Most cow flatulence is the result of human activity. Before cows were domesticated, there was almost no chance cows would ever reach these ridiculous herd sizes in nature (barring natural selection for KILLER COWS).

Also, they did not eat corn or soy-based diets much in nature, which both cause increased methane emissions. Alfalfa and flaxseed are proven to reduce emissions.

http://www.nytimes.com/2009/06/05/us/05cows.html?pagewanted=...