With regard to digestion, fermentation generally refers to an anaerobic (= without oxygen) biological process that occurs in the digestive tract of an animal. In the horse, the main site of fermentation is the large intestine or hind gut. During fermentation, microbes secrete enzymes which break down ingested food (and fibre the horse cannot digest itself), which results in gas production and the formation of other molecules, some of which can be absorbed by the animal and provide energy (e.g. fatty acids). The exact products of fermentation depend on the conditions and the substance being fermented.
Easily fermentable food (e.g. grains with easily accessible, high contents of starch or sugar) result in a fast fermentation and large amounts of CO2, which is acidic when disolved. Constant exposure of the digestive tract to high acid levels can cause irritation. Fibrous foods generally result in a slow fermentation, which can be considered more beneficial for the horse.