Research Highlights

Controlling Palmer Amaranth with a Limited Toolbox

In this article, you’ll find details on:

• A comprehensive North Carolina trial found many herbicide programs without auxin herbicides, dicamba or 2,4-D, that control Palmer amaranth.
• At least two herbicide applications at two different timings provide acceptable Palmer amaranth control. 
• Herbicide programs with strong residual activity in a pre-emergence application and two modes of action that impact Palmer amaranth consistently deliver excellent control. 

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By Laura Temple

The docudrama Apollo 13 showcases the creativity needed to make something work with a drastically limited toolbox. 

For soybean farmers in regions with heavy Palmer amaranth pressure, the limitation and loss of auxin herbicides — dicamba and 2,4-D — for weed control felt a bit like an in-flight explosion. 

“Because of the impressive ability of Palmer amaranth to adapt to environments, conditions and control methods, farmers had quickly adopted soybean systems with new herbicide traits,” says weed scientist Wesley Everman. “Adoption of the dicamba trait had approached 85%, with much of the rest using the 2,4-D trait.”

He knew there was a real chance farmers could have their weed control toolbox depleted. With a “fit a square peg in a round hole” mentality, he started exploring ways to manage Palmer amaranth without auxins at North Carolina State University, with support from the North Carolina Soybean Producers Association. His work started a few years before that loss became reality.

“Farmers in North Carolina have been listening to weed resistance management recommendations and diversifying their weed control,” Everman says. 

That’s part of the reason they were quick to adopt new soybean weed control technology. It also made them sensitive to limitations imposed on their weed management toolbox.

Testing Other Tools

Everman conducted a comprehensive weed control field trial that included three application timings with multiple treatment options. He aimed to compare programs with limited modes of action to those with multiple modes of action, as well as programs with and without residual activity. 

Pre-emergence treatments: 

• No herbicide
• Fierce herbicide, a premix of flumioxazin, Group 14, and pyroxasulfone, Group 15
• Boundary herbicide, a combination of metribuzin, Group 5, and S-metolachlor, Group 15
• Authority MTZ herbicide, which combines sulfentrazon, Group 14, and metribuzin

Early post-emergence treatments, applied at the V2 or V3 growth stage:

• No herbicide
• Liberty herbicide, or glufosinate, a Group 10 herbicide with no residual
• Prefix herbicide, a premix of fomesafen, Group 14, and S-metolachlor
• Warrant Ultra, a combination of acetochlor, Group 15, and fomesafen

Late post-emergence treatments, applied at V6:

• No herbicide
• Liberty herbicide
• Flexstar herbicide, containing fomesafen 

He used a factorial design that tested every possible combination, for a total of 48 different treatments of one, two or three herbicide applications and an untreated check. Everman’s team collected three years of data.

“Many treatments required rain for activation,” he explains. “During the trials, we had lots of weather variability — both dry and wet seasons.”

Identifying Solutions

According to Everman, it is possible to control Palmer amaranth well without auxin herbicides.

“Thirty of the 48 treatments we tested had 90% or better control of Palmer amaranth,” he reports. “That is about two-thirds of the options we explored.”

His analysis of the results provides practical guidance for managing Palmer amaranth in soybeans.

• Herbicide programs with strong residual activity in a pre-emergence application and two modes of action that impact Palmer amaranth deliver excellent control. 
• Two herbicide applications, at any two timings, generally provide acceptable weed control.
• Pre-emergence applications give more flexibility for size of weeds that can be controlled with the second application.
• Weed size does contribute to more variability in weed control when combining early-post and late-postemergence applications. Control can drop to between 60 to 75% when weeds get large.

“Dicamba was a great tool, but we need other options,” Everman says. “This research proves that there are systems and products available for excellent weed control within current agronomic systems.”

Following this trial, he led a state-wide survey of Palmer amaranth surviving until just prior to harvest. His team collected and screened 137 populations for development of herbicide resistance. They found that all the most common post-emergence herbicides, including those used in the weed control trial, are widely effective.

However, fomesafen showed the greatest variability in control, with potentially resistant populations in the screening. Palmer amaranth carrying resistance to PPO inhibitors has been confirmed in North Carolina. Glufosinate-resistant Palmer amaranth, uncontrolled by Liberty, has also been confirmed in North Carolina, though it didn’t show up in this screening.

The screening also found reduced control of Palmer amaranth with S-metolachlor, a Group 15 herbicide. 

“While there are populations of concern, we still have effective options that preserve the ability to manage resistant Palmer amaranth,” Everman says. “Rotation of chemistries in soybeans and other crops is crucial to avoid further development of resistance.”

Additional Resources:

Palmer amaranth – GROW webpage 

Palmer Amaranth Management in Soybeans – GROW fact sheet 

Understanding Biochemistry to Fight Herbicide Resistance in Palmer Amaranth – SRIN article 

Creating a Palmer Amaranth Bioherbicide from Fungal Pathogens – SRIN article 

Research Survey Monitors Herbicide-Resistant Palmer Amaranth in South Carolina – SRIN article

Cover Crops and Residual Herbicides Team Up Against Palmer Amaranth – GROW article 

Sterile Pollen Technique Shows Promise for Palmer Amaranth Weed Control – GROW article

Meet the researcher: Wesley Everman

Published: Jun 30, 2025