The spread of common waterhemp (Amaranthus rudis) and Palmer amaranth (Amaranthus palmeri) has become an increasing concern in Wisconsin (Hammer et al. 2016b). Both species are well-known for their competitive ability, abundant seed production, and propensity for developing herbicide resistance. Herbicide-resistant common waterhemp was first confirmed in Wisconsin in 1999, when a population was found to be resistant to ALS-inhibitors. More recently, glyphosate resistance was confirmed in two waterhemp populations in west-central Wisconsin (Butts and Davis 2015a).

The first occurrence of Palmer amaranth in Wisconsin was documented in 2013 (Davis and Recker 2014). This population was subsequently confirmed to be resistant to glyphosate (Butts and Davis 2015b). Since that time, Palmer amaranth has been found in three additional counties in Wisconsin. Responding to the increasing concern of Wisconsin growers, we have investigated several instances of suspected herbicide-resistant common waterhemp and Palmer amaranth. Our methods and findings are described.