Waterhemp Scores Again

Illinois scientists find new resistance to this tough-to-control weed.

Pamela Smith
By  Pamela Smith , Crops Technology Editor
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Waterhemp is one tough weed. Now it has begun to outsmart a group of important residual herbicides, Image by Pamela Smith

Waterhemp has just thumbed its nose at another group of herbicides.

Resistance to Group 15 herbicides (very-long-chain fatty acid inhibitors) has officially been found in Illinois research plots. It is the first dicot (broadleaf) weed in the world to outmaneuver herbicides within the Group 15 chemical family. While scientists aren’t sure how widespread the issue is, University of Illinois weed scientist Aaron Hager says the discovery is yet another warning to change weed-management behaviors now.

“Waterhemp has now shown the ability to resist seven different herbicide sites of action,” Hager says. “Farmers have been leaning heavily on the Group 15 herbicides across all crops as they battle resistant weeds. This is another example of how important it is to diversify weed-control approaches to keep the effectiveness of this tool.”

Syngenta has been collaborating with Illinois scientists on their findings and providing important background information, confirms Dane Bowers, Syngenta technical lead for herbicides, and Gordon Vail, Syngenta technical product lead on S-metolachlor, one of the Group 15 herbicides.

S-metolachlor is the active ingredient commonly sold by Syngenta under the trade name Dual Magnum. It is also found as a component in many popular herbicide premixes.

“We feel it is important as a manufacturer to alert farmers to these findings and work alongside weed scientists to keep these herbicides effective as long as possible,” Vail says. “The reality is, not any one product or class of chemistry by itself is good enough. We need them all.”

Go down through the sites of action commonly used in corn and soybean crops today, and nearly all have had some level of resistance confirmed, Bowers adds. “This is the age we live in. Resistance to a particular AI (active ingredient) or site of action doesn’t invalidate or devalue that active. It can still be extremely important across a wide geography to control other weed species.”


What makes this discovery unique is the Group 15 herbicides have been fairly resilient. Worldwide, there are only five cases of grass weeds that have been confirmed resistant to very-long-chain fatty acid inhibitor (Group 15) herbicides. Most of those cases are in wheat and rice. The only confirmed Group 15 resistance in the United States had been in monocot (grass) in Idaho, Oregon and Washington.

The Illinois discovery was the first confirmed case in the world in a dicot (broadleaf) weed species, according to the International Survey of Herbicide Resistant Weeds (www.weedscience.org). The University of Arkansas has since listed two Palmer amaranth populations with reduced sensitivity specific to S-metolachlor.

Hager says the resistant populations were found in McLean and Champaign counties. Both populations were already known to be resistant to multiple modes of sites of action--the McLean County population now resists five sites of action, and the Champaign County waterhemp population has six-way resistance.

Both the resistant populations demonstrate resistance to all Group 15 herbicides tested, which included S-metolachlor (Dual Magnum), dimethenamid-P (Outlook), pyroxasulfone (Zidua) and acetochlor (Harness, Warrant, Degree and others), Hager says.

“In field trials, we noticed that the level of control with metolachlor was drastically lower than that of acetochlor,” he reports. “It has always been less, but this was dramatically different.”

Further field trials and greenhouse studies showed resistance ratios that ranged from 4- to 64-fold among the Group 15 herbicides. However, Hager says that doesn’t mean simply switching to another herbicide within this site of action removes the threat of resistance in waterhemp.

“This is yet another example of how relying on chemicals alone to control waterhemp isn’t working anymore,” Hager adds. “It also means we expect the length of residual control is going to continue to decline.” Group 15 herbicides are also important ingredients in preemergence residual programs.


Weed scientists have long warned that new herbicide miracles are not sitting in company pipelines. Bowers and Vail point to Syngenta’s Resistance Fighter program as an example of efforts to educate about the risk of continuously relying on the same weed-control tactics.

However, they also agree the message is deceptively simple in a world where nature keeps evolving. “While this particular example of resistance seems to be limited right now, it is evidence that we need to do things different going forward,” Vail says.

“We cannot continue to rely totally on herbicides and expect to be sustainable long term,” he says. “I think in a lot of cases, we’re doing the right things; but it is getting more and more difficult for these herbicides to be effective in some of these weed species.”


Some key ways to delay herbicide resistance include crop rotation, combining effective herbicides with multiple sites of action and using full rates for all applications in the field. Letting preemergence residual products clean up fields early and using postemergence products as a backup rather than the first line of defense are also important, Bowers says. Cover crops, tillage and increasing crop competition through narrower row spacings are other tactics.

Bowers notes that products such as atrazine and HPPD-inhibiting herbicides, for example, have had known resistance for years but remain important tools. “While the discovery with Group 15 resistance in waterhemp is important, we think that addressing it early and being up front about it is equally important.

“There’s a lot more work to be done to find out how or why this has developed in Illinois,” Bowers says. “What we do know is it is critical to use full rates of products and employ multiple tactics to avoid resistance.”

“Resistance doesn’t happen overnight,” Hager says. “You may notice residual control declining and then small weed breaks.

“Keeping weeds from going to seed can go a long way toward helping keep these products useful,” he continues. “And, yes, sometimes that means using a hoe.”

For More Information:

> Full summary of weeds and countries that have confirmed resistant to long chain fatty acids

> Syngenta’s Resistance Fighter program


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