Abstract
Strong selection from herbicides has led to the rapid evolution of herbicide‐resistant weeds, greatly complicating weed management efforts worldwide. In particular, overreliance on glyphosate, the active ingredient in RoundUp®, has spurred the evolution of resistance to this herbicide in ≥40 species. Previously, we reported that Conyza canadensis (horseweed) has evolved extreme resistance to glyphosate, surviving at 40× the original 1× effective dosage. Here, we tested for underlying fitness effects of glyphosate resistance to better understand whether resistance could persist indefinitely in this self‐pollinating, annual weed. We sampled seeds from a single maternal plant (“biotype”) at each of 26 horseweed populations in Iowa, representing nine susceptible biotypes (S), eight with low‐level resistance (LR), and nine with extreme resistance (ER). In 2016 and 2017, we compared early growth rates and bolting dates of these biotypes in common garden experiments at two sites near Ames, Iowa. Nested ANOVAs showed that, as a group, ER biotypes attained similar or larger rosette size after 6 weeks compared to S or LR biotypes, which were similar to each other in size. Also, ER biotypes bolted 1–2 weeks earlier than S or LR biotypes. These fitness‐related traits also varied among biotypes within the same resistance category, and time to bolting was inversely correlated with rosette size across all biotypes. Disease symptoms affected 40% of all plants in 2016 and 78% in 2017, so we did not attempt to measure lifetime fecundity. In both years, the frequency of disease symptoms was greatest in S biotypes and similar in LR versus ER biotypes. Overall, our findings indicate there are no early growth penalty and possibly no lifetime fitness penalty associated with glyphosate resistance, including extremely strong resistance. We conclude that glyphosate resistance is likely to persist in horseweed populations, with or without continued selection pressure from exposure to glyphosate.
Highlights
The concomitant use of herbicides and genetically modified crops worldwide has altered the face of agriculture in recent decades
Combining data for each resistance category, we found that the frequency of disease symptoms in Experiment A was ~2× greater for S plants compared to level resistance (LR) and extremely resistant (ER) plants, which were similar to each other (Figure 5)
Like many other weed species, horseweed has evolved several mechanisms for glyphosate resistance and the fitness effects associated with specific mechanisms have not been studied to date
Summary
The concomitant use of herbicides and genetically modified crops worldwide has altered the face of agriculture in recent decades. At least 40 weed species have evolved resistance to glyphosate (Heap, 2019). This problem has led growers to use increased amounts of glyphosate and incorporate more toxic herbicides, such as 2,4-D and dicamba, in weed management efforts (Benbrook, 2012, 2016). The extent to which herbicide resistance spreads and persists in weed populations is heavily influenced by whether or not resistance mechanisms are associated with fitness effects in the absence of herbicide treatment (Bergelson & Purrington, 1996; Vila-Aiub, Neve, & Roux, 2011). Understanding fitness effects of resistance genes is important for developing weed management strategies and modeling approaches for mitigating the problem (Neve, 2008; Vila-Aiub, Neve, & Powles, 2009b). When herbicide resistance does not confer a fitness cost, and may even be associated with a fitness benefit in the absence of an herbicide (Beres, Yang, et al, 2018; Wang et al, 2014), this trait has the potential to persist indefinitely in weed populations
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