A just-published analysis of data received from a satellite in 2004 has shown that at least during that year, livestock in the U.S. emitted more methane into the atmosphere than did the oil and gas industry. In their article published in Journal of Geophysical Research: Atmospheres, a team of researchers from Harvard University, California Institute of Technology and the University of California studying the data note that such emissions were far higher than was reported by the U.S. Environmental Protection Agency (EPA).
Specifically, the researchers found satellite data showed livestock emitted 13 million tons of methane over the summer in 2004 (the EPA reported 9.7 million tons). They found the satellite data also showed that the combined emissions of the oil and gas industry amounted to 7 million tons (the EPA reported 9.9 million tons).
Unfortunately the sensor on the satellite was unable to show methane amounts after 2004, thus more data is not available. That will, however, change soon as a new satellite with sophisticated atmospheric gas monitoring sensors aboard is set to launch next year. More information on the role that methane plays in changing our climate can be found here.
Actually it does. Grasslands evolved to be grazed, and before there were cattle, there were very large numbers of bison doing that grazing (plus other animals, but we'll just consider bison here) -- estimates run from 30 million to as high as 50 million animals.http://www.fws.gov/bisonrange/timeline.htm [fws.gov]
Total inventory of cattle in the U.S. last year was about 87 million animals.http://agebb.missouri.edu/mkt/bull12c.htm [missouri.edu]
Bison average about twice the mass of domestic cattle, and all males in the wild grew to fullsized bulls (cows and steers do not achieve nearly the same mass as bulls, and the vast majority of modern male cattle are castrated). By weight alone, one bison equals 2 to 3 modern steers or cows. So at their native population, there were more pounds of bison present than there now are of cattle.
Also, cattle are about 5% more efficient than bison, so they need to eat less to support the same mass of animal... and better feed utilization means less wasted as methane.http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1456&context=greatplainsresearch [unl.edu]
So if the object is to reduce methane production, the evidence indicates that we had best stick with the current situation and domestic cattle, rather than returning to the native natural situation of a somewhat larger and less-efficient mass of bison.