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Attack of the Superweeds

Herbicides are losing the war — and agriculture might never be the same again.

Credit. Illustrations by Kelsey Dake

By H. Claire Brown

  • Aug. 18, 2021

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It was already too late for Darren Nicolet to reverse course last June when he heard that the Ninth Circuit Court of Appeals had overturned E.P.A. approval of three products containing dicamba, a controversial but widely used weed killer. A farmer in Kansas, Nicolet had planned his season around the herbicide, planting his fields with soybeans that were genetically modified to survive being showered with the chemical. He was well aware of dicamba’s tendency to vaporize and drift from field to field, causing damage to crops and threatening nearby wildlife and trees, but he didn’t feel as if he had much of a choice: Dicamba was one of the last tools that provided some control over Palmer amaranth, an aggressive weed that would quickly go on to choke out his sorghum crop — and that threatened to overtake his soybeans too. “There was a little bit of a moment of panic there for a few hours,” Nicolet said; he was worried that a season without dicamba would mean devastation for his farm.

If there’s a plant perfectly suited to outcompete the farmers, researchers and chemical companies that collectively define industrial American agriculture, it’s Palmer amaranth. This pigweed (a catchall term that includes some plants in the amaranth family) can re-root itself after being yanked from the ground. It can grow three inches a day. And it has evolved resistance to many of the most common weed killers, continuing to reproduce in what ought to be the worst of circumstances: A three-day-old, herbicide-injured seedling, for example, can expend its last bit of energy to produce seeds before it withers up and dies. Unchecked, Palmer amaranth can suppress soybean yields by nearly 80 percent and corn yields by about 90 percent.

Nicolet was ultimately allowed to spray dicamba last summer because he purchased it before restrictions took effect. He used it this year too: The Trump administration issued new approvals for some formulations containing dicamba just a week before the presidential election. Still, Nicolet says the weed killer will eventually stop working on his land, another management tool rendered useless by the pigweed’s remarkable onslaught. Whether that day is 10 years in the future or three, he has no idea, but the Palmer amaranth continues to gain ground all the while. This summer, a handful of pigweeds sprouted in a field that had recently been sprayed. Nicolet couldn’t weed the 96 affected acres by hand, so he decided to let them grow. “It’s not really enough to hurt yield this year,” he said. “But you know, you have 100 weeds out there, the next year you’ll have a million.”

When I visited Nicolet’s farm, south of Great Bend, in July 2019, his soybeans were just a few weeks old. Already, Palmer amaranth plants outnumbered them, their narrow, rough-edged leaves intermittently flecked with purple, filling spaces between the rows and wedging themselves between the young soybean plants. I was tagging along with Vipan Kumar, a weed scientist from Kansas State University, and two of his graduate students as they checked in with local growers. Earlier, in the squat concrete greenhouse that functions as his research laboratory, Kumar held in front of me an arm-length seed pod on a top-heavy Palmer amaranth plant, one of many among rows and rows of tall, thin seedlings that shot up from small plastic trays designed to support just a few inches of growth, arcing past light fixtures and fans as they stretched toward the window-paneled ceiling. The plant in his hands was a Palmer amaranth descendant that had demonstrated resistance to 2,4-D, one of two active ingredients in compounds used to defoliate forests during the Vietnam War. Though this development was certainly bad news for farmers, Kumar seemed unable to suppress his awe for the plant’s evolutionary capabilities. “I was excited to see it,” he said, referring to the moment his team discovered the new resistance.

At the farm, Nicolet, clad in faded jeans and a sweat-soaked baseball cap, bent down at a spot where the weeds looked brown and wilted. This field had been sprayed between plantings with an herbicide containing paraquat. A single sip of paraquat can kill a person, and the chemical was implicated in a string of random vending-machine poisonings carried out in Japan in 1985 that killed at least 10 people. “It’s something I don’t want to be around and something that I really don’t want to use,” Nicolet said. But sometimes the pigweed can appear to resurrect itself even after an application of paraquat: Nicolet recalled a recent instance in which the weeds appeared to die, only to start growing again a few days later. “I don’t know what the future will hold,” he said on a phone call later. “If things get worse enough, are we just going to be looking at basically just Palmer everywhere?”

Superweeds — that is, weeds that have evolved characteristics that make them more difficult to control as a result of repeatedly using the same management tactic — are rapidly overtaking American commodity farms, and Palmer amaranth is their king. Scientists have identified a population of Palmer amaranth that can tolerate being sprayed with six different herbicides (though not all at once), and they continue to discover new resistances. By now, it’s clear that weeds are evolving faster than companies are developing new weed killers: Just six years ago, in response to the onset of resistance to its marquee product, Roundup (active ingredient: glyphosate), Monsanto began selling a new generation of genetically modified seeds bred to resist both glyphosate and dicamba. By 2020, scientists had confirmed the existence of dicamba-resistant Palmer amaranth. The agribusiness giant took a decade to develop that product line. The weeds caught up in five years.

For a generation, Roundup worked as a one-size-fits-all approach to controlling weeds. But as resistant weeds spread, no better chemical was brought forward to succeed it. Instead, Monsanto placed its bets on an older weed killer, dicamba, that had problems with drifting. Glyphosate, too, has fallen out of favor outside U.S. farming circles because of its possible links to cancer, and Bayer, the company that acquired Monsanto in 2018, announced in July that it would phase the chemical out of U.S. lawn and garden products to avoid future lawsuits after committing up to $9.6 billion to settle about 125,000 claims that the product caused non-Hodgkin’s lymphoma among users. (Bayer stresses that this change is unrelated to safety considerations.) Glyphosate’s use remains ubiquitous among growers, however. Even though it doesn’t work on all weeds anymore, the alternative — adopting a more integrated approach to weed control — would mean totally rethinking their operations.

A January paper on a Palmer amaranth population shown to resist multiple weed killers put the problem succinctly: “Weed resistance to herbicides, especially multiple-herbicide resistance, poses a serious threat to global food production.” (Herbicide-resistant weeds are generally less of a concern on organic farms, but these make up less than 1 percent of total U.S. acreage.) It’s hard to estimate exactly how much damage has already been wrought by herbicide resistance; the weeds are gaining ground faster than scientists can survey them. But research published in 2016 by the Weed Science Society of America found that uncontrolled weeds could cause tens of billions of dollars of crop losses every year. Bob Hartzler, a retired weed scientist at Iowa State University, estimates that the tipping point when weed killers cease to be effective on some problematic species, including Palmer amaranth, is just five to 10 years away. “There’s general consensus among most weed scientists that the problems we see are just going to continue to accelerate,” he says. “And that’s why we’re sort of pessimistic that we can continue this herbicide-only system.”

In the arms race between biology and biotechnology, the weeds are winning. Worse, Kumar says, growers are clinging to the unrealistic idea that chemical companies will invent a miraculous new herbicide before it’s too late. Even if such a miracle product were close at hand, an even greater threat looms large: Evidence is mounting that weeds can actually metabolize herbicides, breaking them down before they do their work. In other words, Palmer amaranth may have evolved resistance to weed killers that have yet to be invented. “This is not something I just created in a lab,” Kumar says, referring to the onset of herbicide resistance. “It’s all there in nature, happening all over.”

Weeds always adapt to whatever’s trying to kill them. Lawn mowers exert evolutionary pressure on plants until they’re growing outward instead of upward, keeping close to the ground and avoiding the blade. Rice farmers weeding their paddies by hand skip over grasses that look like rice seedlings, allowing the imitators to reproduce — and making hand-weeding all the more difficult. Yet the speed and persistence with which herbicide-resistant weed populations have taken over American farmland is very much an outgrowth of the last few decades of industrial agriculture. Plants like Palmer amaranth evolved widespread resistance to Roundup precisely because it was ubiquitous.

When Monsanto introduced Roundup in the mid-1970s, it worked better than any other weed killer on the market, and it was dirt cheap as well. “It was so good,” Kumar says. “Wherever you put it, it was so effective.” “Top control at a rock-bottom price,” subsequent television ads would crow. “The herbicide that gets to the root of the problem.”

Two decades later, Roundup’s complement, an innovation that caused sales to surge even higher, arrived: Roundup Ready seeds. The genetically modified plants that sprouted from them could survive spray after spray of the herbicide. This enabled farmers to simply plant Roundup Ready seeds, wait until the weeds emerged, then spray the entire field with Roundup. Everything but the valuable crop quickly wilted and died. The development revolutionized weed control: Farmers no longer needed to buy a vast array of expensive herbicides each year or till their land every season.

Monsanto first rolled out Roundup Ready soybeans in 1996. Farmers rushed to adopt the paired products: By 2011, according to the Department of Agriculture’s Economic Research Service, about 94 percent of all soybean acres in the United States were planted with seeds engineered to resist herbicides. Cotton and corn followed similar trajectories. Between 1990 and 2014, the volume of U.S. glyphosate use increased more than 30-fold. “It was just so cheap and effective that that’s all people used for almost 20 years,” says Stephen Duke, a former researcher at the Department of Agriculture.

It turns out that Palmer amaranth was perfectly adapted to evolve resistance and to do so quickly. The plant is native to the Southwest, and its leaves were once baked and eaten by people among the Cocopah and Pima tribes; the Navajo ground the seeds into meal. But as the pigweed spread eastward, the plants began competing with cotton in the South, emerging as a serious threat to the crops by the mid-1990s.

Whereas cash crops are virtually identical — farmers purchase new genetically engineered seeds containing the glyphosate-tolerance trait every year — Palmer amaranth benefits from incredible genetic diversity. It mates sexually (obligate outcrossing, in biology-speak), and female plants produce hundreds of thousands of seeds each year. The plants that sprouted with random mutations that inadvertently equipped them to survive showers of herbicide lived to reproduce with one another. Then, once applications of Roundup annihilated all the weeds in a field except the resistant Palmer amaranth, the pigweed could spread without competition. In one study, researchers planted a single Roundup-resistant Palmer amaranth plant in each of four fields of genetically modified cotton. In three years, the weeds choked out the cotton, and the crop failed.

Watch what you plant: Some native seed mixes contaminated with invasive weed

The Minnesota Department of Agriculture wants landowners to pay more attention to what seeds they plant this spring.

Native seed mixes contaminated with the invasive Palmer amaranth are a big concern for the agency.

"The reason that we're so focused on Palmer amaranth is because of its potential to impact corn and soybean production," said Denise Thiede, a program supervisor in the plant protection division.

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The invasive plant first sprouted in Minnesota in 2016 in a field planted with a native seed mix. It's now found in four counties, where efforts have been undertaken to eradicate the plant before it can get established.

The plant spreads aggressively and is often herbicide resistant, so it can take over fields and reduce corn and soybean yields.

Seed mixes that contain native plants for use in conservation or pollinator habitat often come from a variety of sources and the Department of Agriculture has yet to trace the source of Palmer amaranth in the contaminated seed mixes.

"What I have seen in these investigations is that land owners are really unaware of the labeling law, and in some cases they have allowed unlabeled seed to be planted on their property," said Thiede.

Landowners should always ask a seed vendor for label information before buying seed. Likewise, Thiede said, landowners should make sure any seeding contractors use properly labeled seeds and never buy seed without a label.

"Landowners are the first line of defense, and they have information at their fingertips to help prevent this contamination."

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