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 U.S. each year, according to the Environmental Protection Agency. That makes the hoofed critters the largest source of the heat-trapping gas.

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