Proponents of the new cotton and soybean varieties, engineered by Monsanto to tolerate spraying with multiple herbicides, say they're a much-needed tool. "These weed management solutions will provide farmers with more consistent, flexible control of tough-to-manage broadleaf weeds," said Monsanto in a press release issued after the US Department of Agriculture approved the crops for use last month.

But others think the benefits will at best be short-lived. Weeds may soon become resistant to the new herbicide mixtures, resulting in new generations of ever-more-intractable weeds that will need to be controlled with yet more herbicides.