In the arms race between biology and biotechnology, the weeds win. Worse, says Kumar, 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 around, an even greater threat looms: There is mounting evidence that weeds can actually metabolize and break down herbicides before they do their job. In other words, Palmer amaranth may have developed resistance to weed killers yet to be invented. “This isn’t something I just made in a lab,” Kumar says, referring to the onset of herbicide resistance. “It’s all there in nature, it happens everywhere.”
Weeds always adapt to anything that tries to kill them. Lawnmowers exert evolutionary pressure on plants until they grow outward instead of up, staying close to the ground and avoiding the blade. Rice farmers who hand-weed their paddy fields jump over grasses that look like rice seedlings, allowing the imitators to reproduce — making the hand-weeding all the more difficult. But the speed and persistence with which herbicide-resistant weed populations have taken over U.S. farmland is largely an outgrowth of recent decades of industrial farming. Plants such as Palmer amaranth developed widespread resistance to Roundup because it was ubiquitous.
When Monsanto introduced Roundup in the mid-1970s, it worked better than any weed killer on the market, and it was dirt cheap too. “It was so good,” says Kumar. “Wherever you put it, it was so effective.” “Top control at a bottom price,” subsequent television ads would crow. “The herbicide that gets to the root of the problem.”
Two decades later, Roundup’s complement arrived, an innovation that pushed sales even higher: Roundup Ready seeds. The genetically modified plants that sprouted from it were able to survive one spray after another of the herbicide. This allowed farmers to easily plant Roundup Ready seeds, wait for the weeds to emerge, and then spray the entire field with Roundup. All but the valuable crop quickly withered and died. The development revolutionized weed control: farmers no longer had to buy a large number of expensive herbicides every year or till their land every season.
Monsanto first introduced Roundup Ready soybeans in 1996. Farmers rushed to adopt the paired products: In 2011, about 94 percent of all soybeans in the United States were planted with seeds designed to withstand herbicides, according to the Department of Agriculture’s Economic Research Service. Cotton and corn followed similar trajectories. Between 1990 and 2014, the use of glyphosate in the US increased more than 30 times. “It was just so cheap and effective that people used it for almost 20 years,” said Stephen Duke, a former researcher at the Department of Agriculture.
It turns out that Palmer amaranth was perfectly adapted to develop resistance and do so quickly. The plant is native to the Southwest and the leaves were once baked and eaten by people of the Cocopah and Pima tribes; the Navajo ground the seeds into flour. But as the hogweed spread eastward, the plants began to compete with cotton in the south, and in the mid-1990s it became a serious threat to crops.
While cash crops are virtually identical – farmers buy new genetically engineered seeds with the glyphosate tolerance trait every year – Palmer amaranth benefits from incredible genetic diversity. It mates sexually (mandatory outcrossing, in biology parlance), and female plants produce hundreds of thousands of seeds each year. The plants that sprouted with random mutations that inadvertently equip them to survive herbicide rains lived to reproduce with each other. When applications of Roundup destroyed all weeds in a field except the resistant Palmer amaranth, the hogweed was able to spread without competition. In one study, researchers planted a single Roundup-resistant Palmer amaranth plant in each of four fields of GM cotton. In three years, the weeds choked the cotton, and the crop failed.