See page 51 of http://www.fao.org/docrep/012/k7930e/k7930e00.pdf
"Methane emissions per animal from this assessment are comparable to figures
obtained by Schils et al. (2007b), Cederberg et al. (2009) in OECD countries (ranging from 110 to 130 kg methane per cow per year) and by Herrero et al. (2008) in Africa (ranging between 21 and 40 kg methane per livestock unit per year)."
This source has also mentioned how incomplete studies have been on the actual methane production of cattle because of the various changes in feed, feed type, individuality in cattle, as well as the emissions from manure. Methane is a natural gas in the Earth, and the amount that comes from cattle is actually more insignificant than that naturally produced from such sources as volcanoes, the oceanic floor, swamps, etc.