The silver bullet that wasn’t: rapid agronomic weed adaptations to glyphosate in North America

Authors: Landau, Christopher; BRADLEY, KEVIN - University Of Missouri; BURNS, ERIN - Michigan State University; FLESSNER, MICHAEL - Virginia Tech; GAGE, KARLA - Southern Illinois University; HAGER, AARON - University Of Illinois; IKLEY, JOE - North Dakota State University; JHA, PRASHANT - Iowa State University; JOHNSON, PAUL - South Dakota State University; JOHNSON, WILLIAM - Purdue University; LANCASTER, S - Kansas State University; LEGLEITER, TRAVIS - University Of Kentucky; LINGENFELTER, DWIGHT - Pennsylvania State University; LOUX, MARK - The Ohio State University; MILLER, ERIC - Southern Illinois University; NORSWORTHY, JASON - University Of Arkansas; OWEN, MICHAEL - Iowa State University; NOLTE, SCOTT - Texas A&M University; SARANGI, DEBLIN - University Of Minnesota; SIKKEMA, PETER - University Of Guelph; SPRAGUE, CHRISTY - Michigan State University; VANGESSEL, MARK - University Of Delaware; WERLE, RODRIGO - University Of Wisconsin; YOUNG, BRIAN - Purdue University; Williams, Martin

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/2/2023
Publication Date: 12/5/2023
Citation: Landau, C.A., Bradley, K., Burns, E., Flessner, M., Gage, K., Hager, A., Ikley, J., Jha, P., Johnson, P., Johnson, W., Lancaster, S., Legleiter, T., Lingenfelter, D., Loux, M., Miller, E., Norsworthy, J., Owen, M., Nolte, S., Sarangi, D., Sikkema, P., Sprague, C., Vangessel, M., Werle, R., Young, B., Williams, M. 2023. The silver bullet that wasn’t: rapid agronomic weed adaptations to glyphosate in North America. Proceedings of the National Academy of Sciences (PNAS). 2(12). Article pgrad338. https://doi.org/10.1093/pnasnexus/pgad338.
DOI: https://doi.org/10.1093/pnasnexus/pgad338

Interpretive Summary: Singular solutions to difficult problems can sound great. Glyphosate-resistant (GR) corn, cotton, and soybean commercialized near the end of the 20th century allowed in-crop use of glyphosate, a broad-spectrum, non-selective herbicide. Some believed the technology would completely solve weed control problems in GR crops. Several factors, including initially high glyphosate efficacy, led to a simplification of weed management systems that relied heavily, in some cases exclusively, on glyphosate for weed control. Adoption of glyphosate use on GR crops in North America, and abandonment of other weed management tactics, was extensive and rapid. How did that work out? We analyzed data from thousands of glyphosate evaluation trials from 24 institutions across the U.S.A. and Canada from 1996 to 2021. Throughout North America and for every major weed species analyzed, mean weed control with glyphosate decreased over time. Furthermore, variability in weed control increased over time. Today, continued deterioration of glyphosate performance has rendered the herbicide largely ineffective as a stand-alone treatment. The analyses illustrate the rapid adaptations of agronomically important weed species to glyphosate, the most widely used herbicide in the world. Rather than seeking the next singular solution to solve today’s weed problems, we argue for diverse, multi-faceted strategies that delay weed adaptation to current and future management tactics.

Technical Abstract: The rapid adoption of glyphosate-resistant (GR) corn, cotton, and soybean at the end of the 20th century led to a simplification of weed management systems that relied heavily, in some cases exclusively, on glyphosate for weed control. Glyphosate efficacy has changed, and a greater understanding of such trends would shed new light on weed adaptation, in this case, to a technology that transformed production agriculture and the crop protection industry. The objectives of this research were to: 1) quantify the risk of poor weed control from repeated glyphosate use on troublesome weed species in corn and soybean systems, and 2) determine the extent to which glyphosate preceded by a soil-residual herbicide reduces the risk of poor and inconsistent weed control compared to glyphosate used alone. Herbicide evaluation trials from 24 institutions across the US and Canada, from 1996 to 2021, were collected and compiled into a single database. Two subsets were created; one with postemergence (POST) applied glyphosate as the only treatment and, the other with POST glyphosate following a preemergence (PRE) soil-residual herbicide application. Within each subset, mean and variance of weed control ratings for seven problematic weed species were regressed over time for nine US states and one Canadian province. With few exceptions, mean weed control with POST glyphosate alone decreased over time while the variability in weed control increased. When glyphosate application was preceded by a PRE herbicide effective against the target weed species, there was little change in mean weed control or variability in weed control. These results illustrate the rapid adaptation of agronomically important weed species to a product heralded as a ‘once in a lifetime’ herbicide. Incorporating more diversity into weed management systems is essential to slowing weed adaptation and prolonging the usefulness of existing and future technologies.