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Weed scientist identifies two types of herbicide resistance farmers should watch for.

Herbicide resistance is an evolutionary process – one that began decades ago and grows each season. With new discoveries in herbicide resistance becoming more complex, the weed-control battle proves to be a never-ending uphill climb for U.S. soybean farmers.

But as resistant weeds continue to evolve, ongoing research provides a better understanding of the challenges presented in the types of herbicide resistance we face—target-site resistance and non-target-site resistance—and which control mechanisms will work for each moving forward.

Target-Site Resistance

Historically, the type of resistance that farmers and researchers deal with most often is target-site resistance, according to Aaron Hager, Ph.D., associate professor of weed science at the University of Illinois.

“Every herbicide has a specific target site in a plant,” says Hager. “The herbicide has to bind to that target site and effectively shut it down in order to kill the weed.”

Resistance can occur when changes in the target site stop the herbicide from effectively binding there.

“It doesn’t take a lot for this to occur,” says Hager. “One change to the amino acid residue chain in the protein sequence, and all of the sudden you have resistance.”

Target-site mutations are often prompted by repeated applications of the same herbicide site of action. Today, target-site resistance remains the most common type of herbicide resistance seen around the world, encompassing resistance to ALS inhibitors, ACCase inhibitors and more.

Non-Target-Site Resistance

On the other end of the spectrum, non-target-site resistance, or metabolic resistance, involves more change in the plant than just at the target site alone.

In this type of resistance, weeds develop the ability to rapidly metabolize, or break down, the herbicide before it can cause significant biotoxic effects to the weed.

“Once an herbicide is inside a plant, it’s going to try and control it,” says Hager. “With metabolic resistance, the weed is now behaving like the resistant crop and can rapidly metabolize the herbicide and survive.”

Though less common than target-site resistance, metabolic resistance is a concern on the rise for Hager and his colleagues. With the recent discovery of single gene atrazine metabolic resistance in waterhemp, they’ve only hit the tip of the iceberg on their research with this type of resistance.

“There are still many unknowns about this,” says Hager. “Our ability to predict which other herbicides the weed might also be resistant to goes out the window. This is what we call the game changer.”

Although there are still many questions to be answered, researchers speculate that it takes longer for metabolic resistance to develop and spread compared to target-site resistance.

Controlling all types

But whether you’re dealing with target-site resistance or non-target-site resistance, Hager says the control method remains the same: tank-mixing multiple herbicide modes of action in each application.

“The probability that a weed will be resistant to two different herbicides combined is less than the probability of the weed being resistant to each herbicide individually,” says Hager.

But while he says mixing herbicides is a better strategy than rotating individual products, it doesn’t erase resistance, it only forestalls it. Hager encourages incorporating diversified control methods into your weed-management plan to stave off resistance.

With the close similarity between waterhemp and Palmer amaranth, Hager suspects that farmers may start to see metabolic resistance to Palmer in the future. The time to start your improved weed-control plan is now. Visit for help getting started.

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