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How Superweeds Like Palmer Amaranth Are Changing Agriculture

How Superweeds Like Palmer Amaranth Are Changing Agriculture

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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.

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