Tuesday, June 18, 2024

Effectively Navigating Herbicide-Resistant Weeds

BY GRANT SAUM
UNION CITY, TENNESSEE

Cotton is very susceptible to weed competition in the early season due to cotton’s slow growth habit from emergence to about the 4-node stage. As Mid-South cotton producers move later into the growing season, finding effective ways to manage herbicide-resistant weeds becomes a major focus. Growers in the Mid-South and across the Cotton Belt have seen a growing list of resistant weeds in cotton this season, a problem that Cotton Incorporated is working hard to address. 

According to the International Survey of Herbicide Resistant Weeds, cotton currently has 18 species of herbicide-resistant weeds globally. In the United States, one of the most widely explored resistant weeds in cotton is Palmer amaranth. Published research findings from a study conducted in 20171 state, “Cotton yields decreased linearly from 13% to 54% for 1 to 10 Palmer amaranth plants/9.1 m of row.”  In other words, one Palmer plant per 30 feet of row (that emerged the same time as cotton) would be expected to decrease yield by 13%. Numbers like these are potentially devastating to cotton producers. 

Resistant Palmer amaranth pictured in cotton at the West Tennessee Ag Research and Education Center.

Funding research into herbicide resistance is vital to staying ahead of the ever-expanding list of herbicide-resistant weeds. Cotton Incorporated approaches research related to weed management with the assumption the herbicide resistance is going to continue to get worse, and fewer herbicide options will be effective or available. 

Identifying Resistant Weeds

Cotton Incorporated’s Agricultural and Environmental Research department is working with scientists across the country to help identify resistant weeds and find economical solutions to combat these problems in growers’ fields. Cotton Incorporated provides funding for research on this issue with both core funds and with funding through each state’s respective State Support Program. 

Dr. Gaylon Morgan, director of agronomy and weed control for Cotton Incorporated, said, “We support on-going monitoring through interactions with growers and consultants who report suspected resistance. From there, scientists collect plants or seed samples and then conduct a rate response study on the suspected weeds.”

Timely identification is key to recommending alternative weed management strategies for growers, which will delay the spread of these herbicide-resistant weeds to new fields and regions.

Identifying whether a weed is resistant to a single or multiple modes of action starts with the growers reporting potential resistance. Quick reporting allows farmers to act, and taking swift action can minimize the spread. Scientists then perform rate response studies to confirm herbicide resistance.

“The development of on-going resistance monitoring is crucial and the first step to resolving the weed resistance problem,” said Morgan.  

What’s Next?

While effective monitoring, reporting and testing are critical, the next steps are equally important. Once herbicide-resistant weeds are identified, Extension and industry personnel can develop management recommendations, which are then communicated with growers, consultants and private industry. Additionally, scientists must conduct more basic research for understanding and managing the mechanism of the specific herbicide-
resistant weeds. In some cases, weeds may be resistant to a single herbicide and in other cases many herbicides within a mode of action, which dictates the viable management options moving forward.

Understanding the mechanism of herbicide resistance within a weed may provide an opportunity to reverse herbicide resistance. Cotton Incorporated is supporting this type of research as well, and some promising results have been found in the lab. The next step is to prove the functionality of these practices outside the lab and to actual weeds in the field.

With the real threat of growing weed resistance in the Mid-South and beyond, Cotton Incorporated is dedicated to funding research and providing information to growers in the hopes of protecting producer profitability. For more information on resistant weed management research projects, visit cottoncultivated.cottoninc.com.  

¹Morgan, G., Baumann, P., & Chandler, J. (2001). Competitive Impact of Palmer Amaranth (Amaranthus palmeri) on Cotton (Gossypium hirsutum) Development and Yield. Weed Technology


Grant Saum is The Cotton Board’s Mid-South Regional Communication Manager. Email him at gsaum@cottonboard.org.

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