Washington Post: 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 …