WASHINGTON - One contributor to global warming - bigger than coal mines, landfills, and sewage-treatment plants - is being left out of efforts by the Obama administration and House Democrats to limit greenhouse-gas emissions.

Cow burps.

Belching from the nation's 170 million cattle, sheep, and pigs produces about one-quarter of the methane released in the United States each year, according to the Environmental Protection Agency.

In part because of an adept farm-lobby campaign that equates government regulation with a cow tax, the gas that farm animals pass is exempt from legislation being considered by Congress to limit greenhouse-gas emissions.

The EPA under President Obama has said it had no plans to regulate the gas, even though the agency recently included methane among six greenhouse gases it says it believes are endangering human health and welfare.

The message circulating in Internet chat rooms, the halls of Congress, and farm co-ops was that America's farms would face financial ruin if the EPA required them to buy air-pollution permits as power plants and factories do. The cost of those permits amounted to a cow tax, farm groups argued.

Administration officials and House Democratic leaders have tried to assure farm groups that they have no intention of regulating cows. That effort, however, has done little to ease the concern of farmers and their advocates in Congress about the toll that regulating greenhouse gases will have on agriculture.

Lawmakers and farm groups are now pressing for the climate legislation to guarantee that farmers will be compensated for taking steps to reduce greenhouse gases.

Research has shown that changing cattle diet and boosting efficiency - such as producing the same amount of milk and beef from a smaller herd - can result in less gas, said Frank M. Mitloehner, an associate professor at the University of California at Davis.

"I don't think livestock should be ignored," Mitloehner said. But laws designed to reduce emissions from smokestacks and tailpipes will not work with cattle, he said.

"You cannot capture these gases," he said.