MILWAUKEE » The federal government today proposed eliminating restrictions on corn and soybean seeds genetically engineered to resist a common weed killer, a move welcomed by many farmers but worrisome to scientists and environmentalists who fear it could invite growers to use more chemicals on crops.

The herbicide known as 2,4-D has had limited use in corn and soybean farming because it becomes toxic to the plants early in their growth. The new seeds would allow farmers to use the weed killer throughout the plants’ lives.

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

"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 because it let them spray fields after their plants emerged, killing the weeds but leaving 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," said Ron Moore, who grows 2,000 acres of corn and soybeans with his brother in western Illinois.

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 hasten the evolution of weeds resistant to it.

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

Freese also raised concerns about 2,4-D’s tendency to drift beyond the area where it is sprayed, threatening neighboring crops and wild plants.

Dow AgroSciences has attempted to address that by developing a new version of 2,4-D and new equipment to use with it, company spokesman Garry Hamlin said. The seeds and new 2,4-D have been approved in Canada but not yet sold there.

The company has targeted their release in the U.S. for 2015, pending approval by various federal agencies. In anticipation of that, it has received import approval from multiple nations so that farmers using seeds sold under the Enlist brand name can export their crops and products made from them.

For now, the seeds can only be used in tightly controlled trials.

The public has 45 days to comment on the USDA report published today as part of the deregulation process. The Environmental Protection Agency is conducting a separate review on the impact of expanded use of 2,4-D, although it previously found the herbicide safe.

The EPA plans to release a report in the coming months, and the two agencies are expected to make final decisions simultaneously on use of the chemical and seeds. It was not clear when that would happen.

Dow AgroSciences has asked the USDA to deregulate one corn and two soybean varieties. The corn resists 2,4-D and glyphosate, the generic form of Roundup. Both soybean varieties resist 2,4-D, but they differ in their immunities to other herbicides.

The USDA said farmers could help curb resistance to 2,4-D by using a variety of means to fight weeds and not relying solely on the one herbicide.

Eighty-six percent of corn, soybean and cotton farmers in the South and 74 percent of those in the Midwest have had problems with hard-to-control weeds, Hamlin said, citing private data from agricultural services.

"These growers need new tools," he said.

Moore said he currently uses both glyphosate and another herbicide in an effort to discourage resistance. He was not certain whether he would use the Enlist seeds and 2,4-D if that became an option because he has not had problems with Roundup resistance.

But he said he knows famers who have and some have gone back to tilling. Weeds can’t be left in fields because they suck up the nutrients, water and sunlight meant for crops.

"Those are the three things that plants all need to grow," Moore said. "I don’t want anything competing with my soybeans."

Freese said he would rather see farmers use organic means to control weeds, perhaps by using cover crops that go into the ground after the harvest to hold soil in place and deter weeds until the next year’s growing season.

Among its critics, 2,4-D is best known as a component of the Vietnam War-era herbicide Agent Orange, which has not been produced since the 1970s.

Agent Orange has been tied to health problems in Vietnam veterans, but scientists do not believe 2,4-D was the culprit. Instead, their research focused on dioxin, a cancer-causing substance found in another ingredient known as 2,4,5-T, which was banned by the EPA in 1985.