Farmers in the South started noticing the problem before anyone else. When they sprayed their fields with Roundup weed killer, weeds kept growing anyway. In some areas, fields became so choked with weeds that farmers abandoned them.

Midwestern farmers have been watching the troubles in the South. Roundup, or its ingredient, glyphosate, is used with crops genetically modified to withstand the herbicide and has become the most ubiquitous product in American farming. It has meant less pesticide use. Less environmentally damaging tillage. And it has helped catapult Creve Coeur-based Monsanto, the developer of the Roundup Ready system, into the most dominant player in the seed industry.

But now, this silver bullet of American agriculture is beginning to miss its mark. The herbicide-resistant weeds that have plagued Southern farmers are emerging in Missouri with similar tenacity.

“It’s a serious, serious problem,” said Blake Hurst, a corn and soy farmer in northwestern Missouri and vice president of the board of the Missouri Farm Bureau. “The further north you get, the less of a problem it’s been so far. Farmers here are denying it’s going to happen to them. But guess what? It’s on the way to your farm.”

So far, glyphosate-resistant weeds have been found in at least 22 states. Last month, University of Missouri researchers confirmed that herbicide resistant giant ragweed has been found on 12 farms, bringing the total count of herbicide-resistant weeds in the state to five.

“There is no question glyphosate is a once-in-a-century herbicide,” said Kevin Bradley, a weed scientist with the University of Missouri who conducted the giant ragweed survey. “The problem is that glyphosate has been so good that farmers have gotten spoiled a little bit … We can’t continue to abuse the system, which is just using Roundup Ready soybeans and spraying glyphosate over and over and over.”

Monsanto, and the farmers who use its products, stress that glyphosate is still an effective product, one that controls more than 300 weeds. But the company acknowledges that it may have underestimated how long it would take for weeds to become resistant to the chemical and that it should have educated farmers sooner about the issue.