HERBICIDE SUSCEPTIBILITY AND BIOLOGICAL FITNESS OF TRIAZINE-RESISTANTAND SUSCEPTIBLE COMMON LAMBSQUARTERS (CHENOPODIUM-ALBUM)

Biological fitness and negative cross-resistance to other herbicides m ay be an important factor in managing triazine-resistant common lambsq uarters. Greenhouse experiments examined the sensitivity of a resistan t and a susceptible biotype to foliarly-applied bentazon, bromoxynil, dicamba, pyridate, and thifensulfuron. The noncompetitive vigor of tri azine-resistant and susceptible common lambsquarters also was compared by growing plants in individual containers and harvesting them period ically throughout their vegetative period and at reproductive maturity . In the herbicide susceptibility study, 11 kg ai ha(-1) atrazine had no effect on the growth of the resistant biotype, while it reduced sus ceptible common lambsquarters' biomass by up to 68%, Estimated I-50 va lues indicated the resistant biotype exhibited between 36 and 79% grea ter susceptibility to bentazon, bromoxynil, dicamba, and pyridate than did the susceptible one, while both responded similarly to thifensulf uron. In growth studies, the susceptible biotype achieved greater heig ht, leaf area, and plant dry weight than the resistant population for the majority of harvest dates; however, values equalized between bioty pes as the plants reached maturity. These experiments suggest that alt ernative management programs that exploit reduced fitness and increase d herbicide susceptibility in triazine-resistant common lambsquarters could be developed. However, further studies are needed to determine w hether these results have application for the management of triazine-r esistant weeds in the field.