My memory is rusty and the link doesn't corroborate your figure from what I could see. What I remember is that methane is more potent a greenhouse gas than O2 but it has a significantly shorter half-life in the atmosphere.

Methane has a half-life of < 10 years (7, I think) but a GWP of 25. The primary concern of atmospheric methane is, then, the possibility of a quick build-up that increases CO2 emission from natural sources; there is quite a bit of CO2 stored in bogs that only a bit of methane will release. It's the long-term side-effects of atmospheric methane that become long-term concerns, rather than the methane itself.

Sorry, the link was just to corroborate the fact that cattle produce a lot of methane. For the GWP of Methane: http://www.wolframalpha.com/input/?i=methane+100+year+gwp

[1] doesn't really discuss whether grass-fed cattle produce more, less, or about the same amount of methane as those eating conventional feed.