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Herbicide resistance is a growing issue in waterhemp and Palmer amaranth

by Wyoming Livestock Roundup

            “Farmers need to buckle down to reduce the growing problem of herbicide-resistant strains of weeds, especially waterhemp and Palmer amaranth,” says University of Illinois Weed Scientist Dr. Patrick Tranal.

            In fact, he notes these two weeds are developing stronger defenses against herbicides all across the U.S. and there are very few post-emergence options to treat them.

Multiple resistance

            “Herbicide resistance in these two weeds, among others, has been a factor for several years,” Tranal says. “But the emergence of resistance to multiple modes of action is growing.”

            Tranal notes research in the Midwest has reported the existence of Palmer amaranth populations exhibiting four-way resistance.

            “Waterhemp is even further along,” Tranal states.

            He continues, “We’re seeing multiple resistance and it’s hard to keep track. We have three-way, four-way and five-way resistant populations. I believe there is even a six-way resistant population that’s been reported.”

            Tranal explains these include acetolactate synthase (ALS)-inhibiting herbicides or those with protoporphyrinogen oxidase (PPO) chemistries and glyphosate.

            “Those are the most common ones,” he says. “Then, on top of that, we’re seeing resistance to herbicides with four-hydroxyphenylpyruvate deoxygenate (HPPD) chemistry, primarily corn products such as Callisto.”

            For years agronomists have urged growers to use herbicides with multiple modes of action to prevent development of resistance by weeds. However, Tranal notes this is becoming more challenging since some strains of weeds are exhibiting multiple resistances.

            “Do we have plants resistant to multiple modes of action? Yes, we do, but it doesn’t mean all plants out in the field are,” he says. “One may be resistant to A and B or A, B and C. The more chemistries we throw at these, the more likely we are to kill all of our plants, but we also need to make sure we reduce the chance these weeds survive up to harvest.”

Metabolic and non-target resistance

            The other concern in both of these species, according to Tranal, is metabolic resistance or non-target resistance.

            “With ALS resistance, we have a mutation which encodes the enzyme targeted by the herbicide. What this means, from a practical standpoint, is if we have target resistance, resistance only affects the target, so we can rotate to another herbicide and it will control that,” he says. 

            Other weapons, including mechanical weed suppression systems such as the Harrison Seed Destructor (HSD), will likely play a role in the future war against weed resistances, according to Tranal.

            “The HSD is used during harvest to pulverize weed seeds so they don’t have an opportunity to germinate later,” he explains. “However, farmers should be ever vigilant. We’re getting to the point where we’re literally running out of options.”

Waterhemp – a bigger threat

            According to Greg Ury, an agronomist with BASF, waterhemp is the bigger threat of the two weeds.

            “Most farmers in the Midwest are still trying to get control on resistant strains of waterhemp,” he notes. “There are a few areas of Palmer amaranth, but it isn’t as widespread as waterhemp.”

            He continues, “The easiest way to control waterhemp is to keep it from emerging. The key is to use pre-plant applications with residual activity.”

            Therefore, Ury recommends BASF’s Zidua.

            “Typically, in most cases, the Class 15 residuals work extremely well on waterhemp,” Ury states. “Then growers will usually come back in with Liberty or dicamba application.”

            He notes Class 15 residuals also include herbicides like Dual Magnum, Warrant and Stalwart.

            Hannah Bugas is the assistant editor for the Wyoming Livestock Roundup. Send comments on this article to

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