While Congress considers taxing auto and industrial pollution, a U.N. report fingers another guilty party — cows.

The U.N. accuses our bovine buddies of producing 18 percent of the world's greenhouse-gas emissions — more than planes, trains and automobiles combined.

"Methane is a greenhouse gas. It leads to climate change, that leads to global warming," explains Frank Mitloehner, an air-quality specialist at the University of California, Davis. "What we do is we study how much methane comes from different animal types, let's say dairy cows or steers."

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When cows digest their food, stomach bacteria produce methane, an ozone-forming gas considered 23 times worse than carbon dioxide when it comes to trapping heat in the atmosphere.

"They produce approximately 200 or so pounds of methane a year, and that's per animal," says Mitloehner. "Lactating animals produce twice that."

That makes heifers the Hummers of agriculture. At U.C. Davis, scientists use one-of-a-kind "burp tents" to measure the methane emissions of each animal.

"A lot of people think this gas is coming from the rear end," explains Nancy Hirshberg of Stoneyfield Farms in Highgate, Vt. "Ninety-five percent is actually from the front end, from the burps."

To reduce a cow's carbon hoofprint, farmers in Vermont claim to be designing the cow of the future thru genetics and diet. By replacing corn and soybeans with flaxseed and alfalfa, Stonyfield Farms says it's cut methane emissions by 18 percent.

"It reduces the burps, the methane burps from the cows, and it also improves the nutritional quality of the milk by increasing the omega 3 [fatty acids]," says Hirshberg.

Agricultural producers fear Washington wants to regulate their business. Already, some consumers have stopped eating meat on certain days.

The studies here are meant in part to bring common sense and science to the debate. In the U.S., all livestock only generate about 3 percent of the gases that cause climate change, partly because we have so many more cars and trucks compared to livestock as do other countries.

The dairy industry hopes to save the cow from being labeled the "coal" of agriculture.

"The cows love it, the farmers love it, we love it, it's better for consumers," says Hirshberg. "We've been just thrilled to discover that this could be done. It's been an incredible project for us and we're very excited to share it with the world."