Attack of the Superweeds
Herbicides are losing the war — and agriculture might never be the same again.
Credit. Illustrations by Kelsey Dake
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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.
Vipan Kumar, a weed scientist from Kansas State University. Credit. Benjamin Rasmussen for The New York Times
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.
What is the Meaning of the Parable of the Weeds?
First century Jews lived in an agrarian culture, so it’s no wonder that a lot of Jesus’s teaching used the example of crops and farmers. In the Parable of the Sower, Jesus talks about how people respond to the gospel. In the Parable of the Tenants, He used the story of a vineyard to address the ways Israel had consistently worked against God.
In Matthew 13, Jesus tells another farm-related story:
Jesus told them another parable: “The kingdom of heaven is like a man who sowed good seed in his field. But while everyone was sleeping, his enemy came and sowed weeds among the wheat, and went away. When the wheat sprouted and formed heads, then the weeds also appeared.
The owner’s servants came to him and said, “Sir, didn’t you sow good seed in your field? Where then did the weeds come from?”
“An enemy did this,” he replied.
The servants asked him, “Do you want us to go and pull them up?”
“No,” he answered, “because while you are pulling the weeds, you may uproot the wheat with them. Let both grow together until the harvest. At that time I will tell the harvesters: First collect the weeds and tie them in bundles to be burned; then gather the wheat and bring it into my barn” (Matthew 13:24-30).
The specifics of this story would have made a lot more sense to Jesus’s original audience. When one farmer wanted to sabotage another, it wasn’t unheard of for them to sow bearded darnel into their wheat.
Bearded darnel is a noxious weed that mimics many of the characteristics of wheat—for a while. Before they mature, the two plants are almost identical, but as they grow, the differences become apparent in the fruit. Unfortunately, darnel is poisonous and in big enough doses will kill a person. So it’s not something a farmer wants mixed up in their harvest.
The concerned servants want to remove the darnel, but the farmer is afraid they’ll mistakenly throw out perfectly good wheat. He instructs them to leave the separation to the harvesters whose job it is to remove the darnel.
Interpreting the parable
After Jesus and the disciples leave the crowds, they ask Him to interpret the parable for them:
He answered, “The one who sowed the good seed is the Son of Man. The field is the world, and the good seed stands for the people of the kingdom. The weeds are the people of the evil one, and the enemy who sows them is the devil. The harvest is the end of the age, and the harvesters are angels.
“As the weeds are pulled up and burned in the fire, so it will be at the end of the age. The Son of Man will send out his angels, and they will weed out of his kingdom everything that causes sin and all who do evil. They will throw them into the blazing furnace, where there will be weeping and gnashing of teeth. Then the righteous will shine like the sun in the kingdom of their Father. Whoever has ears, let them hear” (Matthew 13:37–43).
When Jesus first introduces the parable, He says, “The kingdom of heaven is like. “
To really grasp this parable, it’s helpful to understand that Jesus is describing the kingdom of God. Jesus is sowing gospel seeds throughout the world and raising up Christians. But at the same time, the enemy is in the world spreading counterfeit seed. In its immature state, it isn’t always simple to discern the differences between those that belong to the kingdom and those who do not.
The servants want to help the farmer by uprooting the imposters, but they lack the sensitivity of the angelic harvesters. It’s not the job of the servants to make judgments about what is and isn’t actual wheat. Their job is to serve the farmer as He spreads the legitimate seed.
It seems that the main point of the parable is that unlike the disciples’ expectation, the kingdom of God wouldn’t be a restored Israel. It would be a borderless kingdom where the citizens might not immediately appear much different than those in the kingdom of man. Any attempt to separate the two could do damage to God’s kingdom.
The servants should assist Jesus in planting seeds and ensuring that they grow to maturity. At the end of the age, it’s the job of the harvester to judge who is or isn’t a member of God’s kingdom.
If you’re interested in learning more about Jesus’s teaching, check out All the Parables of Jesus for a bird’s-eye view of His story-like lessons.