Resistant Weeds in Your Cotton, Soy Fields? Fight Back

Control weed resistance in cotton fields

Resistance is advancing at an alarming rate – just talk to any weed scientist in the United States.

No new herbicide sites of action have been introduced in the past 30 years, and the industry now knows that over-reliance on a single chemistry is not a sustainable strategy.

But the latest research suggests that growers can help fend off resistance by tankmixing two effective sites of action. A 2015 university study found that fields in which two effective sites of action (SOAs) were tankmixed resulted in these fields being 83 times less likely to develop herbicide resistance.1

What does "effective" mean? It refers to a chemistry to which the weeds in your fields have not shown resistance. With no known resistance in broadacre crops, Liberty® is an excellent resistance management tool, offering a unique site of action (Group 10).* Plus, Liberty is compatible with over 100 tankmix partners.

The History of Resistant Palmer Amaranth

Dr. Peter Dotray, Professor and Extension Weed Specialist of Texas A&M, comments on the industry’s overall approach to weed management over the last few decades.

“Once we transitioned into the opportunity of using glyphosate as an herbicide in crops, I think a lot of other strategies were put on the shelf and growers quickly found that glyphosate followed by glyphosate followed by glyphosate was extremely effective.”

But the industry learned a major lesson. Certain weed populations quickly evolved, no longer responding to the herbicide, and as a result, growers have lost that chemistry as an effective tool for certain weeds.

While no stranger to the southern and eastern regions of the country, Palmer amaranth is often cited as the most concerning driver weed – not only resistant to glyphosate, but now documented to have stacked resistance to multiple chemistries.2

“It’s everywhere – wherever we grow row crops,” said Eric Prostko, a weed scientist at the University of Georgia, adding that Palmer is even becoming a concern in the Midwest, where waterhemp has long reigned.

“In this area, we already have Palmer everywhere, so I don't know where else it can go other than the Gulf of Mexico. And I would bet it probably would grow there, given the time,” he said, with a laugh.

Combine Herbicides and Cultural Practices to Manage Weeds

Cultural practices such as tillage, crop rotation and hand removal of weeds can supplement herbicide treatment, leading to a more sustainable outcome. Dotray cites the importance of using multiple management strategies.

“We’ve taken a step backwards, so to speak, and are visiting now all the current strategies available to develop a more diversified weed management program,” he said. “It’s thinking about herbicides, rates, and incorporation. It’s also thinking about tillage, crop rotation, and the use of biological control where possible.”

Prostko agrees that having multiple weed management strategies is best. He recommends starting clean by using a combination of tillage, cover crops and herbicides to remove pigweeds from the field.

“Make sure that when you get to the field to plant, there are no pigweeds in the field, because if you allow that to happen, you will lose,” he said.

Second, he recommends using a residual, water-activated herbicide that is effective on Palmer amaranth. “It’s going to be almost impossible for growers to manage resistance and typical weed problems without the use of residual herbicides,” he explains. “And when they are activated by moisture, whether through rainfall or irrigation, they work great.”

Finally, he advises being timely with post-emergence herbicides. Waiting until weeds are taller will make them more difficult to manage. But growers who begin soon after crop emergence, when weeds are smaller, can achieve better results.

“Whatever herbicide you choose … you want to spray those pigweed plants before the biggest pigweed in the field is 3 inches tall.”

Combine multiple, effective herbicide sites of action, plus cultural practices such as crop rotation and cultural approaches, to manage or delay resistance. Know your fields and know your weeds. Talk to a specialist about herbicide recommendations for your farm.

Plant Herbicide-Tolerant Cotton and Soybean Varieties

Herbicide-tolerant seed traits and compatible herbicides offer another option for soybean and cotton growers seeking protection from weeds and resistance. Bayer offers LibertyLink® cotton and soybean varieties, including FiberMax® and Stoneville® cotton and Credenz® soybeans.

Crops enabled with the LibertyLink trait allow growers to spray Liberty herbicide for control of resistant weeds like Palmer amaranth. Results show 98% control of key driver weeds like Palmer amaranth when used as part of a complete weed management program.**

In addition to the LibertyLink trait, cotton growers may choose varieties with a GlyTol® trait, which allows the use of glyphosate. Growers who plant FiberMax or Stoneville cotton varieties with the GlyTol plus LibertyLink traits can spray glyphosate or Liberty herbicide to control a wide range of tough-to-manage and glyphosate-resistant weeds.


*The active ingredient in Liberty is a Group 10 herbicide, which is the only broad-spectrum herbicide that effectively controls grasses and broadleaf weeds, and it has no known resistance in U.S. broadacre crops.

**Results based on 5 years of trials where Liberty was applied according to S.T.O.P. Weeds with Liberty guidelines and as part of a complete weed control program, where an effective residual product is used followed by Liberty.

Work Cited

  1. Evans, J. A., Tranel, P. J., Hager, A. G., Schutte, B., Wu, C., Chatham, L. A. and Davis, A. S. “Managing the evolution of herbicide resistance.” 2016.

  2. “Palmer Amaranth (Pigweed).” Take Action Pesticide Resistance Management. 2018.

Tags: cotton, soybean

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