ACCase inhibitor herbicides in Manitoba

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Abstract
Since 1976, seven acetyl coenzyme-A carboxylase (ACCase) inhibitors (referred to as Group 1 herbicides) have been registered in western Canada for wild oat (Avena fatua L.) and green foxtail (Setaria viridis L.) control. In 1990, Group 1 resistant wild oat populations were identified from fields in Manitoba which had been repeatedly sprayed with these products during the previous 10 yr. Since the occurrence of resistance is directly related to the frequency of herbicide use, the purpose of this study was to compile herbicide use histories on a province-wide basis using data included in the Manitoba Crop Insurance Corporation (MCIC) database. The database was used to determine the relative importance of Group 1 herbicide use in major crops compared with other products, and to identify individual townships at low, medium and high risk for developing Group 1 resistance. Low, medium and high risk townships were arbitrarily defined as those in which Group 1 products were used on less than 30%, on 30 to 50% and on more than 50% of the sprayed fields, respectively. From 1981 to 1993, Group 1 herbicide use increased from 15 to 50% of the sprayed area, and since 1990, these products have been used on one out of two sprayed fields on an annual basis, with the most intensive use in flax. In the early 1980s, fewer than 5% of the townships were at high risk. These were located near the towns of Swan River, Dauphin and Treherne, and the city of Winnipeg, which were the locations where Group 1 resistant wild oat and green foxtail were first identified. Between 1989 and 1993, more than 40% of the townships were considered to be at high risk. Should the trend toward increased use of Group 1 herbicides continue through the 1990s, the resistance problem will inevitably worsen. Key words: Herbicide use, selection, herbicide rotation

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