How much methane do you get out of 1kg biomass vs how much co2 if you burned it? Methane is indeed a more potent (if shorter lived) greenhouse gas, but one would guess that some amount end up either in the cow or as manure which then act as a natural fertilizer. Natural fertilizer is also an object which commonly get classified as having zero carbon footprint, which again, is a bit odd. The manure has zero carbon if used as natural fertilizer, but the meat from the same cow does not.