Fecundity and survival of flumetsulam-resistant and -susceptible <italic>Ranunculus acris</italic> plants with and without exposure to the herbicide.

Variable efficacy of the acetohydroxyacid synthase (AHAS) inhibitor herbicide, flumetsulam, against dairy pasture populations of <italic>Ranunculus acris</italic>, has been attributed to resistance. We compared, over two years, the fitness of two resistant (R) clones and nine susceptible (S) clones treated or not with flumetsulam by measuring seed yields and survival of plants. Without the herbicide, all plants grew and survived. The seed yields of the R and S plants without the herbicide were similar (327 and 217 seeds/plant, respectively in Year 1, and 257 and 323 seeds/plant in Year 2) indicating no fitness penalty to the resistance. With the herbicide, growth and survival of the R plants was unaffected, whereas all S plants were defoliated before seedhead formation but 96% regrew from their rhizomes in Year 2. Seed yields of the treated R and S plants in Year 1 were 347 and 0 seeds/plant, respectively and 243 and 205 seeds/plant in Year 2. Survival of flumetsulam-susceptible <italic>R. acris</italic> plants following spraying would maintain susceptible AHAS alleles in the population, while without a fitness penalty, resistant plants would persist in non-spraying years facilitating further selection when the herbicide is used again. [ABSTRACT FROM AUTHOR]