Over the past 20 years, an important discovery in herbicide chemistry has been herbicides that inhibit acetolactate synthase (ALS). Although there are 15 classes of chemistry which have been described as inhibitors of ALS, only four classes have been commercialized: sulfonylurea, imidazolinone, triazolopyrimidine, and pyrimidinylthiobenzoates (Saari et al. 1994). These four classes of herbicides have been widely used due to their relatively low use rates, limited environmental impact, low mammalian toxicity, wide crop selectivity, and high efficacy. The rapid adoption and persistent use of these herbicides has selected for weeds with resistance to ALS-inhibiting herbicides. Currently there are 43 monocots and 20 dicots reported worldwide to be resistant to ALS-inhibiting herbicides (Heap 1999). Within the Midwestern states, there are 14 species with reported resistance to ALS-inhibiting herbicides (Heap 1999).

Giant foxtail with putative resistance to ALS inhibitors was identified recently in Wisconsin (WI), Minnesota (MN), and Illinois (IL). These populations were identified in fields with a history of ALS inhibitor use in both corn and soybean. In addition, eastern black nightshade and green foxtail with putative resistance to ALS inhibitors were also identified recently in Wisconsin. Therefore, our objectives were to confirm and quantify resistance of giant foxtail from WI, MN, and IL, eastern black nightshade from WI, and green foxtail from WI to imidazolinone and sulfonylurea herbicides.