How ridiculous, if they are contributing to global warming it is probably only by a very small amount. 40 million bison roamed the great American plains when the first European settlers arrived, and there is no evidence that the methane they produced affected the climate. Not forgetting African mega-fauna and cattle elsewhere in the world. I'd be more concerned about the vast quantities of methane currently being released from the melting Siberian perma-frost and in the Arctic than cows, not that there is anything anyone can do about it.
A few years ago a French professor produced a theory that the dinosaurs may have poisoned themselves by releasing vast quantities of methane thus raising greenhouse gasses to a perilous levels by trapping Co2 in the atmosphere. His work has not exactly been accepted by the scientific comunity.
