Probably less because we are feeding the cheapest source of feed to cows, not what they can digest the best. The food that your body fails to digest will be digested in the big intestine which can lead to bloating.
> A number of past studies have found lower greenhouse gas emissions associated with the feedlot system. One reason is that grass-fed cows gain weight more slowly, so they produce more methane (mostly in the form of belches) over their longer lifespans.