The first confirmed case of herbicide resistance in Wisconsin was atrazine resistant common lambsquarters in 1979 (Heap 2015). Since then, herbicide resistance has been confirmed in 12 other weed species in Wisconsin. Resistance to photosystem II inhibitors such as atrazine and other triazine herbicides has been confirmed in smooth pigweed (1985), kochia (1987), and velvetleaf (1990), in addition to common lambs quarters in 1979. Resistance to ACCase inhibitors has been confirmed in only two species: giant foxtail (1991) and large crabgrass (1992). In contrast, resistance to ALS inhibitors has been confirmed in many species including kochia (1995) and eastern black nightshade, giant foxtail, green foxtail, and common waterhemp, all in 1999. More recently, resistance to ALS inhibitors has been found in giant ragweed (Marion et al. 2013; Stoltenberg et al. 2015) and common ragweed (Butts et al. 2015).
Glyphosate resistance in Wisconsin is a relatively recent occurrence compared to the instances of photosystem II inhibitor, ACCase inhibitor, and ALS inhibitor resistance noted above. The first confirmed case of glyphosate resistance occurred in 2011 in a giant ragweed population in Rock County (Glettner et al. 2012; Stoltenberg et al. 2015). Glyphosate resistance was subsequently confirmed in horseweed populations found in Jefferson County (Recker et al. 2013) and Columbia County (Recker et al. 2014). Following confirmation of glyphosate-resistant common waterhemp populations in Eau Claire and Pierce Counties (Butts and Davis 2015a, 2015b) and Palmer amaranth in Dane County (Butts and Davis 2015b, 2015c), glyphosate resistance concerns in Wisconsin have focused mostly on pigweeds (Amaranthus spp.). In 2015, there were 18 new reports of suspected glyphosate-resistant common waterhemp populations, bringing the total to 30 counties in which glyphosate resistance has been investigated since 2012 (Figure 1). In addition to the previously confirmed glyphosate-resistant common waterhemp in Eau Claire and Pierce Counties, molecular screening indicated glyphosate resistance in seven more counties in 2015. Glyphosate resistance in these seven cases has yet to be confirmed by whole-plant dose-response analysis at UW-Madison, but preliminary research indicates that whole-plant dose-response results are consistent with findings from molecular screening.