A vast majority of soybeans and corn planted in this country, and in much of the world, are genetically engineered, and the technology is rapidly pushing its way into many more crops.

For farmers, the benefits are real with these seeds they can spend less time plowing and cultivating and can use more benign agricultural chemicals to kill weeds. But according to a recent report from the National Research Council, there are also signs of trouble, chief among them the appearance in various parts of the country of herbicide-resistant weeds.

Such weeds could undermine the main purpose of genetically engineered crops: their ability to tolerate spraying with glyphosate, an environmentally benign herbicide marketed by Monsanto, one of the major producers of genetically engineered seeds, under the name Roundup. As ever, nature is finding its way around our defenses.

There were no glyphosate-resistant weeds when genetically engineered crops were introduced. A farmer could plant Monsantos seeds, spray with Roundup, kill the weeds and enjoy the harvest. Now there are many such weeds, and they are tenacious. Some farmers using genetically engineered cotton are facing the prospect of m