Justin Cariker is battling a pigweed invasion on his Dundee (Miss.) farm Maude Clay for Bloomberg Businessweek

Justin Cariker grabs a 7-foot-tall Palmer pigweed at his farm, bending the wrist-thick stem to reveal how it has overwhelmed the Cotton plant beneath it. This is no ordinary weed: Over time it has developed resistance to Monsanto’s best-selling herbicide, Roundup. Hundreds of such “superweeds” are rising defiantly across this corner of the Mississippi Delta. “We’re not winning the battle,” Cariker, owner of Maud Farms in Dundee, Miss., says as he looks at weeds that tower over his infested cotton field like spindly green scarecrows.

Cariker’s superweeds represent a growing problem for Monsanto, whose $11 billion of annual sales are anchored in Crops genetically engineered to tolerate Roundup, the world’s best-selling weedkiller. The use of Roundup Ready seeds has transformed farming in the 15 years since their debut, allowing growers to easily dispatch hundreds of types of weeds with a single herbicide while leaving crops unscathed. “When the Roundup system first came out, to a farmer this was the best thing that ever happened,” says Cariker, who used the labor-saving technology to double his planted acreage, to 5,000. “Farmers thought we had died and gone to heaven.”

Not exactly: It turns out the widespread use of Roundup has led to the evolution of far-tougher-to-eradicate strains of weeds. As a result, rivals such as Dow Chemical, DuPont, Syngenta, and Bayer see an opportunity. They hope to revive sales of older herbicides still able to kill many Roundup-resistant weeds, allowing them to challenge Monsanto’s dominance in genetically modified crops. Still, the substitutes could eventually create weeds that survive multiple chemicals, just as increased use of antibiotics in pigs and chickens has led to the evolution of bacteria that resist multiple drugs, says Charles Benbrook, chief scientist at the Organic Center in Troy, Ore. “It’s akin to putting gasoline on a fire to put it out,” he says. “It’s a very high-risk gamble for the U.S. biotechnology and pesticide industry to go down this road.”

Crops created to survive Roundup or generic glyphosate (its active ingredient) now comprise 94 percent of soybeans, 73 percent of cotton, and 72 percent of corn grown in the U.S. Glyphosate is applied at seven times the rate of all other herbicides combined in U.S. soybean fields and 1.6 times the rate of all others in cotton fields, according to agriculture consultant Cropnosis.

Dow Chemical, DuPont, Syngenta, and Bayer are engineering crops to withstand alternative herbicides that can kill the weeds Roundup no longer can. Dow expects to begin collecting $1.5 billion in additional profit in 2013 by selling seeds for crops that tolerate a reformulated version of 2,4-D, a herbicide the U.S. first registered for sale in 1948 and one of the chemicals used in the Vietnam War-era defoliant Agent Orange.

Unrelenting Roundup use has caused 11 weed species to evolve glyphosate resistance in 26 U.S. states, with Palmer pigweed and horseweed the most widespread, according to the International Survey of Herbicide Resistant Weeds. They have invaded 14 million U.S. acres of cotton, soybean, and corn, and that will double by 2015, says Chuck Foresman, Syngenta’s head of corn crop protection. A Dow study this year found as many as 20 million acres of corn and soybeans may already be infested.

Monsanto Chief Executive Officer Hugh Grant says competitors’ efforts to develop their own herbicide-tolerant crops isn’t a threat to the company’s flagship business. Seed companies will cross-license each others’ genetics to create crops able to withstand multiple weedkillers, he says, and spraying fields with a mix of herbicides will kill the superweeds and give Roundup Ready Crops new life. Monsanto itself is adding resistance to dicamba, an older weedkiller, to Roundup Ready crops for sale by 2015. “The cavalry is coming,” Grant says.

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