Waterhemp, Palmer amaranth and some other tough broadleaf weeds and grasses are no longer slipping past just single herbicides. Across the Corn Belt and beyond, they are tolerating entire herbicide programs. Weed scientists say that pattern points to a critical issue more farmers are facing: metabolic resistance.
Unlike traditional target-site resistance, which is often specific to a single herbicide class, metabolic resistance is even worse because it can confer cross-resistance to multiple, unrelated herbicide groups.
Aaron Hager, University of Illinois Extension weed scientist often warns that when a tough weed like waterhemp learns to metabolize one herbicide, it becomes easier for it to “learn” to detoxify others. That ability has helped lead to the 7-way resistance with waterhemp seen in some Illinois counties, according to weed scientist Patrick Hanel, one of Hager’s colleagues.
At least 13 states have reported having some degree of “highly suspected” or confirmed cases of metabolic weed resistance. Here are three of the broadleaf weeds demonstrating metabolic resistance and states where they’re located:
Target-site resistance can be identified through DNA tests. But metabolic resistance is a “guessing game” involving hundreds of potential enzymes working in tandem, making it difficult for scientists and farmers to know which products will still work in their specific fields.
Tommy Butts sees the trend for metabolic resistance taking root in Indiana. He says PPO resistance in waterhemp is “getting widespread,” and the failures are expanding to other chemistries as well.
“We had more complaints last year about things like mesotrione or Callisto starting to fail, which is really scary in the corn acres,” says Butts, Purdue University Extension weed scientist. “Corn is supposed to be our easy year to control waterhemp, and now, all of a sudden, we start losing Callisto.” He addresses this in detail in the latest Purdue Crop Chat.
The bad news does not stop there.
“You start talking auxins and glufosinate, and we have confirmed resistance in the state to those,” he says. “I wouldn’t say that’s as widespread, but it’s definitely popping up.”
With metabolic resistance chipping away at PPOs, HPPDs, atrazine partners, auxins and glufosinate, the old playbook of “just switch products” no longer works well.
“Hammer With Residuals” And Build Effective Combinations
Butts’ first message to corn and soybean farmers is straightforward: no more solo herbicide passes in the field.
“We have to hammer weeds with effective residuals and then mix up our posts as much as possible,” he says.
In his view, that means at least two things for row-crop growers. First, use layered residual programs that keep fields clean as long as possible and reduce the number of emerged weeds that ever see a post pass. Second, use post-emerge applications that combine multiple, truly effective modes of action at full labeled rates.
Cutting rates, he warns, is exactly how growers “train” metabolism-based resistance to take root.
With soybean trait systems, he pushes hard against relying on a single flagship product.
“If we’re growing Enlist soybeans, don’t just rely on Enlist and don’t just rely on Liberty,” Butts advises. “Do the tank mix. The tank mix trumps everything.”
Pay More Up Front To Avoid Making Expensive “Revenge Sprays”
Metabolic resistance can thrive when weeds are hit with chemistry they can partially tolerate. That is why Butts keeps coming back to strong, early, soil-applied programs.
He hears pushback from farmers every year on using multiple products in the tank.
“A lot of people tell me, ‘Well, it costs way too much up front with $20 for a pre. Corn gets even more expensive,’” he acknowledges.
However, Butts points to work by Purdue University Extension and other states showing those dollars pay off when the entire season is measured.
“If you can get a strong residual program out and get it activated, the whole-season economics of it makes sense,” Butts says. “It’s consistently shown that if you have that strong pre up front, you don’t have what I like to call the revenge sprays in August, where we’re going across the field three different times trying to kill waist-high waterhemp.”
Check out this tool from GROW on how to address waterhemp specifically.
Protect Herbicide Tools To Extend Their Use
As more herbicide modes of action come under pressure, Butts singles out metribuzin as an example of a product that still pulls its weight in soybeans.
“Metribuzin is a big one in soybeans, because we don’t have a lot of resistance to that,” he says.
“I will also put in the plug for AMS in general, across the board,” Butts says. “That always helps with some of those products… when we start getting later in the season, we get more stressed weeds. AMS even tends to help there.”
Underlying all of it is a blunt warning about what happens if growers decide to skimp on their weed control efforts.
“If you let it go even one year, now you’ve made yourself a mess for the next five to 10 years,” he says. “You’ve got to try and stay on top of weeds as much as possible.”
5 Practical Recommendations To Address Metabolic Resistance
Because metabolic resistance is so unpredictable, weed scientists have shifted their advice away from “rotating chemicals” toward a “zero-threshold” approach to control. The following metabolic resistance management recommendations have been presented by Aaron Hager, University of Illinois Weed Scientist, and Beck’s agronomists:
1. The primary focus of metabolic resistance management should be on decreasing the weed seed bank. This means that weeds must be eliminated before they ever go to seed.
2. A robust residual herbicide program should be used, not because residuals represent a different herbicide family but because they eliminate weeds at the earliest growth stages – slashing contributions to the weed seed bank.
3. Physically cutting weeds out of the crop must be included in the management plan, because physical elimination of weed escapes further slashes contributions to the weed seed bank.
4. Post-herbicide programs should shift from calendar-based timing to scouting-based timing. Once weeds break through a pre-emerge residual program, they must be eliminated. Such early targeting further slashes contributions to the weed seed bank.
5. Mechanical techniques, field cultivators, etc., should be used where possible to further the cause of decreased seed production.
