Herbicide Resistance or Tolerance? Know Your Weed Spectrum
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Certain weeds like morningglory have a natural tolerance to glyphosate.
Identifying your weed spectrum by field is critically essential for control success. Yet, to reduce current and future problems, you need to know which weed species is resistant or tolerant to specific herbicides.
Evolving weed genetics and pollen flow across fields also help species survive. This is especially true when controlling the toughest Corn Belt weeds, such as waterhemp, marestail, lambsquarters, Palmer amaranth, and common and giant ragweed. Individuals within each species have also shown they can develop resistance to multiple herbicide sites of action
Resistance vs. Naturally Tolerant
Weed researchers define herbicide resistance as the inherited ability of a weed to survive a herbicide dose that’s usually lethal to weeds that haven’t been overexposed to a herbicide.
Herbicide tolerance is described as the inherent ability of a weed species to survive and reproduce after herbicide treatment, where no selection or genetic manipulation occurs. It is naturally tolerant. For example, morningglory, yellow nutsedge, field horsetail, prairie cupgrass, wild buckwheat and dayflower species contain some natural tolerance to glyphosate. Labeled rates can be less effective against these naturally tolerant species.
“Tolerance is hard to quantify, as we don’t have weed seed from 20 years ago to test,” says Nevin Lawrence, weed scientist from University of Nebraska in Scottsbluff. “There was a study several years ago in which researchers gathered common lambsquarters populations worldwide that had not been previously exposed to glyphosate. When they sprayed the weeds with glyphosate, they discovered a high level of natural tolerance among some lambsquarters, yet some couldn’t tolerate it.”
To distinguish between resistance and tolerance accurately, you need to know the history of the weed population. As this research paper (linked above) points out, defining resistance or tolerance would require elaborate molecular genetic studies. So, the author believes it’s best to use the term resistance (not tolerance) to avoid further confusion.
Regional Weed Differences
In Lawrence’s area of western Nebraska on sandier soils, redroot or Palmer amaranth pigweed species are relatively easy to control with glyphosate — if they don’t have herbicide resistance. “On the other hand, kochia and lambsquarters are notoriously difficult to control, and they’ve always been that way,” Lawrence says.
On tolerant weeds, growers can try switching adjuvants, product rates or application timing to often achieve control. “But if you’re dealing with true herbicide-resistant weeds, these tactics will not improve control,” Lawrence says.
Judging the severity of herbicide resistance becomes more daunting when determining between single resistance, cross-resistance and multiple resistance. Single resistance means resistance to only one herbicide family or one mode of action. Cross-resistance involves one or more families within the same mode of action, like waterhemp-resisting sulfonylurea and imidazolinone herbicides — both in the ALS mode of action. Multiple resistance is defined as weeds that that are not controlled by two or more herbicides that belong to at least two modes of action.
New Risk Management Tool
In 2020, University of Wyoming weed scientist Andrew Kniss, along with Lawrence and University of Idaho Extension specialist Albert Adjesiwor, designed a herbicide resistance risk calculator tool for western Nebraska and eastern Wyoming.
To help farmers make challenging decisions, this interactive online tool allows them to choose their most difficult weed and enter their four-year crop rotation with the herbicides they applied. The tool then estimates the effectiveness of each herbicide program, the risk of resistance developing to each herbicide mode of action and the cost of each program.
“While the program was built for this western Corn Belt area with University of Nebraska data, it’s still a good exercise for Midwest farmers to understand the risks of using the same products frequently,” Lawrence says. “Regional differences will vary somewhat on weed risks, herbicide efficacy and costs, but it would still provide good information to discuss with a local weed control adviser.”
How to Determine Resistance
Knowing if fields have herbicide-resistant weeds will help you fine-tune your weed control program.
To help rule out weed resistance, frequent scouting before and after application should be conducted to examine for factors such as weather, misapplication, improper timing and weed emergence after nonresidual herbicide use.
If herbicide resistance appears a likely possibility, check these application details:
- If only one weed species shows signs of resistance, are the other weeds on the product label controlled?
- If more than one species shows resistance signs, check other application factors, as it may not be herbicide resistance.
- Did this field area show signs of resistance last year, especially if the same herbicides or sites of action were used?
- Has this field received the same herbicides or sites of action repeatedly?
Contact your local retailer, crop consultant, Extension weed specialist or Corteva Agriscience representative to develop a comprehensive program to manage the issue.
For a deeper dive into avoiding weed resistance, check out these herbicide resistance resources.
Content provided by DTN/The Progressive Farmer
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