Multiple resistance to flumetsulam and MCPA in two clones of Ranunculus acris

Herbicide dose-responses of seedling progenies of dairy pasture populations of Ranunculus acris varying in herbicide exposure history indicate that this weed has evolved multiple resistance to phenoxycarboxylic acid and acetolactate synthase (ALS)-inhibitor herbicides. To test this hypothesis, we conducted a replicated experiment in which 11 clones of R. acris (6 from Tasman and 5 from Canterbury) varying in parental population exposure to these two mode-of-action herbicides were treated with full- and half-label doses of MCPA and flumetsulam. For comparison, aminopyralid and glyphosate (pyridine carboxylic acid and 5-enolpyruvylshikimate 3-phosphate (EPSP) synthase-inhibitor mode-of-actions respectively and with label recommendations for R. acris control) were included in the experiment. Clones A and J, two of three from field populations with MCPA- and flumetsulam-exposure histories and anecdotal records of field control failures, were unaffected by flumetsulam and many plants survived treatment with MCPA, providing conclusive evidence for multiple herbicide resistance in R. acris and its occurrence in the same individuals. By contrast, there was no evidence among the 11 clones for resistance to either aminopyralid or glyphosate (86-100% mortality in all clones). The herbicide-resistant and -susceptible clones of R. acris provide a unique opportunity to investigate the biochemical mechanism(s) and genetics of phenoxycarboxylic acid and ALS-inhibitor resistance in this species.