Some orchard and vineyard weeds are resistant.
Horseweed, also known as marestail, has been showing resistance to glyphosate in California, Oregon, and Washington. Pictured left to right: in bloom, as a young stalk, and as a rosette.
photos Courtesy of Tim Miller, Washington State University
Many scientists said weeds could never develop resistance to glyphosate, but in the late 1990s they were proven wrong.
“As weed scientists, we were flabbergasted,” Dr. Bradley Hanson, extension weed specialist with the University of California, Davis, recalled during a weed management seminar in Wenatchee, Washington, this winter.
Resistance to glyphosate was thought unlikely because of the herbicide’s unique mode of action and behavior in plants. But there are now at least 13 weed species in the United States that have evolved resistance to glyphosate. Horseweed, also known as marestail (Conyza canadensis), is one orchard and vineyard weed that has been showing resistance to glyphosate in California, Oregon, and now Washington. Some California populations of a related weed, hairy fleabane (Conyza bonariensis), are resistant to both glyphosate and paraquat.
What happened? Two things, Hanson says. Roundup-Ready soybeans, introduced in 1996, soon accounted for 90 percent of the country’s 60 million acres of soybean plantings. Then came other Roundup-Ready crops, such as corn, cotton, alfalfa, and sugar beets, which are also grown on millions of acres. Roundup-Ready crops are genetically modified so that the herbicide’s target site in the crop plant is unaffected while the weeds are vulnerable. While the resistant crops do not directly cause resistance in weeds, they create an opportunity for in-crop use of a formerly nonselective herbicide, which dramatically increases selection pressure for resistant biotypes.
The other factor was that glyphosate became much cheaper after the Roundup patent expired in 2000, and many generic formulations came onto the market. That led to a tremendous increase in use of the product. Glyphosate cost $100 a gallon in the 1970s, compared with $50 in 2008. Today, growers can buy it for $15 a gallon or even less, Hanson said.
About 16 million pounds of glyphosate are used annually in California, and glyphosate accounts for 40 percent of all herbicide active ingredients used. The situation is probably similar in Washington and Oregon.
Resistance develops as a result of slight genetic mutations in weeds that can make them unaffected by the herbicide. These mutations occur naturally and are not caused by herbicides, Hanson said. Occasionally, one of these mutations enables a weed to survive exposure to the herbicide and continue to reproduce while susceptible weeds die. When the herbicide continues to be applied, populations of these resistant plants increase. These are weeds that used to be controlled but no longer are, even at higher herbicide rates.
There are two types of resistance: target-site and nontarget-site. Herbicides usually affect plants by disrupting the activity of an enzyme that plays a key role in some biochemical process in the plants. Target-site resistance occurs when the enzyme becomes less sensitive to the herbicide, usually because of a mutation in the gene coding for the protein. Nontarget-site resistance develops without involving the active site of the herbicide in the plant. There are several ways this can happen. A common type of nontarget-site resistance develops when the plant becomes better able to metabolically degrade the herbicide or move it away from the target site.
In the United States, about 125 weeds have developed resistance to 15 herbicide families. Some types of herbicides are more prone to resistance than others.
Resistance has been reported to triazine herbicides, which are Photosystem II inhibitors, Hanson said. These were introduced in the late 1960s and were widely used in the early 1970s. Growers switched to ALS inhibitors, which were introduced in the 1980s, but resistance was already seen by the 1990s. This is now one of the most common classes of herbicides facing resistance.
Resistance to protoporphyrinogen oxidase (PPO) inhibitors, which are widely used in tree fruits and grapes, is starting to show up, Hanson said. Products with this mode of action include Goal (oxyfluorfen), Aim (carfentrazone), Treevix (saflufenacil), Kixor, and Chateau (flumioxazin).
Resistance to glycines, including glyphosate, is also causing concern, although it is still relatively minor compared with resistance to other herbicide classes. In Oregon, Italian ryegrass has shown some resistance to Rely (glufosinate).
“That’s trouble brewing,” Hanson said. “That’s something we’re keeping an eye on.”
Practices that lead to resistance include not rotating crops, not using tillage, having a weakly competitive crop, and not using herbicides with different modes of action in rotation, Hanson said.
“For example, maybe I plant trees, don’t use tillage, and only use Roundup. That would be a bad way to manage resistance,” he said. On the other hand, a complex rotation utilizing tillage, hand weeding, and use of multiple herbicide modes of action will minimize selection of resistant biotypes.
Since growers of perennial crops, such as tree fruits and grapes, can’t easily rotate crops or till the ground, herbicide rotations or tank mixes of herbicides with different modes of action are the best option.
The weeds most likely to develop resistance are annuals that produce a lot of seeds and have little seed dormancy but some seed longevity, so that the ones that don’t germinate right away can persist for a while. The worst weeds develop through two or three generations per year.
The types of herbicides most likely to lose effectiveness because of resistance are those that have a single mode of action, are highly effective, are used frequently and at high rates, and have a long residual life. The more individuals that are selected with the herbicide, the greater the chances of finding resistant mutants, Hanson said. “It boils down to a numbers game.”
Resistance management is based on reducing selection pressure by rotating herbicides with different modes of action—not just different active ingredients or families of herbicides, he stressed.
Tank mixes help as long as the herbicides target the same weeds. Applying a herbicide that targets grasses with one that targets broadleaf weeds is not managing resistance, but managing the weed spectrum, Hanson said.
Keep good records of what you have used and where you’ve seen failures, he advised. Not every weed control failure is due to resistance, but if healthy plants are intermixed with dying plants of the same species, it’s a strong sign of resistance. A patch of uncontrolled weeds that is spreading from year to year can also be a sign of resistance. Monitor your orchard and control escapes before they become large problems, he suggested.
For more information, download the publication “Selecting Pressure, Shifting Populations, and Herbicide Resistance and Tolerance” from www.ipm.ucdavis.edu/PDF/PUBS/hanson-herbicideresistance.pdf.