To fight them, farmers throughout the East, Midwest and South are being forced to spray fields with more toxic herbicides, pull weeds by hand and return to more labor-intensive plowing methods.

Farm experts say that such efforts could lead to higher food prices, lower crop yields, rising farm costs, and more pollution of land and water.

"It is the single largest threat to production agriculture that we have ever seen," said Andrew Wargo III, the president of the Arkansas Association of Conservation Districts.

The first resistant species to pose a serious threat to agriculture was spotted in a Delaware soybean field in 2000. Since then, the problem has spread, with 10 resistant species in at least 22 states infesting millions of acres, predominantly soybeans, cotton and corn.

The superweeds could temper American agriculture's enthusiasm for some genetically modified crops. Soybeans, corn and cotton that are engineered to survive spraying with Roundup have become standard in American fields. However, if Roundup does not kill the weeds, farmers have little incentive to spend the extra money for the special seeds.

Roundup -- originally made by Monsanto but now also sold by others under the generic name glyphosate -- has been little short of a miracle chemical for farmers. It kills a broad spectrum of weeds, is easy and safe to work with, and breaks down quickly, reducing its environmental impact.

But farmers sprayed so much Roundup that weeds quickly evolved to survive it. Now, strains of horseweed and giant ragweed that have developed Roundup resistance are forcing farmers to go back to more expensive techniques that they had long ago abandoned.

A particularly tenacious species of glyphosate-resistant pigweed can grow three inches a day and reach seven feet or more. In an attempt to kill the pest before it becomes that big, farmers in western Tennessee are plowing their fields and mixing herbicides into the soil.

That threatens to reverse one of the agricultural advances bolstered by the Roundup revolution: minimum-till farming. By combining Roundup and Roundup Ready crops, farmers did not have to plow under the weeds to control them. That reduced erosion, the runoff of chemicals into waterways and the use of fuel for tractors.

If frequent plowing becomes necessary again, "that is certainly a major concern for our environment," Ken Smith, a weed scientist at the University of Arkansas, said.

Source: Minneapolis Star Tribune