Rumination is the process by which cows regurgitate previously consumed feed (cud) and chew it further. Larger particles are processed in the mouth again to decrease the size of the particles. This cud is then swallowed and digested through a fermentation process in the rumen. In this digestive/fermentation process supported by numerous species of useful bacteria, the food turns from being not particularly digestible into something of high biological value for the animal, becoming milk and meat for the body to grow. As a side effect, cows produce methane, considerably contributing to the greenhouse effect. Each litre of milk produced corresponds to about 35 litres of methane gas that is released into the atmosphere through belching or flatulence.

This analysis has been confirmed by several sources. According to British scientists, each cow produces between 500 and 600 litres of methane per day. In the United Kingdom alone, cows account for 3% of all greenhouse gas emissions. According to the University of Pennsylvania, as much as 37% of methane emissions (one of the most insidious greenhouse gases) originate in intensive animal farming. In California, the Environmental Protection Agency, which deals with frequent excesses of ozone, has already instituted regulations on polluting emissions that require cattle farms to control waste and change the diets of their livestock.