Variable herbicide responses among Illinois waterhemp (Amaranthus rudis and A. tuberculatus) populations

To determine the extent of herbicide resistance in the common and tall waterhemp populations of Illinois, USA, research was conducted to survey 59 waterhemp populations from 28 counties in Illinois for responses to several herbicides. Herbicides used in the survey included two acetolactate synthase (ALS)-inhibitors, glyphosate, a triazine, and a diphenylether. Using greenhouse experiments, six plants from each waterhemp population were treated with each herbicide and phenotypically evaluated 2 weeks after application. Herbicides were applied at rates that resulted in 50% injury (I₅₀) of a control population in preliminary experiments. Using I₅₀ rates of atrazine at 1.3 kg ai ha^-1, fomesafen at 33 g ai ha^-1, glyphosate at 210 g ai ha^-1, or imazethapyr at 35 g ae ha^-1, significant differences in herbicide responses were identified among the waterhemp populations to atrazine, glyphosate, and imazethapyr, but not fomesafen. Several populations were identified with differential responses to the two ALS-inhibiting herbicides, suggesting different mutations conferring ALS-inhibitor resistance. Results of this study confirmed that waterhemp is highly variable in response to herbicides, and numerous resistant populations exist in Illinois.