BIOTYPES OF PALMER AMARANTH (AMARANTHUS-PALMERI) AND COMMON WATERHEMP(AMARANTHUS-RUDIS) ARE RESISTANT TO IMAZETHAPYR AND THIFENSULFURON

Seeds of suspected herbicide-resistant Palmer amaranth and common wate rhemp were collected in Clay County and Douglas County, KS, respective ly. An experiment was established in a greenhouse to determine if thes e species had developed resistance to imazethapyr and thifensulfuron. Imazethapyr was applied pre- (PRE) and postemergence (POST) at 1x, 2x, 4x, and 8x the suggested use rate (70 g/ha), and thifensulfuron was a pplied POST at 1x, 2x, 4x, and 8x the suggested use rate (4.5 g/ha). B oth species had developed resistance to all rates of these herbicides. The occurrence of resistance at the Clay County site (Palmer amaranth ) fit the typical pattern for the development of herbicide resistance, i.e., multiple applications of the same class of herbicide for severa l years. However, the Douglas County (common waterhemp) site had a lim ited history of use of ALS-inhibiting herbicides and did not follow ty pical models of resistance development.