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How Super Herbs Like Palmer Amaranth Are Changing Agriculture


In the arms race between biology and biotechnology, weeds are winning. Worse still, Kumar says, growers cling to the unrealistic idea that chemical companies will invent a miraculous new herbicide before it’s too late. Even with such a miracle crop close at hand, an even greater threat looms: There is growing evidence that weeds can actually metabolize herbicides and break them down before they can do their job. In other words, Palmer amaranth may have developed resistance to weed killers that haven’t been invented yet. “This is not something I just created in a lab,” Kumar says, referring to the onset of herbicide resistance. “There is everything in nature, it happens everywhere.”

Weeds always adapt whatever is trying to kill them. Lawnmowers put evolutionary pressure on plants until they grow outward rather than upwards, staying close to the ground and avoiding the blade. By weeding their paddy by hand, rice farmers bypass weeds that look like rice seedlings, allowing copycats to breed and making manual weeding even more difficult. Still, the speed and persistence of herbicide-resistant weed populations taking over American farmland is a result of the last few decades of industrial agriculture. Plants such as Palmer amaranth have developed widespread resistance to precisely Roundup. because was everywhere.

When Monsanto introduced Roundup in the mid-1970s, it worked better than any other weed killer on the market and was also very inexpensive. “It was very good,” Kumar says. “Wherever you put it, it was very effective.” “Best control at lowest price” sang the television commercials that followed. “The herbicide that gets to the root of the problem.”

Twenty years later, an innovation that drove sales even higher came with Roundup’s complement: Roundup Ready seeds. Genetically modified plants that sprout from them can survive spraying after spraying the herbicide. This allowed farmers to simply plant Roundup Ready seeds, wait until weeds appeared, and then spray the entire field with Roundup. Everything but the valuable crop quickly withered and died. This development revolutionized weed control: Farmers no longer had to purchase large numbers of expensive herbicides on their land each year or every season.

Monsanto first introduced Roundup Ready soybeans in 1996. Farmers rushed to adopt paired crops: by 2011, according to the Department of Agriculture’s Economic Research Service, about 94 percent of all soybean acres in the United States are planted with seeds designed to resist herbicides. Cotton and corn followed a similar path. Between 1990 and 2014, the volume of US glyphosate use increased more than 30 times. “It was so cheap and effective that all humans have been using it for almost 20 years,” says Stephen Duke, a former researcher in the Department of Agriculture.

It turns out that Palmer amaranth is perfectly adapted to improve resistance and do it quickly. The plant is native to the southwest and its leaves were once cooked and eaten by people between the Cocopah and Pima tribes; The Navajo turn seeds into food. But as hogweed spread eastward, plants began to compete with cotton in the South and emerged as a serious threat to crops by the mid-1990s.

While the cash crops are pretty much the same – farmers purchase new genetically engineered seeds containing the glyphosate tolerance trait each year – Palmer amaranth benefits from incredible genetic diversity. They mate sexually (which makes crossing necessary in biology), and female plants produce hundreds of thousands of seeds each year. Plants that sprouted through random mutations and inadvertently equipped them to survive a rain of herbicides lived to reproduce with each other. Later, hogweed could spread without competition when applications of Roundup had destroyed all weeds in a field except the resistant Palmer amaranth. In one study, researchers planted a single Roundup-resistant Palmer amaranth plant in each of four fields of genetically modified cotton. Within three years, weeds suffocated the cotton and the crop failed.



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