No evidence for early fitness penalty in glyphosate-resistant biotypes of Conyza canadensis: common garden experiments in the absence of glyphosate

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 40x the original 1x 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 9 susceptible biotypes (S), 8 with low-level resistance (LR), and 9 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.  N...