It's well-known that methane produced by cows (and sheep, goats, etc - the world's livestock) is a contributing factor to the greenhouse gases affecting the climate. To what extent is a matter of debate, but flatulent cows play their part.
I read a story - click here - earlier in the week which trumps the bovine wind farm, though. Sauropods, the long-necked giants of the Mesozoic, may have generated more methane than is currently being pumped out by both natural and industrial sources. An adult cow will weigh about half a ton, while a smaller sauropod would have been 40 times that. Diplodocus, the old favourite, weighed in at maybe 40-50 tons and the largest sauropods may have broken the 100 ton mark.
You wouldn't really want to stand downwind of an Argentinosaurus.