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Genetically engineered crops that could sharply increase the use of two powerful herbicides are now unlikely to reach the market until at least 2015 because the Department of Agriculture has decided to subject the crops to more stringent environmental reviews than it had originally intended.

The department said on Friday that it had made the decision after determining that approval of the crops “may significantly affect the quality of the human environment.”

The crops in question are Dow Chemical’s corn and soybeans that would be resistant to the herbicide 2,4-D and Monsanto’s dicamba-resistant cotton and soybeans.

Many farmers say they would welcome the new crops because it would give them a way to kill the rapidly growing number of weeds that have become resistant to their main herbicide – Roundup, known generically as glyphosate. Most of the corn, soybeans and cotton grown in the United States are genetically engineered to tolerate glyphosate, allowing farmers to spray the chemical to kill weeds without hurting the crops.

But opponents all say that approval of the crops would spur big increases in the use of 2,4-D and dicamba, which they say are more damaging to the environment and possibly human health than glyphosate.

Some fruit and vegetable growers and canners have been concerned that their crops would be damaged by 2,4-D or dicamba drifting over from nearby corn or soybean farms. The Agriculture Department said Friday that both chemicals had “been safely and widely used across the country since the 1960s.”

The department had already prepared shorter environmental assessments on two of the Dow crops and put them out for public comment. It did not say how long the more complete environmental impact statements would take, though past experience suggests it could be 15 months to more than two years.