Ruminants
Ruminant farm animals like cows, sheep and goats are very different from other farm animals like chickens because of the way they digest food.
Ruminants have four stomachs that are used to consume vast amounts of food. Rumen is the largest stomach of the four and from which Ruminants derive their name.
This stomach acts as a large fermentation vat that breaks down raw plants that ruminants consume and sends some of the byproduct onto the other three stomachs for further processing into nutrients. In cows, it can hold up to 25 gallons of plant material.
But one byproduct of this fermentation, Methane, is expelled from the ruminants body. With cows, 95% of it escapes through the mouth as burps and the other 5% is expelled through flatulence. Because cows are large ruminants, they make a lot of Methane.
The indelicate nature of these natural facts are amusing to us all but their contribution to global warming is no laughing matter.
There are approximately 1.5 Billion cows in the world at any given time. A single cow emits 165 to 266 lbs of Methane a year. Times 1.5 Billion cows and you get 249 to nearly 400 Billion pounds of Methane emitted into the air annually that we could eliminate or drastically reduce if we limited or gave up consuming beef and dairy products.
https://extension.umn.edu/dairy-nutrition/ruminant-digestive-system
http://www.fao.org/ag/againfo/resources/en/publications/tackling_climate_change/index.htm