Monsanto rolled out seeds genetically engineered to withstand its Roundup herbicide back in the mid-1990s. Today, Roundup Ready crops blanket U.S. farmland. According to USDA figures, 90 percent of soybeans and 60 percent of corn and cotton planted in the United States contain the Roundup-resistant gene.

Back-of-the envelope calculations tell me that total land devoted to Roundup Ready crops equals nearly 200,000 square miles — about two-thirds the size of Texas.

Roundup Ready’s conquest of U.S. farmland has been an unmitigated boon for Monsanto shareholders. Not only could the company charge a fat premium for its biotech seeds, but sales of the Roundup herbicide surged. By 2008, Monsanto was clocking more than $1 billion annually in profit from Roundup sales alone.

For years, Monsanto presented its economic triumph as an environmental gift to humanity. According to company talking points, the sudden ubiquity of Roundup, whose active ingredient is a chemical called glyphosate, eliminated the need to use other, more toxic herbicides. Moreover, Roundup Ready technology allows farmers to control weeds without having to till, dramatically reducing soil erosion.

Now, however, the Roundup juggernaut has run off course. Roundup’s status as ecologically benign is turning to dust. A study by France’s University of Caen last year found that the herbicide’s allegedly “inert” ingredients magnify glyphosate’s toxic effects. According to the study, “the proprietary mixtures available on the market could cause cell damage and even death” at levels commonly used on farm fields.

The annual cascade of Roundup on vast swaths of prime farmland also appears to be undermining soil health and productivity, as this startling recent report cited by Tom Laskawy shows.

Meanwhile, the endlessly repeated claim that Roundup Ready technology saves “millions of tons” of soil” from erosion appears to be wildly trumped up. According to Environmental Working Group’s  reading of the USDA’s 2007 National Resource Inventory, “there has been no progress in reducing soil erosion in the Corn Belt since 1997.” (The Corn Belt is the section of the Midwest where the great bulk of Roundup Ready corn and soy are planted.) “The NRI shows that an average-sized Iowa farm loses five tons of high quality topsoil per acre each year,” EWG writes.

Finally, there’s the ecological liability that will likely doom Roundup Ready’s dominance of U.S. farmland. It’s perhaps the least surprising, most-anticipated major development in the history of U.S. agriculture: After years of annually dousing millions of acres of farmland with a single herbicide, farmers are finding that certain weeds have developed resistance to that herbicide.