Weeds are winning in the war against herbicide resistance

For farmers, protecting fields from pests and plagues is a constant battle fought on multiple fronts. Many insects have a taste for the same plants humans do, and pathogenic microbes infect leaves, shoots and roots. Then there are the weeds that compete with crops for soil and sun.


Although academics and companies are looking for technical alternatives such as sprays made from biological compounds, a recent review by researchers at North Carolina State University cautions that society may not be able to science its way out of this thorny problem. There is a “considerable chance,” the authors write, “that the evolution of pest resistance will outpace human innovation.” Addressing the situation requires a collective effort between funding agencies, regulators, farmers and others, the authors add in the review, published in Science. “We need to approach things from more than a single technical fix,” says co-author Jennifer Kuzma, co-director of the Genetic Engineering and Society Center at NC State. While regulatory action seems unlikely to happen anytime soon at the federal level, several efforts are underway to figure out how to tackle the problem.

Herbicide resistance dates at least as far back as the 1950s. But the idea that weed control is a collective problem requiring collective action emerged relatively recently, says George Frisvold, an agricultural and resource economist at the University of Arizona, who was not involved in the new review. “People thought that weeds aren’t as mobile as insect pests,” Frisvold says. “But then more and more research came out to suggest that even if they aren’t as mobile, they’re still mobile enough” to spread resistance.

Herbicide resistance can work in many different ways; take glyphosate-resistant Palmer amaranth (Amaranthus palmeri), or pigweed, for example. Glyphosate works by targeting a key protein in plants. Most plants have two copies of the gene that makes this protein. But resistant pigweed has evolved to have between five and 160 copies, which means it can make more of the protein and is thus simply harder to kill. To make things worse a single pigweed plant can produce half a million seeds, each of which has the genetic machinery to sprout into more hardy weeds. In some states including Minnesota and Arkansas, herbicide-resistant pigweed is so persistent that farmers have resorted to hiring laborers to handpick or blowtorch the weeds.

Just last year Frisvold and Adam Davis, a research ecologist at the United States Department of Agriculture’s Agricultural Research Service, wrote a paper suggesting that herbicides were only ever a temporary solution—and, possibly, the sort of technology that comes along once every century or so. Rather than assuming effective new herbicides will be developed, Frisvold and Davis argued for taking a longer view of weed control that includes multiple approaches at once—like crop rotation and curbing the spread of weed seeds. “We sounded the alarm pretty strongly,” Davis said.

The review from the NC State team suggests that government regulation and subsidies—like Environmental Protection Agency regulations already in place to curb insecticide resistance, and federal subsidies that encourage crop rotation—could address herbicide resistance. But both would require political buy-in, and that seems unlikely given the current deregulation trends at the EPA and bogged-down negotiations on the Farm Bill.

Another option could be a push for federal funding to support massive agricultural studies—for example, observing resistance over thousands of acres to see how it spreads across farms and landscapes. “If the funding agencies put more money to study the problems when the regulatory agencies are more conducive to taking action, we’d have the evidence to back up which action is appropriate,” Kuzma says.

Davis and Frisvold are part of an ongoing collaboration between the USDA and academia on herbicide resistance that stretches across 15 states in the South, mid-Atlantic and upper Midwest—one of the first efforts to explore weed control on such a large scale. Part of the project focuses on an Australian machine called the Harrington Seed Destructor, which can be towed behind a combine to catch and crush weed seeds. The scientists are testing the machine on a range of species, including pigweed. They are also examining how the machine could integrate with other techniques including herbicides and crop rotation, since weeds will evolve to resist any single approach.

Farmers and their communities will also have to create some of their own rules regarding herbicide use, the NC State team says. Work is needed to explore how farmers perceive the risks associated with pesticide resistance; the answers could help tailor ideas in a way that would make farming communities more open to trying them. “These problems are actually being generated within the agricultural community” and by chemical companies, says Carol Mallory-Smith, a weed scientist at Oregon State University, who did not work on the review. “There should be a responsibility within the agricultural community and within those private industries to make some of these good management decisions, without having to be incentivized to do them.”

There are several new and existing farmer networks trying to tackle the issue at the local and regional levels. For example, the IDEA Farm Network (an Illinois-based collaboration of farmers, scientists, nongovernment organizations, companies and consumers) has regular meetings to talk about farming—including strategies to lower herbicide use, such as planting cover crops. The Iowa Pest Resistance Management Program has a new project dedicated to herbicide resistance that will include a network of farmers, landowners, companies, scientists and others. And in Arkansas a community-based program called Zero Tolerance, launched several years ago, has been spreading tips for controlling pigweed. These include identifying local leaders in the farming community to get growers on board and to help them monitor one another to make sure they are using best practices.

Still, despite these steps, resistance will continue to be a problem. And the biggest challenge may be getting a critical mass of farmers to adopt whatever practices the research ultimately suggests are needed. “Some growers are still holding out for that silver bullet,” Frisvold says, “that technology will bail them out.”