Which is a bigger methane source: cow belching or cow flatulence?

Contrary to common belief, it’s cow belching due to enteric fermentation. (“Enteric fermentation” is the digestive process of converting sugars into simple molecules for absorption into the bloodstream, which produces methane as a by-product.) However, a small percentage of methane is also produced in the cow’s large intestine and then expelled. Settling ponds and lagoons for processing manure also produce copious amounts of this greenhouse gas.