AUBURN, Ala.–Evolution is turning modern weeds into monsters, as a phenomenon–– herbicide resistance–– makes control increasingly difficult for farmers. Dr. Steve Li, Alabama Extension weed scientist, called the development a “Pandora’s box” that cannot be closed. Li offered a nuanced but grim outlook on the subject.
“In the beginning, glyphosate killed almost everything out there. It’s cheap, and it doesn’t last long in the soil. It was almost the perfect herbicide,” he said. “Unfortunately, the good days didn’t last long.”
Glyphosate, the active ingredient in RoundUp™ and similar herbicides, was first released in 1974. Its utility in the world of agriculture took off in 1996 with the introduction of “RoundUp ready” crops, which withstood glyphosate. These crops were the first genetically modified organisms–– commonly referred to as GMOs–– on the market.
Now, farmers could grow crops resistant to the herbicide and spray their entire field with glyphosate. Weeds withered while their crops remained unscathed. It seemed too good to be true–– and it was.
“Some people in the field originally thought that no weeds would develop resistance to glyphosate,” Li said. “That didn’t last long, either.”
Eight years after the advent of glyphosate-resistant crops, extension agents at the University of Georgia first identified glyphosate resistance in pigweed. These hardier pigweeds spread to neighboring states, costing hundreds of million dollars in lost profit. Not only is pigweed a strong competitor against crops, it is also a prolific producer that can generate up to 100,000 seeds per plant.
Now, many pigweed populations are completely resistant to glyphosate. In 2016, a test of 58 locations in Alabama found only one field free from glyphosate-resistant pigweed, compared to 57 infected fields. Resistance to other herbicides, including PPO-inhibitors, is also on the rise. Other weeds, meanwhile, have acquired resistance to glyphosate, including horseweed, ragweed, and rye grass.
How did this happen?
Acquiring resistance is a long but simple task for a species. Even the most thorough herbicides can’t exterminate 100 percent of the population, and a tiny fraction always survives. The few weeds that made it through the glyphosate fallout of 1996 likely did so because of an oddball mutation that resisted the herbicidal mode of action. Those weeds reproduced with one another, and their offspring inherited the same mutation. Over the course of the following decade, the weeds with the gene for resistance had a monopoly on survival. It’s no surprise that this led to evolution in overdrive.
Li cautioned against blaming the problem entirely on genetically modified crops. “One of the myths is that this is a GMO problem,” he said. “We had weed resistance issues way before GMOs were released.” Indeed, the natural path of evolution indicates that glyphosate resistance would have increased no matter what. GMOs did not create the problem, Li explained–– they only accelerated it.
Li also blamed regulatory pressure on chemical companies for causing a lack of incentive to find solutions. “Let’s say there is a new chemical out there like glyphosate,” he said. “As of now, chemical companies don’t have enough incentives to discover and commercialize it. The way the patent law is, there’s not enough time to make a good profit.” Besides, he added, most of the easy targets for herbicides–– including enzyme inhibitors and hormone mimics–– are already on the market.
Of course, searching for a new chemical is a cat-and-mouse game. As with glyphosate, resistance will always develop over time.
What options for farmers?
Li offered some solutions to the epidemic, though most came with a qualifier. “You can use alternative strategies, but keep in mind, everybody does this to make money,” he said. “You don’t have an unlimited budget.”
Specialty and vegetable crop farmers may have the money to send their labor force out to hand-pull weeds. Because of low commodity prices, row crop farmers don’t have this option. Cover crops in the winter help, as can crop and herbicide rotation. “You just don’t have that many chemistries to rotate,” said Li, “and you don’t always have that many crops to rotate, either.”
Using residual herbicides, cover crops and preventing resistant weeds from producing seeds will alleviate weed resistance problems. However, it is unlikely these tactics will resolve the issue in a long run.
“It’s a very difficult question to answer. There’s no easy solution. If there were, we’d know it already.”
What has the impact been on Alabama?
Alabama Extension agricultural economist Max Runge emphasized that the monetary impact on Alabama has been negligible. “It has affected profit margins,” he said, “but we’ve been fortunate in Alabama that it’s not a tremendous issue so far for most farmers.
“However, some areas have more problems than others, and it’s not likely to go away.”
Though Li noted the limits of crop rotation, Runge said that it still gives Alabama an advantage over the Midwest. “We can rotate corn, soybeans, and cotton in the north, and throw in peanuts down south. You use different chemicals for different crops, and that helps prevent resistance,” he said. “In the Midwest, they’re just growing corn and soybeans.”
Rotating herbicides–– using glyphosate or glufosinate one year and a PPO-inhibitor based chemical the next, for example–– prevents any single resistant mutation from gaining dominance in the gene pool. Crop rotation makes this a realistic solution for the time being.
Runge also praised the extension program in Alabama for educating farmers about weed resistance. He said that farmers in less affected regions are diligent in stemming the flow of resistant weed seeds. “Farmers are careful to clean equipment brought in from out of state and wash off residual seeds,” Runge said. “Education is just another factor helping our state.”
Featured image by Joseph LaForest, University of Georgia, Bugwood.org
Close up of pigweed by Ross Recker, University of Wisconsin-Madison, Bugwood.org
Image of Li at field day by Maggie Lawrence, Alabama Extension