Methane is produced as a by-product of enteric fermentation and is particularly common within ruminating animals (that regurgitate semi-digested plant-based food and then chew it again). Cattle are the principal animal producers of methane, while emissions from livestock categories other than cattle, sheep and swine tend to be limited.
The amount of methane released depends on the breed, age and weight of an animal, the quality and quantity of their feed, and the animal’s energy expenditure. The higher the volume of food intake, the greater the emissions, with feed intake positively related to an animal’s size, its growth, whether or not it is pregnant, or its milk production.
Eurostat, "Using Official Statistics to Calculate Greenhouse Gas Emissions. A Statistical Guide", Publications Office of the European Union, Luxembourg, 2010