Disease-causing bacteria isn't the only place where humans are fighting a losing battle against rapid evolution: Farmers across the country are increasingly finding it difficult to kill "super weeds" as they become resistant to the most popular herbicides.

When "Roundup ready" crops became popular in the mid 1990s, farmers were enamored with the genetically-modified seeds built to withstand glyphosate, the active ingredient in Roundup, the most popular commercial weed killer. But after years of constant exposure, certain invasive plants have also developed a resistance, leading farmers to use more of the chemical. In some cases, the weeds have grown completely tolerant to the chemical, giving farmers fits.

"I was talking to a farmer from Arkansas and he's got weeds that are now eight feet tall, they're the diameter of my wrist, and they can stop a combine in its tracks," says Gary Hirshberg, chairman of Just Label It, an organization fighting for mandatory labeling of genetically modified foods. "The only way they can stop them is to go in there with machetes and hack them out."

The problem is most pronounced in the southeast where farmers have been growing "Roundup ready," cotton for years, according to Charles Benbrook, a Washington State University researcher who studies herbicide use. Farmers who grow genetically modified crops use about 25 percent more herbicides than farmers who use traditional seeds, he says. It's no different than the overuse of antibiotics that has led to resistance from bugs such as gonorrhea in the past year.

"It's very much the same dynamic in play. By genetically altering corn, soybeans, and cotton so they can be sprayed throughout the growing season, farmers are now spraying their fields three times a season," he says. "You're exposing the weed populations to the same herbicide active ingredient three times a year."

Inevitably, some of the weeds are able to survive several courses of Roundup and pass their resistance on to the next generation, he says.

"It's getting to the point where some farmers have no effective herbicides left," Benbrook adds.

Says Hirshberg: "This is not a science fiction thing, this is happening right now. We're creating super weeds."

Farmers are turning to more traditional methods of weed management, such as using cover crops, crop rotations, and physically pulling weeds to lessen their reliance on Roundup. But those solutions are time consuming and expensive, and farmers are still using the Roundup Ready seeds, which can cost up to six times as much as traditional seeds, Benbrook says.

"The farmers' choice of seed dictates their choice of herbicide," he says. "The seed industry hasn't acted preemptively to produce a larger portion of [cheaper] seeds without the Roundup Ready gene in it so [farmers] can diversify."

Roundup became popular because crops treated with the chemical have been shown to be relatively safe for human consumption and, until weeds began showing resistance, was an effective herbicide. But as Roundup loses effectiveness, farmers are turning to 2,4-D, an older herbicide that some say is more dangerous to humans and has been shown to cause birth defects in animals exposed to the chemical. The Environmental Protection Agency has evaluated 2,4-D for possible cancer-causing effects several times and has been unable to come up with a conclusion. The chemical made up one half of the controversial Agent Orange herbicide used during the Vietnam War.

Though the problem is most pronounced among cotton farmers in the southeast, super weeds are starting to show up in fields in the Midwest as well. Experts say two specific weed varieties are proving to be especially hard to kill: Palmer Amaranth infests as much as two million acres of cotton and soybean fields in the southeast and horseweed, which can grow up to five feet tall.

"Both of them are very prolific and can release up to 400,000 seeds," Benbrook says. "Those seeds blow around and move through water — that's what's leading to this rapid geographic spread."

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