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.