The 2002 census of cattle and calves in the U.S. was 95,497,994. That isn't much higher than the estimated number of buffalo (60-80 million) that inhabited the plains of North America before the evil white man wiped them out. Buffalo are generally bigger than cattle, eat more and, naturally, fart more. So there is no significant net difference in ruminant produced methane. I expect the same could be said for Africa where fantastically large herds of animals roamed a century or two ago. If anything today's domestic livestock are a good deal fewer in number than the wild herds of old.
The methane comes mainly from the burps; farts are more likely to be hydrogen sulfide.
