Weed Control: Resisting Resistance

10 Resistance-Fighting Tips

1. Start scouting after harvest in the fall.
2. ID and test weed escapes for herbicide resistance.
3. Use preemergence herbicides with multiple modes of action.
4. Control weeds when they are small.
5. Use the right nozzles and gallonage for the product.
6. Calibrate sprayer and nozzle spacing.
7. Keep booms at the right height.
8. Manage turns and corners without overlap or skips.
9. Be diligent with cleanout of sprayer tank and booms.
10. Always follow label directions.

Whether it’s Palmer amaranth in the South, horseweed in northern states, or waterhemp in the Midwest, weeds resistant to multiple herbicides are showing up in record numbers. Keeping them from robbing yields can feel like a never-ending battle.

The United States has the dubious distinction of having the most weed species listed as resistant—161—according to the International Survey of Herbicide-Resistant Weeds. Canada, with 68 resistant species, ranks third globally. 

“Herbicide-resistant weeds have been documented for the last 60 years, but [they] never had the significant impact on agriculture that we’ve experienced in the past few years,” says Kevin Bradley, Extension weed scientist at the University of Missouri. 

Bradley is on the front lines of the fight against resistance, since Missouri has the distinction of identifying the only documented six-way resistant waterhemp—currently isolated to one field. Multiple-resistant weeds are those which exhibit resistance to three or more herbicide families, including—but not limited to—glyphosate, ALS inhibitors, PPO inhibitors, triazines and 2,4-D. “It’s certainly more challenging to manage [such] multiple-resistant weeds than single- or even three-way resistant weeds,” he says. “But we do have strategies to recommend.”

Scout and test problem weeds

Scouting before and after herbicide application is key. “It’s important to proactively monitor all fields, starting in the fall after harvest,” says Christy Sprague, Extension weed scientist at Michigan State University. “Note what weed escapes you see and, if possible, get them tested for resistance.”

Growers should think about managing resistant weeds before they even put a planter in the field in the spring, she adds. Regardless of weed species, most strategies include preemergence herbicide applications. “Preemergence herbicides will help control some herbicide-resistant weeds and give growers a head start on a good weed management,” Sprague explains. 

Preemergence products offer the benefit of two or more effective modes of action—such as how the product affects a specific enzyme or disrupts photosynthesis—against weeds, adds Bradley. “It’s one of the best options we have right now,” he says. “Anything that allows the weed to survive contributes to resistance. It’s just a numbers game.”

Once weeds do develop, controlling them when small is crucial. “One of the most common incorrect practices is spraying weeds that are too big,” Bradley notes. “If the label says to spray at 4 inches or lower, pay attention. If you wait until weeds are 8 inches, you won’t get anywhere close to the level of control needed, and that weed will likely go to seed and be a bigger problem next year.”

Set up for success

How growers set up equipment also plays a major role in weed management success. “Think carefully about nozzle type and gallonage used,” Sprague says. “For example, if you are growing LibertyLink^® soybeans and spraying Liberty^® herbicide, you’ll need to ensure good coverage. If you are applying a registered dicamba product in [Roundup Ready 2] Xtend^® soybeans, specific nozzles that produce larger droplet sizes to reduce drift are needed.”

Sprayer tank and boom cleanout also are extremely important, as is boom height. “It’s critical for growers to know where their boom is in relation to the crop canopy or target weed,” says Bradley. “Operators tend to keep it higher than what is ideal. You don’t want the boom high enough to allow off-target movement.” Drift, off-target movement or poor canopy penetration can result in less than a lethal dose reaching the weed, setting up potential resistance.

Other common applicator errors also lead to escaped weeds, so it’s paramount to apply herbicides correctly, says Sprague. She advises properly calibrating sprayers before each treatment, including optimizing nozzle spacing for proper spray pattern overlap, to make sure operators apply the right amount of product with the best coverage. Again, boom height needs to be correct. Booms set too low can reduce nozzle overlap, leading to inadequate coverage.

“Sprayer speed is another factor,” says Sprague. “Going too fast leads to non-target impact. We recommend operators travel at 15 mph or less.”

Managing turns and corners without overlap is also vital for maintaining adequate spray coverage. “Applicators use different strategies for making turns without overapplying product,” says Sprague. “Newer equipment has built-in systems for turn compensation and squaring corners.”

In the future, equipment solutions will play an even bigger role in the fight against herbicide resistance. “We expect things like robotic scouting, weed destructors, and sprayers with sensors that detect the weeds and spray accordingly,” says Bradley. 

At the moment, Bradley is looking for anything that makes application more uniform and will result in less off-target movement. “A combination of cultural practices, proper herbicide application and new herbicides is a start,” he says. “There is no one formula, but we’ve seen cleaner fields in the past year or two with all of these practices being adopted together.

“Herbicide resistance is not going away,” he concludes. “The only thing we can do is try to keep it contained.”