Animals are usually classified into two groups on the basis of their digestive physiology; ruminants and monogastric (or non-ruminants). This is really an over-simplification, since some non-ruminants, in addition to their simple stomachs, have an important symbiotic relationship with a microbial population in their hind-gut. Animals such as the horse and the rabbit are able to take advantage of the nutrients from many of the same feeds as ruminants. The major difference between these animals and ruminants is that their fermentation takes place at the end of the gastrointestinal tract while in the ruminant it occurs at the beginning. These non-ruminant herbivores also have requirements for dietary essential amino acids and vitamins, since although they are synthesized by the microbes, the animal's ability to absorb them from the large intestine is severely limited.

Canines and felines have simple stomachs, short intestinal tracts and undeveloped caeca. All ingested food passes quickly through the digestive system with little opportunity for microbial fermentation. Thus, these animals are carnivores, feeding almost entirely on animal tissue although some nutrients can also be obtained from high quality plant material like hulled grains or tubers that contain starch rather than cellulose. Omnivores, like humans or pigs, also possess simple stomachs but their digestive systems are capable of extracting nutrients from readily digested plant tissue if the plant cell wall is thin or the fibrous coat surrounding grain seeds or nuts is disrupted or removed. Omnivores vary considerably in ability to obtain nutrients from plant cell walls. In pigs, the sacculated colon and caecum make up about half of the total digestive capacity, providing substantial volume for microbial fermentation. This allows extensive break-down of complex plant tissues and liberation of sufficient nutrients from