JEFFERSON CITY, Mo. (DTN) -- Farmers who plan to spray a glufosinate herbicide for postemergence weed control this season, take note: The weed you're likely trying hardest to kill is fighting back.

Weed scientists in two states have confirmed waterhemp populations that can survive being sprayed with glufosinate, an active ingredient previously considered effective at controlling the troublesome weed.

In late December 2025, weed scientists at the University of Illinois and Southern Illinois University reported they had confirmed glufosinate resistance in three waterhemp populations through greenhouse dose-response experiments. In mid-March, Debalin Sarangi, University of Minnesota Extension weed scientist, confirmed that his team had identified a glufosinate-resistant population in Dodge County in southeast Minnesota.

"We've lost a couple battles to waterhemp," said Rodrigo Werle, University of Wisconsin Extension cropping systems weed scientist, during a recent webinar discussing strategies for waterhemp control. "I still think we can win the war."

University of Illinois weed scientist Aaron Hager reported the Illinois waterhemp populations were 2.2- to 3.4-fold resistant to glufosinate when compared to a known susceptible population in the greenhouse. Sarangi said the Minnesota population was 2.8- to 3-fold resistant. This means it required two to three times more herbicide to kill the resistant weeds. Both weed scientists noted that while these resistance ratios were relatively low compared with waterhemp resistance to other herbicides, they were still high enough to result in incomplete control.

STRATEGIES FOR PRESEASON SUCCESS

While some waterhemp is now showing resistance to glufosinate, Werle said it's not time to throw in the towel. Instead, farmers need to protect this herbicide technology from losing any more efficacy -- as new modes of action for postemergence application aren't likely to be available any time soon. The first step is a robust preemergence herbicide package.

"Just putting a preemergence herbicide out there is not enough," he said. "I am a big proponent of front-loading residual herbicides. Do not skimp on your residuals up front. This idea of saving some of your preemergence residual herbicide up front to put it with your postemergence program, according to our Wisconsin data, I think that's a mistake."

Werle suggested building a preemergence program that includes multiple modes of action, especially a Group 14 herbicide and a Group 15 herbicide. A Group 5 herbicide such as metribuzin can also help with residual control of waterhemp, assuming a farmer's soil type allows for its use. These products should be applied at full rates based on soil properties. He also recommended adding a Group 2 herbicide to the preemergence program to help control larger-seeded weeds such as common ragweed, giant ragweed and grasses.

PERFECT THE POST PASS

Applying glufosinate and other postemergence herbicides at full rates under the best application conditions will help ensure the products work as intended. As with preemergent residual herbicides, Werle recommended applying an effective, two-way postemergence mix.

"The vast majority of our soybean acres here in Wisconsin currently are Enlist acres, so the big question is, 'Do I go with Enlist One (2,4-D)? Do I go with Liberty (glufosinate)? How about mixing?'" he said. "Some work we did a couple years ago shows that when you mix these two chemistries together, we are seeing more effective and more consistent control of waterhemp."

That control is accentuated when weeds are still small, he added. When applying the standard labeled rate of glufosinate, it is recommended to spray when weeds are less than 3 inches tall.

"The more you delay, the lower the likelihood of success. The more we delay our applications, the more our crop is growing and the weeds are growing, the more they're going to be intercepting your spray solution," Werle explained. "So, if you have smaller weeds growing adjacent to your crop rows, or if you have smaller weeds growing underneath other big weeds, those weeds are being shielded. And then they end up being exposed to sublethal rates of herbicides. And that's a major problem.

"And that's why we don't want to delay our applications because the more we delay them, the more spray interception occurs, the less the rate of the herbicide that we'll be getting on these plants or parts of plants that are shielded."

Werle noted herbicide formulation does matter and it can greatly impact weed control. Using a low-cost generic version of glufosinate without any adjuvants, for example, will not provide effective control. Be sure to check the label of the product you are using to make sure what adjuvants are needed, he said.

"When you're spraying Liberty or any other glufosinate-based product, you must have AMS (ammonium sulfate) in that tank. That AMS is conditioning the water. That's the first thing," he said. "Second thing, that AMS is a source of nitrogen that increases uptake and translocation of our foliar-applied herbicides. The third aspect, glufosinate controls weeds in part because of the accumulation of ammonia. When we're spraying our herbicides with AMS, there is additional ammonia being provided."

In 2025, a retrospective research analysis published in the journal, Weed Science, looked at how weather affects the probability of successful weed control with one postemergence application of glufosinate. The analysis identified weather variables before, during and after application that most influence glufosinate efficacy, including solar radiation (i.e. amount of sunshine), relative humidity, air temperature and rainfall.

The analysis found that glufosinate performs best in sunny conditions and high temperatures. Humidity at the time of application also is important. Below 70% relative humidity, the probability of successful weed control decreased, while higher levels of humidity improved weed control, especially for waterhemp.

Read more from DTN here: https://www.dtnpf.com/… .

Listen to Werle discuss strategies for effective waterhemp control here: https://youtu.be/….

Read more about preserving glufosinate efficacy here: https://blog-crop-news.extension.umn.edu/….

Jason Jenkins can be reached at jason.jenkins@dtn.com

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