EDIT: it must be part of cow digestion. I just remember that cow burbs are bigger methane problems, but cow farts are fine. Still not sure about methane from grass dying normally, and maybe there’s lots extra grass being grown for cows that wouldn’t grow normally?
Aerobic decomposition, exposed to oxygen like dead grass on the ground, produces mostly CO2 and H2O (plus some other O based stuff like SO4, PO4, NO3). Anaerobic decomposition, like in a cow's stomachs, produces mostly CH4 and CO2 (plus some other H based stuff like NH3, H2S, PH3).
EDIT: it must be part of cow digestion. I just remember that cow burbs are bigger methane problems, but cow farts are fine. Still not sure about methane from grass dying normally, and maybe there’s lots extra grass being grown for cows that wouldn’t grow normally?