Field and greenhouse studies were conducted to evaluate the responses of several imidazolinone (IMI)-resistant Amaranthus hybridus populations to various acetolactate synthase (EC 220.127.116.11)-inhibiting herbicides. In 1996 field studies, IMI resistance was confirmed in one A. hybridus population (R4) that was not cross-resistant to the sulfonylurea herbicides CGA-277476, chlorimuron, or thifensulfuron. Amaranthus hybridus control with triazolopyrimidine herbicides was ≤ 35%, but control with cloransulam-methyl or flumetsulam plus cloransulam-methyl was higher than with IMI herbicides. Follow-up greenhouse studies were conducted in 1997 and 1998 to investigate the response of one IMI-susceptible (S) A. hybridus population collected near Painter, VA, and four IMI-resistant A. hybridus populations (R1, R2, R3, R4) collected from fields in Somerset County, MD, to postemergence imazethapyr, chlorimuron, thifensulfuron, pyrithiobac, and cloransulam-methyl applications. Resistance to imazethapyr was confirmed in all R populations, and no practical level of cross-resistance to chlorimuron, thifensulfuron, pyrithiobac, or cloransulam-methyl was detected. Based on resistance ratios, R populations were slightly more tolerant to chlorimuron and slightly more sensitive to pyrithiobac, thifensulfuron, and cloransulam-methyl than the S population.
Nomenclature: CGA-277476, 2-[[[[(4,6-dimethyl-2-pyrimidinyl)amino]carbonyl]amino]sulfonyl]benzoic acid,3-oxetanyl-ester; chlorimuron; cloransulam-methyl, flumetsulam, imazethapyr, pyrithiobac, thifensulfuron; Amaranthus hybridus L. AMACH, smooth pigweed.