Unlike humans, horses are able to extract energy from the fiber in their diet. The fiber is converted to volatile fatty acids (VFAs) by a bacterial fermentation process in the hindgut; the VFAs are then used to synthesize glucose (sugar) for immediate energy or are converted to fat for storage.
Horses need to eat a large amount of fiber each day to keep their gut active and free of problems, such as colic. A strong urge to chew on fibrous material also means horses may develop vices such as wood-chewing and stall-walking if they become bored without hay or pasture to consume. Since horses can handle only small amounts of concentrated energy in the form of grain, the bulk of the diet should be free-choice forage (hay or pasture), preferably with a lot of natural long-stem matter as opposed to pelleted hay or small cubes.