Resistance of Ranunculus acris to flumetsulam, thifensulfuron-methyl and MCPA in New Zealand dairy pastures

Research in the 1980s showed that Ranunculus acris had evolved resistance to the phenoxy herbicides MCPA and MCPB in New Zealand. Since then two ALS-inhibitor herbicides, flumetsulam and thifensulfuron-methyl, have been used but recently flumetsulam has been reported as being less effective. The mortality responses of seedling progeny of 15 R. acris populations, differing in historical exposure to phenoxy and ALS-inhibitor herbicides, were compared using a log-series of five doses of MCPA, flumetsulam and thifensulfuron-methyl. The resulting mortalities were higher than expected at lower doses, meaning LD50 values were not reliably estimated. In a second experiment, the responses of one population with no previous exposure to herbicides and one population with high exposure to flumetsulam were compared using a wider range of doses of flumetsulam (0.04 to 25 times). The LD50 values differed 5.3-fold between the resistant and susceptible biotypes. In a third experiment, four populations varying in past exposure to both herbicide groups were treated with either flumetsulam or MCPA. Resulting LD50 values were higher in populations with high previous exposure to both groups. These results confirm that resistance to both phenoxy and ALS herbicides can occur in R. acris populations through repeated use. Since these are the only two available groups for the selective control of this weed in dairy pastures in New Zealand, the options for control are reduced.