One reason glyphosate-resistant weeds are multiplying is lack of early detection. Because glyphosate has been so effective, busy farmers often skimp on scouting after spraying, says Jeff Stachler, Extension weed scientist at North Dakota State University and the University of Minnesota. As a consequence, “we’re not detecting weed changes from year to year.”
Another impediment to early detection: Glyphosate resistance looks quite different from older cases of herbicide resistance, making it tougher to spot, he says. Weeds that have overcome ALS inhibitors, ACC-ase inhibitors or triazines usually show little damage from the herbicide. Sensitive plants die; resistant plants survive and thrive. “It tends to be black or white, dead or alive.”
By contrast, there’s a much wider range of weed responses to glyphosate, Stachler says, “from dead to nearly normal in appearance, and every response in between.” This variability makes it tricky to distinguish glyphosate resistance from other causes of weed control failure, like application mishaps, weather or certain weeds’ natural ability to tolerate glyphosate.
“Good integrated pest management hinges on scouting,” says Vince Davis, University of Wisconsin Extension weed scientist. Yet, it’s quite common for farmers to miss the early warning signs of glyphosate resistance, he says. Weed escapes “get chalked up to poor control for some other reason” than genetic changes in weeds. “It can take a couple of years before growers realize it’s resistance, and by then, it’s too late to keep populations low.”
There’s definitely a need for “better understanding of how to scout for glyphosate-resistant weeds,” agrees Doug Holen, a University of Minnesota Extension crops specialist. “Resistant weeds can be hard to see, and you have to want to find them.”
Knowing what field patterns and individual plant responses to look for can greatly aid in identifying low-level resistance, he says. Here are some questions to keep in mind while scouting:
• Has the field been sprayed repeatedly with glyphosate? Applying glyphosate year after year puts tremendous pressure on weeds to adapt to the herbicide, says Stachler, who works with a lot of farmers who grow Roundup Ready soybeans, corn and sugar beets. Fields where glyphosate has been used continuously and exclusively are at very high risk for resistant weeds, he says.
•Have glyphosate-resistant weeds been confirmed in your region? Keep up to date on resistant weed developments, so you know “which weed species have a greater likelihood of being glyphosate resistant,” Davis says. If you are having trouble controlling any of those species, you could have a resistance issue, even if weeds are not present in large numbers, he says.
And keep in mind that farm borders are no protection from glyphosate-resistant weeds. Resistant seeds and genetics can spread to new fields by wind, water, tillage, equipment or animals. In Michigan, for example, pockets of multi-herbicide-resistant Palmer amaranth appeared in 2010, says Christy Sprague, Michigan State University Extension weed scientist. “This is not a common weed in Michigan,” she says, and was probably brought in from another state in livestock feed. Likewise, floodwaters have spread waterhemp to northwestern Minnesota, where the weed had never been a problem before, Stachler says.
• Do you see a mix of uncontrolled weeds? If several weed species are present in the field 10-14 days after a glyphosate application, “you can be quite confident that it’s not a resistance problem,” Doug Holen says.
Rather, the weed escapes are due to other factors that affect glyphosate performance, such as incorrect rates for the weed size or species; improper timing; incomplete spray coverage or skips; weather-, tillage- or insect-stressed weeds; late weed flushes; or adverse weather during or after spraying.
To rule out glyphosate application factors, “You have to know the specifics at spraying time,” says Minnesota Crop Consultant Darrol Ike, Delano, Minn. “Were you rushed by rain? Was there an interfering wind? Was dust a problem?” Were weeds too large to be controlled? “You really need an understanding of the quality of the spraying job. Weed resistance may not be the issue.”
One of the most common application errors that Ike sees is the incorrect conversion of acid equivalent units to ounces-per-acre. “It’s where a lot of growers go wrong,” and often results in too low a rate for good control, he says.
• Did a single weed species escape control? “That’sa pretty good sign thatherbicide-resistant biotypes may be present,” Stachler says. If low-level resistance has developed, you will see dead plants next to fairly healthy plants of the same species about 14 days after spraying.
However, if you haven’t paid close attention to weed escapes for a couple of years, Michigan’s Sprague notes, “a lot of the susceptible plants may have been removed from the population” by the time you realize what’s happening.
• Is there a range of glyphosate injuries among the surviving weeds? A continuum of plant responses – from dead to almost normal – is the key tip-off that the weed population is overcoming glyphosate, Stachler says. The reason for this range of responses is genetic diversity within a resistant population.
For example, individual plants with stronger genetic resistance to glyphosate will usually show minor injuries. Leaf yellowing in young leaves is common. Plants may be stunted, with shortened internodes, but will have a healthy meristem.
Biotypes with weaker resistance will show more severe injuries. The main stem or growth point might be burned off, but the axillary nodes below can still produce healthy branches.
Even badly damaged weeds can go on to produce seeds. While these survivors probably won’t cut crop yields, they perpetuate the selection process and increase the probability of resistant traits, Stachler says.“If you see a spotless field with the exception of a handful of weeds or a patch – and you see a continuum of plant responses after two glyphosate applications – I can just about guarantee it’s resistant.”
The only way to confirm resistance is to collect weed seeds and do a greenhouse test. However, visual identification is actually quite reliable, he adds. “About 75-80% of weeds we suspected were resistant,” based on visual inspection, were in fact confirmed resistant in greenhouse tests, he says.
“Resistance starts with a single plant,” Stachler reminds growers. A weed like waterhemp or Palmer amaranth can produce over a million seeds per plant, packing the soil with resistant genetics. Unchecked, resistant plants can dominate the field in short order.
That has already happened in many southern states, Davis notes. “The good news for Upper Midwest farmers is that we still have the opportunity to change our practices before resistance gets away from us.”
What does glyphosate resistance look like?
•Single species surviving
•Dead plants next to living plants of the same species
•Range of glyphosate injury symptoms
•Patchy distribution, often in stressed areas or along field borders
Look for resistant weeds in the most likely places
You can’t walk every row of every field looking for signs of glyphosate-resistant weeds.
Extension weed scientist Jeff Stachler suggests focusing first on parts of the field that have high weed populations or stressful growing conditions. Stressful environments, like salt pockets, low spots that flood or droughty hilltops, intensify selection pressure on weeds, thereby increasing genetic diversity. “Areas of the field that allow for greater genetic diversity of weeds are where you’re likely to see resistance first.”
Fence lines and weedy field borders are another place where problem weeds like waterhemp thrive, says Minnesota crop consultant Darrol Ike, AgroInfoMN, Delano. Often, these areas don’t get a full dose of herbicide, either, further intensifying selection pressure. As you’re scouting, Ike adds, be alert to the stage of weed growth so you know how many days you have to eliminate weeds before they flower.
Glyphosate resistance complex
Different weeds have different genetic mechanisms for overcoming glyphosate herbicide, explains weed scientist Jeff Stachler.
Resistant horseweed (marestail), for example, has a gene that sequesters the herbicide in one part of the plant, allowing the reproductive parts of the plant to keep growing.
Resistant pigweeds have multiple copies of the gene that produces EPSPS, an essential enzyme that plants use to make amino acids. Glyphosate normally disrupts this enzyme.
These varying mechanisms show the complexity of glyphosate resistance, Stachler says.
1. Scouting and identifying weeds late in the season is just as important as scouting at the beginning of the season. Learn why in this short video with Vince Davis, University of Wisconsin weed scientist:
2. Learn the telltale signs of glyphosate resistant weeds in this short video with Jeff Stachler, weed scientist at North Dakota State University and the University of Minnesota: