The recent finding of a new mechanism by which Palmer amaranth pigweed develops resistance to or tolerance of glyphosate is big news in the scientific community, but far from good news for farmers in the Southeast.
Working with glyphosate resistant pigweed seed from Georgia, researchers at Colorado State University have determined that pigweed have the ability to mass copy the gene that glyphosate attaches to and uses to block a key passageway. Long story short, glyphosate shuts down the plant and rapidly kills it — in non-resistant pigweed.
Veteran Weed Scientist Phil Westra, a professor at Colorado State University says, “Technically what happens is the molecular target of glyphosate in pigweed is the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS, EC 188.8.131.52). This enzyme is a component of the shikimate pathway. By attaching to the (EPSPS, EC 184.108.40.206) target enzyme , glyphosate is able to shut down the shikimate pathway, rapidly breaking down the plant and killing it.
“The shikimate pathway links metabolism of carbohydrates to biosynthesis of aromatic compounds. The penultimate enzyme of the pathway is the sole target for glyphosate.
“A typical plant will have two copies of the gene. In a glyphosate resistant plant we have found up to 160 copies of the gene that produces the enzyme to which glyphosate attaches,” Westra explains.
The pigweed plant pumps out these gene copies, which in effect dilutes glyphosate to the point of being ineffective in controlling these weed pests. “When glyphosate is sprayed on one of these resistant plants, some of the enzyme is tied up, but plenty more is around and not affected by the herbicide,” Westra explains.
This occurrence of gene amplification as a herbicide resistance mechanism in a naturally occurring weed population is particularly significant because it could threaten the sustainable use of glyphosate-resistant crop technology, Westra contends.
The molecular trait in pigweed is passed along via pollen. Pollen can easily move it 300-400 feet in a cropping season. Westra says this explains why growers usually see a pigweed or two sticking up in a field one year and the next year have an oval-shaped clump of weeds that may be 20-30 feet across.
This latest discovery of a new mechanism by which herbicide resistance develops adds a new chapter to the overall story of resistance problems that continue to grow for farmers worldwide. Previously, three primary mechanisms were known to produce herbicide resistance in weeds and target plants.
The most commonly occurring mechanism is via an altered target site in which there is mutation for a certain enzyme in a gene. This is the classic mechanism for plant resistance to ALS inhibitors and other urea-based herbicides.
Altered translocation pattern is another less common mechanism. When this occurs in a weed, horseweed is a good example, the herbicide is re-directed to the leaf edges or other areas of the plant where it can do little damage. Growers in the Southeast are becoming all to familiar with glyphosate resistant horse weed.
A third mechanism, common in other parts of the world, but not known in the U.S., occurs when a plant chemically breaks down the herbicide and renders it inactive.
“I don’t think we have the final answer on Palmer amaranth resistance. The consensus is that there are two or three mechanisms for resistance. The tests we ran came from seed from Stanley Culpepper in Georgia (weed scientist at the University of Georgia) and these have a high level of resistance. Ken Smith in Arkansas (professor and weed scientist at the University of Arkansas) and Alan York in North Carolina (weed scientist and professor emeritus at N.C. State University) have indicated some of their pigweed may not have the same level of resistance,” Westra says.
This new finding accentuates the need for farmers to have a good herbicide resistance avoidance and/or management plan for all crops. Though glyphosate resistance is the biggest newsmaker, it’s far from the only herbicide resistance problem faced by farmers.
Knowing the active ingredient and/or mode of action in all herbicides is a basic ingredient of a good crop production plan. Managing crop rotations and rotating herbicide families used in each crop is already common among most successful farmers in the Southeast. The latest scientific revelation as to how glyphosate resistance develops is another good reason to intensify crop and herbicide rotation strategies.
One beneficial side effect of the Colorado State findings is the development of a rapid assay test to determine whether a pigweed plant is glyphosate resistant. Building on an earlier assay for the altered target site mechanism, USDA researcher Dale Shaner has developed a similar test to detect the relative sensitivity of a weed to glyphosate. .
Until recently, the best scientific option for determining glyphosate resistance was to grow seed in a greenhouse, then spray them with glyphosate and see which ones survive. By the time scientific evidence is in, too many farmers are past the point of no return with resistant pigweed.
The best way to determine glyphosate resistance quickly is common sense. If you know you sprayed a weed with a high enough rate of glyphosate to kill it and it didn’t die, it’s likely resistant to the herbicide, or tolerant of it in a technical sense.
Shaner’s new assay test in non-invasive, can be used on a small leaf sample and generates results in a couple of day.
Though Southeast cotton and soybean farmers may think they have been singled out by Mother Nature and plagued with weeds resistant to herbicides, they are mistaken. Farmers in other parts of the country share their plight — slightly different melody and verse, but same dire consequences.
In southern Illinois, pigweeds are resistant to ALS inhibitors, glyphosate, PPO inhibitors and triazines. Four distinct modes of action and water hemp is resistant to all four in 23 counties in southern Illinois.
“In Missouri we only have ‘triple-stacked’ weeds, quips University of Missouri Weed Scientist Kevin Bradley. Bradley says the occurrence of resistance to glyphosate, PPO-inhibitor, and ALS-inhibitor herbicides in one weed is still rare throughout the Midwest. However, the implications are dire for farmers, if this level of resistance spreads rapidly through soybean-producing areas of the country, he adds.
The take home message from this latest glyphosate resistance finding by Westra, Shaner and other researchers working at Colorado State University is that the Palmer pigweed/glyphosate resistance story is not going away. Not only is it not going away, it is likely to increase in severity and at a more rapid pace, unless growers take some well thought out actions to manage glyphosate resistant pigweed.
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