News Release

Research unveils genetic basis for S-metolachlor resistance in Waterhemp

Peer-Reviewed Publication

Cambridge University Press



Weed Science

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Credit: WSSA

WESTMINSTER, Colorado – 11 January 2024 – Recently published research in the journal Weed Science provides new mechanistic insights into S-metolachlor resistance in waterhemp (Amaranthus tuberculatus). Specifically, the study points to a single major gene that controls metabolic resistance to S-metolachlor (the active ingredient in Dual Magnum and Dual II Magnum), in the Stanford, Illinois resistant (SIR) population, which represents a relatively new and recent type of non-target-site resistance in waterhemp—to soil-applied, Group 15 herbicides.

“Waterhemp has ascended to its current status as the worst weed threatening Corn Belt crop production during the last thirty years,” says Dean Riechers, Professor, University of Illinois Urbana-Champaign. “In that time, waterhemp has developed resistance to seven herbicide sites-of-action classes, creating extremely difficult management challenges to farmers as they try to control it.”

The good news is that the germplasm derived from this research can assist in identifying the gene(s) and gene mutations conferring resistance to S-metolachlor in waterhemp. This is significant, due to the inherent difficulty in studying weed resistance to soil-applied, residual herbicides (like S-metolachlor and other Group 15 herbicides) compared to investigating foliar-applied resistance. In addition, published research on the inheritance of resistance to Group 15 herbicides in dicot weed species is lacking compared to weedy grasses.

“This study’s results are important because they clearly define the crosses and populations needed to properly investigate inheritance of S-metolachlor resistance in the SIR waterhemp population,” says Riechers. “We’ve identified a single, major gene that confers resistance, and a second, recessive gene that may also modify S-metolachlor resistance in SIR, which are both new discoveries.”

The study’s findings will also help establish a baseline for future molecular-genetic studies to pinpoint metabolic resistance traits in other dioecious weedy amaranths, such as A. palmeri. More information is available in the article, “Inheritance of resistance to S-metolachlor in a waterhemp (Amaranthus tuberculatus) population from central Illinois.”

The research is featured in Volume 71, Issue 6 of Weed Science, a Weed Science Society of America journal, published online by Cambridge University Press.

About Weed Science

Weed Science is a journal of the Weed Science Society of America, a nonprofit scientific society focused on weeds and their impact on the environment. The publication presents peer-reviewed original research related to all aspects of weed science, including the biology, ecology, physiology, management and control of weeds. To learn more, visit

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