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.

Dow had initially hoped to have its 2,4-D-resistant corn on the market this year, though it then pushed it back to 2014. On Friday, it said approval was now not likely until 2015. It had not expected its soybeans to be ready for market until 2015 anyway.

Monsanto, which called the Agriculture Department decision “unexpected,” had been hoping to start selling its soybeans in 2014 and cotton in 2015.

The department was likely to be sued had it not taken the new course.

The federal approvals of genetically engineered alfalfa and sugar beets were rescinded by a federal judge a few years ago. The judge, in response to lawsuits filed by the Center for Food Safety, said the Agriculture Department had not adequately considered the environmental impacts.

Still, the department said on Friday that, under its regulatory authority, the decision on whether to approve the crops would rest solely on whether they are plant pests. That raised questions about what influence, if any, the environmental impact statements would have.

One environmental group, the Pesticide Action Network, applauded the delay. “Farmers across the country have been voicing their growing worries about these seeds, which have been designed to be used with toxic drift-prone herbicides,” Marcia Ishii-Eiteman, a senior scientist, said in a statement.

However, she said the fact that the approval decision would be based solely on the plant pest risk rather than the overall environmental impact “illustrates gaping flaws in our regulatory system.”

The Biotechnology Industry Organization, a trade group representing biotech crop developers, said the decision set a bad precedent.

These crops “have already been subjected to multiple delays in the approval system,” Cathleen Enright, executive vice president for food and agriculture, said in a statement. “No new scientific issues about potential risks have been raised.”

Dow and Monsanto said they would cooperate with the Agriculture Department and use the extra time to better prepare for the introduction of the crops.

“Glyphosate-resistant and hard-to-control weeds have spread across our nation’s farmland,” Dow said in a statement. “Twenty-five states are now affected and the number of new acres infested in 2012 increased by 50 percent over the previous year. These adverse trends will continue without new state-of-the-art solutions like the Enlist Weed Control System.”

Enlist is Dow’s name for the crops resistant to 2,4-D and the accompanying herbicide.