The digestive process of a horseIn the scientific field, horses are classified as animals in the opisthokont category of the unikont clade. Horses are mammals. They are non-ruminant herbivores or hindgut fermenters; this means that “their primary and often exclusive food source is plants” (Huntington). This species evolved over time to graze small amounts of roughage frequently throughout the day, typically for 15 to 20 hours. Domestication modified these feeding behaviors to adapt them to the schedules of the caretakers. Normally, people tend to feed large amounts of grains once or twice a day. If people do not own pasture in which horses can graze, the animals are expected to also eat large quantities of grass and/or alfalfa hay. This makes it much more difficult for horses to maintain their health because it compromises their proper digestion abilities. It has been found that this problem can be resolved by feeding small amounts of food often in order to assimilate the natural grazing habits of the species. Horses “…have a relatively small stomach, which holds about as much as a 5 liter bucket of ice” (Reynolds). The stomach is physically capable of holding more substance than this, but it begins to empty once it is two-thirds full. It performs this task as a safety measure as they are unable to regurgitate food. Therefore, if they eat too much or consume something toxic, vomiting is not possible. The digestive system can be divided into two distinct sections: ; the first being more human-like, and the second more accurately resembling a cow's rumen. Their small stomach has the potential to store two to four gallons of the stuff for the average horse passing through the middle of a paper. If oil is added to horses' feed, it has been found to slow the flow of food into the small intestine; thus giving digestive enzymes more time to process starches, proteins and fats. In turn, this increases the total amount of nutrients processed into usable forms for horses and maximizes the digestive efficiency of the small intestine. Horses do not have gallbladders. For this reason, it is difficult for them to digest and utilize high-fat diets. They can handle about 20% fat in their diet, but it takes about three to four weeks to adjust to this change if the owner has not previously been consistent in this aspect of their diet. Normally, rations in a typical horse's diet contain only about three to four percent fat. Also due to the absence of a gallbladder, bile constantly flows from the liver into the small intestine..