How much methane does a cow produce?
See page 51 of
"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.