California's weeds are getting more stubborn. Tom Lanini, a UC Davis weed ecologist, estimates that Roundup-resistant weeds now affect 5,000 acres of California farmland.

Roundup is a trade name for glyphosate, an herbicide that slows weed growth by inhibiting protein synthesis. California's agricultural industry used 5.6 million pounds of glyphosate last year. And Roundup Ready crops, genetically engineered to withstand Roundup, are increasing the herbicide's popularity.

But Lanini worries what overuse of Roundup will do to weeds. "Using a single herbicide over a number of years, you're selecting for plants that aren't controlled by that material," he says. Genetic selection means the few weeds that survive Roundup treatment will produce similarly resistant offspring. "You build up tolerant populations," says Lanini.

He also cautions that Roundup Ready crops could add to the resistance problem. "Crops are becoming super-weeds," he said. "For example, if we plant a field with Roundup Ready cotton one year, and then plant Roundup Ready corn there the next year, any volunteer cotton that comes up with the corn will be a Roundup-resistant weed. How do you control that?"

Getting to the answer is part of Lanini's job -- he's helping farmers find alternate treatments for their resistant weeds. For example, almond growers can let geese eat resistant rye grass in their orchards. Farmers can combat resistance with varied cultivation practices and different herbicides.

Lanini wants to avoid the sort of widespread resistance that has made some medical antibiotics ineffective. "I'm really interested in saving Roundup," he says. "It's been an awesome product, and I'd like to see it remain that way as long as possible."