Herbicide resistant weeds threaten conservation agriculture

Glyphosate-resistant horseweed growing in a Michigan soybean field.
Glyphosate-resistant horseweed growing in a Michigan soybean field. Photo by Christy Sprague, Michigan State University.

East Lansing, MI – Soybean farmers are turning away from conservation agriculture practices that protect soil and environmental health to manage herbicide resistant weeds, say researchers in Michigan State University’s (MSU) Department of Agricultural, Food, and Resource Economics (AFRE).

In a new study published in the American Journal of Agricultural Economics, Braeden Van Deynze (now a postdoctoral research associate at the University of Washington), Scott Swinton (MSU), and David Hennessy (MSU), examined the herbicide and tillage records of thousands of soybean farmers across the United States. The researchers found that where herbicide resistant weeds are more common, farmers are less likely to utilize conservation tillage practices. 

Authors of the study, Braeden Van Deynze, Scott Swinton, and David Hennessy.
Authors of the study – Braeden Van Deynze, Scott Swinton, and David Hennessy.

Van Deynze says, “Our research shows that the era of single-tool weed control is coming to an end. Farmers are turning back to intensive tillage, and to other herbicides, to keep their fields weed-free.” Van Deynze adds, “While we have known for a long time that weeds are evolving to resist glyphosate herbicide, this research is the first to show how herbicide resistance has affected other practices like tillage.”

A return to more tillage to control weeds has significant implications for the future of conservation agriculture in the United States. No-till practices reduce soil erosion, improve stream water quality, and are cited as a potential way to sequester more carbon in agricultural soils in an effort to offset greenhouse gas emissions. 

Van Deynze says, “Glyphosate-based weed control systems have had huge effects both on and off farms, saving farmers time and money while reducing the need for environmentally damaging practices like intensive tillage, which can have major off-farm impacts on water quality and the climate.”

Decrease in percentage of soybean acres under conservation tillage attributed to glyphosate-resistant weeds. Figure 6 in Van Deynze et al. 2021.

The policy implications of the study’s findings could be substantial. The United States Department of Agriculture (USDA) spends millions of dollars annually to encourage farmers to practice conservation agriculture.  If weed control methods compatible with conservation practices are becoming less and less effective, Van Deynze says, “Programs are likely to need to offer larger subsidies to farmers to protect soil and water than they might have if glyphosate were as effective as it was fifteen years ago.”

For individual farmers, Van Deynze says, “While it might be too late for glyphosate, farmers should consider on-farm measures that can delay weeds from adapting to other herbicides, including rotating herbicides or spraying more selectively.”

The research was conducted with support from the National Science Foundation via the Long-term Ecological Research project at the Kellogg Biological Station (KBS-LTER) and by MSU AgBioResearch.

Note for media: Please include the following link to the study in all online media coverage: http://doi.org/10.1111/ajae.12243 and contact author Braeden Van Deynze vandeynz@uw.edu with any questions or follow-up.