Yet scientists warn that there are significant ecological consequences to stacked resistance. "It isn't a very sustainable system, and it keeps breaking down," says Bruce Maxwell, a plant ecologist at Montana State University in Bozeman. Today's weeds are resistant to Roundup, and the weeds of the future will be resistant to multiple herbicides, says Maxwell, just as some bacteria are now resistant to multiple antibiotics. In fact, it's happening already, as scientists discover more weeds resistant to Roundup and other herbicides.

The USDA, though, says the risk doesn't warrant disapproval of the new seeds. The agency's main criteria for approval of a new seed technology is whether it poses a risk to other plants. Companies submit field trial data to the USDA to prove their new seeds do not. If that burden of proof is met, after two rounds of public comment, the seeds get approved. The Environmental Protection Agency has a voice in the process only if the new seeds will change how a pesticide is used. Since some stacked-resistant seeds do involve new uses, the EPA will evaluate how they could affect the environment and human health. To date, though, it has consistently approved herbicide-resistant seeds.

The Agriculture Department realizes that using seeds resistant to multiple herbicides will encourage weeds resistant to multiple herbicides. So the agency encourages alternative tactics, like educating farmers to use crop rotations, weed-munching insects, cover crops, tilling and other tools, a process known as "integrated weed management."

Most weed scientists are already doing this, though, to little avail. Idaho's Morishita has long encouraged sugar beet farmers in his region to rotate beets with wheat, till periodically, and spray herbicides other than Roundup, so weeds are less likely to become resistant. He'd like to see glyphosate, which is relatively benign as herbicides go, remain a useful chemical. "Roundup-ready is really a wonderful technology. But it works so well, it's easy to just stick with it."

Critics say USDA's reliance on education as a strategy to prevent resistance is about as helpful as the agency crossing its fingers or knocking on wood. "Of course, the companies are happy to keep this going," says Bill Freese, of the Center for Food Safety. "They always want a new product to sell."

MSU's Maxwell also worries that increased use of chemicals more potent and dangerous than Roundup will harm the environment, with effects like the release of cancer-causing chemicals and more deformities and deaths among amphibians.

Many farmers, however, claim that crop rotations and other techniques are inefficient. Iowa farmer Dave Miller says it comes down to economics. "(We are told) we need to do some different rotations. Occasionally I hear references (that hearken to the past), 'Grandpa did a five-crop rotation.' Well, Grandpa probably had 30 milk cows and 10 sows. That's not today's production agriculture at all. That's like saying, if you've got a computer virus, why don't we all go back to typewriters?"

Herbicide-resistant seeds may not save farmers as much as they hope, though. In an eight-year trial, the USDA found that farms practicing the kind of rotations recommended by weed scientists were just as profitable or even more so than their corn-soy-focused neighbors -- while causing less pollution.

Seed and herbicide companies remain confident that resistant weeds will always be beatable, that a new chemical solution can always be found. Although agrichemical companies now admit that there's a problem, as recently as 2004, a Monsanto advertorial informed farmers that the true solution to resistant weeds was applying Roundup at a full dose. "The goal is to kill all the weeds, because we know that dead weeds will not become resistant," the ad copy read.

We all know how that turned out.