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No-tillage corn (Zea mays L.) and soybean Glycine max (L.) Merr. production has been widely accepted in the mid-Atlantic region, favoring establishment of horseweed Conyza canadensis (L.) Cronq.. Within 3 yr of using only glyphosate for weed control in continuous glyphosate-resistant soybeans, glyphosate failed to control horseweed in some fields. Seedlings originating from seed of one population collected in Delaware were grown in the greenhouse and exhibited 8- to 13-fold glyphosate resistance compared with a susceptible population. There were no differences between the isopropylamine or diammonium salts of glyphosate.
Mark J. VanGessel (Thu,) studied this question.