Strong selection pressure from overreliance on herbicides has resulted in the rapid evolution of herbicide resistant weeds. Frequent applications of glyphosate, the main active ingredient in RoundUp, have spurred the evolution of increasingly resistant weed populations. Our previous research demonstrated that certain maternal biotypes of Conyza canadensis, a highly self-pollinating weed common in no-till soybean fields, tolerate glyphosate applications of 40 times the manufacturers’ original recommended application rate with >80% survival. These extremely resistant (ER) biotypes that escape agricultural fields and establish in non-agricultural habitats such as creek beds, roadsides, and parks raise questions about long-term persistence when selection for glyphosate resistance is eliminated. We hypothesized that these ER biotypes will have no fitness costs or growth differences compared to low-level resistant (LR) and susceptible (S) biotypes when grown in the absence of glyphosate. During the summers of 2016 and 2017, we tested for growth and fitness-related costs associated with glyphosate resistance in common garden experiments in Iowa. Using previously screened maternal biotypes collected in Iowa, we grew 9 S, 8 LR, and 9 ER biotypes at two field sites with 25 replicates per field and no exposure to glyphosate.
Results/Conclusions
Approximately 40% (2016) and 78% (2017) of plants across both field sites displayed phenotypic abnormalities after bolting including: thickened stem ribboning, flowers forming in clusters, and/or sudden death. Plants displayed no visual abnormalities prior to bolting, so we focus on early growth measures. Therefore, rosette size served as a proxy for lifetime fitness. Overall differences among S, LR, or ER biotypes were determined using a nested ANOVA followed by Tukey’s multiple comparisons. In 2016, there were no significant differences between S biotypes and ER or LR biotypes, but ER biotypes were significantly larger than both other resistance categories in 2017. Furthermore, ER biotypes bolted significantly earlier than S and LR biotypes in both years, and S biotypes had a greater frequency of disease symptoms than LR or ER biotypes (ER: 26% and 75%, LR: 31% and 67%, S: 55% and 89% for 2016 and 2017, respectively). These results indicate there is no growth penalty, and possibly no lifetime fitness penalty, associated with ER as these biotypes grow just as well, if not faster than, LR and S biotypes. This indicates that glyphosate resistance is likely to persist in both agricultural and non-agricultural habitats even without selection pressure from glyphosate applications.