Are the Weeds in Your Fields Herbicide-resistant?

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Soybean weeds

Keeping your corn and soybean fields clean is one of the most important steps toward getting a good yield at harvest each season. However, some years, weed escapes may thwart your plans for perfectly clean rows. An important question to ask yourself when this happens is: “Did something go wrong with my weed control program or are the weeds I’m seeing herbicide resistant?” 

Corteva Agriscience North America Herbicide Biology Leader Kelly Backscheider has advice to help you answer this question. Because, knowing whether you’re facing resistant weeds can help you create the right program to control them – and maximize corn and soybean yield potential. 

Why Herbicide Resistance Develops

Backscheider says there are several reasons weeds will develop resistance to herbicides. 

“Some herbicide modes of action will develop herbicide resistance more easily than others based on the way the herbicide works in the plant. Some weed species will be more likely to develop resistance due to the characteristics of that weed,” Backscheider says. “However, generally, herbicide resistance develops by the repeated use of the same herbicide or repeated use of herbicides from the same mode of action.” 

Backscheider says resistance can develop in a certain location quickly or slowly, depending on multiple factors. One of those being the weed species.  

For example, she says waterhemp and Palmer amaranth can become resistant more quickly than some other weed species, because they’re dioecious with separate male and female plants. This allows for more reproductive variation, “you can have a resistant female pigweed pollinate a susceptible male pigweed and by next year, the offspring are all resistant as a result.”   

“You can have a resistant female pigweed pollinate a susceptible male pigweed and by next year, the offspring are all resistant as a result.” 

This isn’t the only example that can impact the speed at which herbicide resistance progresses. 

“Additionally, herbicide resistance often might not be noticed until it has been present for a few years. It might just be one escaped weed in year one that quickly turns into hundreds of resistant weeds in a matter of a few years,” Backscheider says. “Or, in one situation, you might have a weed that gradually builds up resistance to a level where the labeled rate no longer controls it. In other situations, there are mechanisms of resistance such as gene amplification where a 100x rate won’t control the resistant weed, so resistance happens very quickly.” 

Are Your Weeds Herbicide resistant?

While herbicide resistance is certainly a problem, Backscheider says, the problem on your farm might not be resistance. Your weed control program might have just been ineffective, leading us back to the original question in this article: Are you seeing weed escapes because of resistance or because of an unsuccessful program? 

“A weed control program might be unsuccessful for a number of reasons. Maybe you didn’t have a solid program in place for the weed species that you have. It also might be that the environmental conditions you experienced made it difficult to manage weeds in that particular season,” Backscheider says. “If it’s been too dry, maybe your residual herbicide didn’t get activated and weeds emerged sooner than they should have. Maybe it was cold and cloudy when you applied a postemergence herbicide that needs sunlight to work in the plant. Maybe your weeds were too large for the application.” 

She says resistance will be evident when you have the right environmental conditions with an effective and timely herbicide program and yet you still do not control the weeds. There are other signs of resistance you can look for as well. 

“An unsuccessful weed control program often only happens once and then you are able to control the weeds,” Backscheider says. “Pay attention to your fields. Scout fields a few weeks after making an application. Are there survivors? In a resistance situation, you will often see a dead plant within a short distance of a surviving plant of the same size. 

Backscheider says you also can have certain weeds tested to see if they’re resistant to certain herbicides.  

“Farmers can send in suspected resistant weeds to university labs for testing. Waterhemp and Palmer amaranth are usually able to be tested, but so can other weed species such as marestail or giant ragweed. Keep in mind that there might not yet be testing for resistance to a particular herbicide though,” Backscheider says. “Your best bet is to assume you have resistance. If you have heard resistance being talked about in your state and you are having trouble controlling weeds, it’s best to assume you have it. Plan your weed control program accordingly.”  

“If you have heard resistance being talked about in your state and you are having trouble controlling weeds, it’s best to assume you have it. Plan your weed control program accordingly.” 

Mitigating Herbicide Resistance

While it’s a good idea to have weeds tested to help keep resources like the International Herbicide-Resistance Weed Database up to date, Backscheider says taking steps to prevent or mitigate herbicide resistance is always the right move.  

Here are some reminders to do that: 

  • Create an effective weed control program. 
    • Include burndown, and pre- and postemergence applications. 
    • Use multiple modes of action in each pass. 
    • Overlap residual activity. 
  • Utilize other weed control tools, including: 
    • Crop rotation 
    • Cover crops 
    • Tillage 

 “Herbicide resistance isn’t going away,” Backscheider says. “So companies are focusing on new traits, new herbicide products and new tools that can help manage resistant weeds so farmers can maximize yield.” 

And while you keep a lookout for those new tools and technologies, Backscheider says it’s also important to keep your ear to the ground about resistance. She says if you don’t see it in your fields yet, you’ll probably be able to predict if you will. 

“Pay attention to your neighbor’s fields. Pay attention to your university weed science social media accounts, blogs, podcasts, etc.,” Backscheider advises. “Pay attention to your neighboring state(s). If you live close to a state line and resistant weeds are showing up across the border, it’s likely just a matter of time before you will have resistance as well.” “ 

You can find powerful solutions to prevent and mitigate herbicide resistance – and protect your yield potential – by visiting soybean herbicides portfolio and corn herbicides portfolio pages on