I think ruminants including cows do significantly contribute to methane emissions by burping.
"Cows don't emit 400 quarts of daily flatulence, as the term is usually understood. According to Professor Johnson, they emit 400 quarts' worth of burps,known in polite circles as eructation."
http://www.straightdope.com/columns/read/832/do-cow-and-term...
And it is fairly self-evident when you think of it: cows are ruminants, and the process of rumination means fermentation before digestion. Methane is released in this fermentation, so it naturally comes as burping, not flatulence.
"Cows don't emit 400 quarts of daily flatulence, as the term is usually understood. According to Professor Johnson, they emit 400 quarts' worth of burps,known in polite circles as eructation." http://www.straightdope.com/columns/read/832/do-cow-and-term...