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.

Nomenclature: Corn; Zea mays L.; horseweed; Conyza canadensis (L.) Cronq. ERICA; soybean; Glycine max (L.) Merr.; glyphosate.