High Levels of Glyphosate Resistance in Conyza canadensis from Agricultural and Non-Agricultural Sites in Ohio and Iowa

Zachery T. Beres,Allison A. Snow,Micheal D. K. Owen,Mark M. Loux,Bruce A. Ackley,Emily E. Ernst

doi:10.1038/s41598-018-28163-w

Zachery T. Beres, Allison A. Snow + Show 4 more

Open Access

https://doi.org/10.1038/s41598-018-28163-w

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Abstract

Glyphosate is an important herbicide worldwide, but its efficacy has been compromised where weed species have evolved glyphosate resistance. To better understand evolutionary outcomes of continued and strong selection from glyphosate exposure, we characterized variation in resistance in self-pollinating Conyza canadensis (horseweed) in Ohio and Iowa, where glyphosate resistance was first reported in 2002 and 2011, respectively. In 2015, we collected seeds from a total of 74 maternal plants (biotypes) from no-till soybean fields vs. non-agricultural sites in each state, using one representative plant per site. Young plants from each biotype were sprayed with glyphosate rates of 0x, 1x (840 g ae ha−1), 8x, 20x, or 40x. Resistant biotypes with at least 80% survival at each dosage were designated as R1 (1x), R2 (8x), R3 (20x), or R4 (40x). Nearly all Ohio agricultural biotypes were R4, as were 62% of biotypes from the non-agricultural sites. In Iowa, R4 biotypes were clustered in the southeastern soybean fields, where no-till agriculture is more common, and 45% of non-agricultural biotypes were R1–R4. Our results show that resistance levels to glyphosate can be very high (at least 40x) in both states, and that non-agricultural sites likely serve as a refuge for glyphosate-resistant biotypes.

Highlights

® tion of many agricultural weeds[1,2]
Detection of R4 plants surviving in no-till soybean fields illustrates the challenges that growers face when attempting to manage glyphosate-resistant horseweed, which requires the use of other herbicide modes of action and/or other control methods such as tillage
The prevalence of highly resistant biotypes in Ohio and Iowa may be related to a history of evolved glyphosate resistance dating back to at least 2002 and 2011, respectively[8]

Summary

Introduction

® tion of many agricultural weeds[1,2]. Glyphosate, the active ingredient in Roundup , is a broad-spectrum, systemic herbicide that inhibits 5-enolpyruvlyshikimate-3-phosphate synthase (EPSPS; EC 2.5.1.19), an enzyme in the shikimic acid pathway, thereby blocking the production of three essential aromatic amino acids (tryptophan, tyrosine, and phenylalanine) and other downstream metabolites[3,4]. By 2016, herbicide-resistant crop varieties in the USA represented 94% of soybean production, 89% of maize, and 89% of cotton[7], mainly due to transgenic glyphosate resistance. At least 37 weed species are reported to have evolved resistance to glyphosate[8]. As the abundance and distribution of GR weeds increases over time, growers often use greater herbicide dosages, more frequent applications, and/or use other herbicides and herbicide-tolerant crops with stacked resistance traits[2]. Understanding how quickly high levels of glyphosate resistance evolve and spread has practical applications for weed management and is interesting from an evolutionary biology standpoint. Davis et al.[16] reported widespread glyphosate resistance in Indiana horseweed populations for samples collected from no-till soybean fields in 2003–2005. We studied horseweed populations in Ohio and Iowa, where GR biotypes were first reported in 2002 and 20118, respectively, and may have been present prior to these reports

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