Differential sensitivity of Kansas Palmer amaranth populations to multiple herbicides

A field survey was conducted in 2014 to determine the response of Palmer amaranth (Amaranthus palmeri S. Wats.) populations from 16 counties in southcentral Kansas to commonly used herbicides. The main objective was to evaluate the sensitivity of 28 randomly collected populations to field-use rates of glyphosate, chlorsulfuron, 2,4-D, dicamba, atrazine, and mesotrione herbicides; and further characterize the sensitivity levels in two populations. The response of all tested populations was categorized as less sensitive (<= 59% injury), moderately sensitive (60-89% injury), and susceptible (90-100% injury) based on visible injury at 21 days after treatment (DAT). Results from herbicide screening experiments indicated that 47, 36, 7, 75, and 32% of tested populations were less sensitive (putative resistant) to glyphosate, chlorsulfuron, 2,4-D, atrazine, and mesotrione herbicide, respectively. None of the tested populations showed less sensitivity (<= 59% injury) to dicamba herbicide. Whole plant dose-response assays on two populations collected from Kiowa (KW2) and Pratt (PR8) counties indicated that the KW2 population had 3.0-, 2.3-, 5.8-, and 8.4-fold less sensitivity to 2,4-D, chlorsulfuron, atrazine, and mesotrione, respectively, compared to a known susceptible (MHS) population. In contrast, the PR8 population was 2.0-, 8.6-, 10.6-, 3.7-, and 2.8-fold less sensitive to 2,4-D, glyphosate, chlorsulfuron, atrazine, and mesotrione, respectively, compared to MHS population. Overall, these results suggest that reduced sensitivity to glyphosate, chlorsulfuron, atrazine, and/or mesotrione is highly prevalent in southcentral Kansas. Growers should adopt diversified weed control methods to manage these difficult-to-control populations on their production fields.