Variation in MCPA‐resistance in Ranunculus acris L. subsp. acris and its correlation with historical exposure to MCPA

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Abstract
Summary: In seven field experiments in which the effect of MCPA on Ranunculus acris subsp. acris in dairy pastures in New Zealand was studied, control varied between sites from 93% to 11% reduction in ground cover. To determine if the populations varied in resistance to MCPA, seedling progeny were sprayed with a range of doses of MCPA in a glasshouse. The LD50 values ranged from 0.36 kg MCPA ha−1 to 1.5 kg MCPA ha−1 confirming the existence of genetic variability in resistance between the populations. Correlations between the LD50 values and the control of the populations in the field, suggested that the differences in resistance contributed to the differences in control. A correlation between the LD50 values and historical exposures of the populations to MCPA suggested that selection for resistance had occurred. No evidence of a difference in competitive ability between the least and the most susceptible biotypes was detected, suggesting that resistance would not decline if spraying ceased.