Figures from the United States Department of Agriculture show herbicide use skyrocketing in soybeans, a leading G.M. crop, growing by two and a half times in the last two decades, at a time when planted acreage of the crop grew by less than a third. Use in corn was trending downward even before the introduction of G.M. crops, but then nearly doubled from 2002 to 2010, before leveling off. Weed resistance problems in such crops have pushed overall usage up.

To some, this outcome was predictable. The whole point of engineering bug-resistant plants “was to reduce insecticide use, and it did,” said Joseph Kovach, a retired Ohio State University researcher who studied the environmental risks of pesticides. But the goal of herbicide-resistant seeds was to “sell more product,” he said — more herbicide.

Farmers with crops overcome by weeds, or a particular pest or disease, can understandably be G.M. evangelists. “It’s silly bordering on ridiculous to turn our backs on a technology that has so much to offer,” said Duane Grant, the chairman of the Amalgamated Sugar Company, a cooperative of more than 750 sugar beet farmers in the Northwest.

He says crops resistant to Roundup, Monsanto’s most popular weedkiller, saved his cooperative.

But weeds are becoming resistant to Roundup around the world — creating an opening for the industry to sell more seeds and more pesticides. The latest seeds have been engineered for resistance to two weedkillers, with resistance to as many as five planned. That will also make it easier for farmers battling resistant weeds to spray a widening array of poisons sold by the same companies.

Growing resistance to Roundup is also reviving old, and contentious, chemicals. One is 2,4-D, an ingredient in Agent Orange, the infamous Vietnam War defoliant. Its potential risks have long divided scientists and have alarmed advocacy groups.

Another is dicamba. In Louisiana, Monsanto is spending nearly $1 billion to begin production of the chemical there. And even though Monsanto’s version is not yet approved for use, the company is already selling seeds that are resistant to it — leading to reports that some farmers are damaging neighbors’ crops by illegally spraying older versions of the toxin.

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