The federal government on Friday proposed eliminating restrictions on the use of corn and soybean seeds that are genetically engineered to resist a common weed killer, a move welcomed by many farmers but feared by scientists and environmentalists who worry it will encourage growers to use more chemicals.

Farmers have been eager for a new generation of herbicide-resistant seeds because of the prevalence of weeds that have become immune to Monsanto's Roundup. The new genetically altered "Enlist" corn and soybeans developed by Dow AgroSciences would allow farmers to use the weed killer throughout the plants' lives.

But skeptics are concerned that use of the new seeds and the herbicide known as 2,4-D will lead to similar problems as weeds acquire resistance to that chemical as well.

"It's just so clear. You can see that you have this pesticide treadmill effect," said Bill Freese, a chemist with the Washington, D.C.-based Center For Food Safety, which promotes organic agriculture.

Most corn and soybeans grown in the U.S. are already genetically engineered, largely to resist Roundup, which was introduced in 1976. Before that, most farmers tilled their fields prior to planting, flipping the soil over and burying the weeds to kill them. The technique also exposed tilled earth to the air, creating problems with erosion and runoff.

Herbicide-resistant seeds permitted most farmers to stop tilling since they could spray fields after their plants emerged, killing the weeds and leaving their crops unharmed.

The new generation of plants "allowed us to do a better job of controlling the weeds, and therefore, we've been able to do a better job of preserving the soil, which is our primary natural resource," Ron Moore, who grows 2,000 acres of corn and soybeans in western Illinois, told The Associated Press.

The U.S. Department of Agriculture's plant-inspection agency concluded that the greatest risk from the new seeds developed by Dow AgroSciences was increased use of 2,4-D, which could accelerate the evolution of weeds resistant to it.

But, the agency said, resistance could develop anyway because 2,4-D is already the third most-used weed-killer in the nation.