Fecundity and survival of flumetsulam-resistant and -susceptible Ranunculus acris plants with and without exposure to the herbicide *

Ranunculus acris L. (giant buttercup), a perennial rhizomatous plant common in old pastures and hay meadows in Europe (Harper and Sagar 1953), has naturalised in New Zealand (Webb et al. 1988) where it is an economically significant weed of dairy pastures (Bourdôt et al. 2003). The herbicide flumetsulam, an inhibitor of the acetohydroxyacid synthase enzyme (AHAS; also known as acetolactate synthase or ALS), was registered for use in pastures in New Zealand in 1997 (Harris and Husband 1997). It provided dairy farmers with an alternative to the phenoxycarboxylic acid herbicides 2,4-D, MCPB and MCPA which were failing to control the weed due to resistance (Bourdôt and Hurrell 1988; Bourdôt et al. 1989; 1990, 1994, 2007). Many dairy farmers began using flumetsulam, but poor results were reported after a few years. Some of these ‘control failures’ were due to the selection for resistance as implied by the higher LD₅₀ values for flumetsulam of seedling progeny of R. acris plants in dairy pastures with high historical exposure and poor field-scale responses to the herbicide compared to seedlings from unexposed pastures (Lusk et al. 2015). Circumstantial evidence for the selection for resistance is provided by the large variation between different dairy pastures in the Golden Bay district in the responses of the weed to the herbicide (Bourdôt et al. 2019). This was substantiated by the occurrence of resistant R. acris plants in a dairy pasture that had been treated regularly with flumetsulam for 12 years and where the landholder reported the herbicide now gives ‘poor’ control of the weed (Jackman et al. 2022).

Poor results from flumetsulam applied to control R. acris populations may not necessarily be due to the selection of resistant plants. It could, for example, be due to seedlings replacing plants killed by the herbicide. Seedlings arise predominantly in winter and spring in dairy pastures in the Golden Bay district promoted by winter pugging of the soil. Despite their low levels of survival (3%–7%), seedlings have been shown to produce 2–7 new plant recruits per square metre per year in dairy pastures (Lusk et al. 2009). Another explanation, and potentially mutually inclusive of seedling recruitment, is that the R. acris plants that are exposed to, and defoliated by flumetsulam, survive, and regrow from their undamaged rhizomes. This was observed to occur in large R. acris plants in the first field evaluations of flumetsulam in dairy pastures in New Zealand (Harris and Husband 1997). Further support for the hypothesis that R. acris plants susceptible to foliar damage by the herbicide, may regrow from unaffected rhizomes was found in a recent experiment where 72% of pot-grown flumetsulam-susceptible plants treated with the herbicide in the spring, and that were completely defoliated, retained viable rhizome growing points (Jackman et al. 2022). These authors did not determine if these ‘susceptible’ plants survived the winter to regrow the following year.

Towards a better understanding of the increase of flumetsulam-resistance in R. acris populations in dairy pastures, we have investigated the genetics and ecological fitness of plants that are either resistant or susceptible to the herbicide. We have focused on 11 clones, previously compared for their responses to flumetsulam. These include two clones of the resistant (R) phenotype where the plants exhibit no symptoms of herbicide damage when treated with the label-recommended application rate of flumetsulam, and nine of the susceptible (S) phenotype where the plants stop growing and are completely defoliated by the herbicide (Jackman et al. 2022). Here we present data for two criteria of fitness (seed yield and plant survival) in the presence and absence of exposure to flumetsulam from an experiment conducted over two complete growing seasons where pot-grown plants of the R and S phenotypes of R. acris were treated or not with the herbicide in the first year.

*Published version has note: This article has been corrected with minor changes. These changes do not impact the academic content of the article