Hyper Resistance – This Waterhemp Defies 6 Modes Of Action – Scary

The slow, insidious march of herbicide resistance has reached yet another dubious milestone – six-way resistance has turned up in a population of waterhemp (Amaranthus tuberculatus) in Randolph County, Missouri.



The waterhemp population is resistant to atrazine, 2,4-D, chlorimuron, fomesafen, glyphosate and mesotrione. That makes it the first population resistant to six different herbicidal modes of action.

According to Missouri Extension weed scientist Kevin Bradley, the resistant population was first investigated several years ago after waterhemp escaped a 2,4-D burndown application on a farmer’s field. At the time, only one other known population of waterhemp had been found with resistance to 2,4-D, a synthetic auxin.

After researchers confirmed that the waterhemp population was resistant to 2,4-D, the news just got worse as one herbicide after another failed to control the population.

According to an article in Weed Science, 16% of the tested Missouri population contained genes stacked for resistance to six classes of chemistry. Only 1% of the population was found to have resistance to only one herbicide, which happened to be glyphosate. The six classes are:

  • ALS inhibitors (group 2 herbicides)
  • Synthetic auxins (group 4 herbicides)
  • Photosystem II inhibitors (group 5 herbicides)
  • PPO inhibitors (group 14 herbicides)
  • EPSP synthase inhibitors (group 9 herbicide)
  • HPPD inhibitors (group 27 herbicides)

Fortunately, the waterhemp population was not resistant to glufosinate or dicamba. Bradley says the producer who farmed the field where the waterhemp was found shifted to LibertyLink soybeans, which are tolerant to glufosinate, and began applying residual herbicides.

“Luckily for us, we have not seen or heard of it getting outside of that area where we first found it,” Bradley said. Currently, the most common resistance combination in waterhemp in Missouri is to glyphosate, ALS inhibitors and PPO inhibitors, Bradley adds.

Scientists are also looking into the possibility that metabolism-based resistance could be at work in the 6-way resistant waterhemp population. In metabolism-based resistance, enzymes that break down one herbicide in a weed are able to break down other herbicides, as well. “For us in weed science, metabolism-based resistance mechanisms are scary,” Bradley says.

Is Palmer Pigweed Next?

University of Tennessee Extension weed scientist Larry Steckel wasn’t surprised at the discovery of six-way resistance in waterhemp and says the southern version of the pigweed species, Palmer amaranth, also could be headed toward higher levels of multiple resistance.

“We already have ALS-, glyphosate- and PPO-resistant Palmer amaranth, and we have some that is DNA-resistant, so we have some with 4-way resistance,” Steckel says. “And we’re 95% planted to Xtend soybeans in West Tennessee this year. A lot of glyphosate and dicamba went out this season and that’s pretty much all we’ve relied on for pigweed on 95% of our soybean acres. It won’t be long before we add dicamba to the list.”

Bradley concurs that dicamba could be the next to fall. “In the past two seasons, I have witnessed growers who are in the process of creating a dicamba-resistant weed. I wouldn’t be surprised at all if we didn’t start seeing that in a couple of years.

“It would have seemed implausible 10 years ago that we would commonly have weeds with resistance to three or four herbicides. Well, that’s true today,” Bradley adds. “Ten years from now, I sure am hoping that I’m not saying that five- and six-way resistance is common.”

How To Slow Down The Progression Of Resistance

To slow down the march of resistance, Steckel advises producers to rely more on practices “that you don’t pour out of a jug.”

“What we’ve been doing is relying on one herbicide until it doesn’t work anymore,” he explains. “First, it was the ALS herbicides like Scepter and Pursuit. When they stopped working, we went to Roundup. When Roundup stopped working, we went to Flexstar. When Flexstar stopped working, we went to dicamba. If all we do is rely on dicamba to do most of the heavy lifting on Palmer amaranth, we’re going to add it to the list, as well, and in a very short time.”



The discovery of a six-way resistant waterhemp population is a cautionary tale for producers, according to Bradley.

“I’ve been at the University of Missouri fighting waterhemp almost 16 years, and it’s worse now than when I began,” Brandley says. “Waterhemp finds a way. We need to stop relying so much on herbicides and start thinking about other practices.”

Best Practices Going Forward

Bradley and Steckel provided several tips to help producers manage the two most unforgiving weeds in U.S. agricultural history, Palmer amaranth and waterhemp:

Take a zero-tolerance approach.

“We don’t want any of the resistant weeds sticking above the canopy,” Bradley stresses. “Don’t let any weed seeds go into the soil seed bank after harvest. It may mean you have to go out and rogue some weeds by hand, but we have to prevent seed production.”

Hit ‘em with residual herbicides that have multiple modes of action.

“In this case (six-way resistance), we would focus on glufosinate and dicamba and residual herbicides,” Bradley notes.

Rotate crops.

This gives the producer more herbicide options, particularly when the rotation includes corn.



Plug cover crops and other non-herbicide weed suppressing practices into the plan.

Don’t rely on the cover crop alone and don’t rely on the herbicide alone. But the two together can really be beneficial.

Switch technologies.

“Go to Enlist (crops that are tolerant to 2,4-D) and get it into the mix,” says Steckel. “Make sure you’re using LibertyLink technology in your cotton. Don’t just rely on dicamba as your backstop. Then make sure you use residuals to keep them down.”

Pull out the hoods.

“Get hooded sprayers into the field again and start using some herbicides like Direx, MSMA and Gramoxone,” Steckel specifies. “It may be hard to get across all your acres with a set of hoods, but there are some very effective hooded sprayer applications that provide a different mode of action that can help a lot.”