Unlike cattle which have several stomachs (and there own interesting digestion system), horses have only one stomach. Like cattle their natural diet is grass. With feed provided by people, horses can run into issues they don’t experience from their natural diet. Fresh grass is fibrous and slows down digestion. If feed is used the feeding should be spread out several times during the day, due to the horses digestive system.
The small intestine is the main site of digestion and absorption of protein, energy, vitamins and minerals.
The cecum is located after the small intestine of a horse and it functions much like the rumen of a cow (as a fermentative vat housing microbes which aid digestion). These microbes break down nutrient sources that would otherwise be unavailable to the horse.
The cecum and colon house bacterial, protozoal and fungal populations which function in microbial digestion of feed material in the digestive tract. Many different products of microbial digestion are absorbed by the horse.
Among other benefits, incorporating long-stem forage into rations increases particle size of ingested matter, thus slowing rate of passage. It also increases dry matter intake, thus stimulating water intake.
Nutrients and Common Feed Sources for Horses from the extension service (USA land grant universities)
Carbohydrates provide the majority of a horse’s energy. Non-structural carbohydrates, such as starch and glucose from grains and gums and pectins from fiber, are readily utilized as energy sources for the horse. The enzyme amylase breaks down non-structural carbohydrates into glucose and simple sugars, which are absorbed in the small intestine.
Structural carbohydrates, such as cellulose and hemicellulose in plants, can only be broken down by bacterial enzymes in the cecum and colon. The microorganisms convert these carbohydrates to volatile fatty acids (acetate, propionate, butyrate), which can provide 30 to 70 percent of the horse’s energy requirement.
Fibrous feeds are a very important part of the horse’s diet. They provide nutrients for both the horse and microbes in the hindgut as well as stimulate muscle tone and activity of the gastrointestinal tract.
Mineral supplements are usually required in the horse’s diet. Macrominerals are added to a horse’s diet to balance the ration to meet mineral requirements.
Human raised horses usually have some grazing but get some or much of there food needs from feed. Those feeds often supplement normal food for wild horses with beets, apples, carrots and other sources. In addition the horse food supplements include minerals, fiber and even pre-biotics and pro-biotics (just like our processed food does).
As a general rule, horses need 1 to 2 quarts (2 to 4 liters per kilogram) of water per pound of dry matter consumed. Of course, other factors can increase the water need, such as exercise (since it results in water loss through sweating).
Related: Great Webcast Explaining the Digestive Systems – Energy Efficiency of Digestion – Tracking the Ecosystem Within Us