Escalating levels of glyphosate resistance in Iowa populations of Conyza canadensis (horseweed): implications for fitness effects

Glyphosate, sold as RoundUp, is the most commonly used herbicide worldwide and more than 30 weed species have evolved resistance to it. In Conyza canadensis (horseweed or marestail), glyphosate resistance was first reported in 2000 in Delaware, while Iowa has only seen resistance since 2011. The goals of this experiment were to 1) test for habitat differences in the level of glyphosate resistance, and 2) test for differences in fitness traits (rosette size and biomass) between susceptible and resistant biotypes in Iowa. I hypothesized that greater levels of glyphosate resistance would be found in agricultural populations than non-agricultural, and that resistance would not be associated with slower growth. Seeds were collected from 28 populations in 2013, with one maternal line (biotype) per population, representing 14 biotypes from soybean fields and 14 biotypes from non-agricultural habitats (e.g., roadsides, parks, etc.). Seedlings were grown in a greenhouse and sprayed with glyphosate after six weeks using three dosages: 1X (= 0.84 kg ae glyphosate/ha), 8X, and 20X, with 0X as a control. With 24 plants in each dosage treatment, per cohort, biotypes with at least 80% survival at each dosage were designated as “resistant” (1X), “highly resistant” (8X), or “extremely resistant” (20X), respectively. I found that 93% of non-agricultural biotypes and one agricultural biotype were susceptible at 1X. Also, 36% of agricultural biotypes were extremely resistant compared to none from non-agricultural habitats, confirming my first hypothesis. Differences in rosette size between susceptible vs. resistant biotypes were significant (p < 0.0001), resulting in larger resistant plants, and differences in biomass were not significant. My results suggest that glyphosate resistance is likely to persist and may increase in frequency and strength if selection pressures from glyphosate applications continue and if seeds of resistant biotypes disperse into non-agricultural areas. Worldwide, the spread of glyphosate-resistant weeds is causing growers to use herbicide mixtures that include other modes of action.