Response of ALS-Resistant Common Ragweed (Ambrosia artemisiifolia) and Giant Ragweed (Ambrosia trifida) to ALS-Inhibiting and Alternative Herbicides

Three studies were conducted in 1999 and 2000 to determine whether acetolactate synthase (ALS)-resistant common ragweed and giant ragweed biotypes were present in Ohio. Results of field studies indicated that biotypes of both species had cross-resistance to three chemical families of ALS-inhibiting herbicides. Cloransulam-methyl applied postemergence at 9, 18, and 36 g/ha controlled more than 85% of two susceptible populations of common and giant ragweed 28 d after treatment, whereas less than 35% control of resistant populations was achieved at the same rates. Fomesafen, lactofen, and glyphosate applied alone at the recommended rates provided the most effective control of ALS-resistant common and giant ragweed. Mixtures of cloransulam-methyl with either fomesafen or lactofen did not significantly increase ALS-resistant common and giant ragweed control compared with each diphenylether herbicide used alone. Dose-response bioassays conducted in the greenhouse indicated that susceptible common and giant ragweed tended to be more sensitive to cloransulam-methyl and chlorimuron than to imazamox. ALS-resistant common ragweed demonstrated a high level of resistance to all the herbicides tested because${\rm GR}_{50}$values were not reached with rates 1,000 times higher than the recommended rate. ALS-resistant giant ragweed treated with 13,000 g/ha of chlorimuron and 18,000 g/ha of cloransulam-methyl was not inhibited enough to obtain a${\rm GR}_{50}$value, thus also demonstrating a high level of resistance. The${\rm GR}_{50}$for ALS-resistant giant ragweed treated with imazamox was 1,161 g/ha. Results of these studies confirmed the presence of ALS-cross-resistant populations of common and giant ragweed in Ohio and suggest that herbicides with different mechanisms of action will be required to manage these weeds effectively.