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.
Industrial scale "farms" also concentrate manure etc in such quantities that natural processes have a hard time to break them down properly.
Another thing about industrial farms is the use of corn as feed. It fattens up a cow faster than grass and is cheaper. But a cow is not adapted to eating corn and it causes major issues, including the generation of large amounts of methane. Even a cow fed grass generates methane, all mammals and some insects do, but not as much.
If it's not breaking down, it's not generating methane... But the fact is, manure experiences excellent demand as a base for fertilizer (about half the commercial fertilizer starts life as manure, but there's not enough manure to satisfy this market). It gets either used on the farm or trucked off to a fertilizer plant, like Bandini in Los Angeles. Mountains of it? Yeah, between production and being hauled off... Most people have NO idea how much herbivores eat, let alone how much they shit.