A mature horse's entire digestive tract, if stretched from beginning to end, is about 100 feet or roughly a third of a football field. The equine's (horse's) gastrointestinal tract is divided into two sections: the foregut and the hindgut. The foregut consists of the stomach which is about the size of a rugby ball and the small intestine which is approximately 70 feet in length. The hindgut, also called the large intestine, is about 30 feet long and consists of the cecum and the colon.

The equine's stomach is small in relation to its body size and it can only hold 2-3 gallons at a time. The main function of the stomach is to add gastric acid to aid in the breakdown of the food, to secrete the enzyme pepsinogen to begin protein digestion and to help regulate food passage to the small intestine. It is in the small intestine where true digestion takes place and is the main site where nutrient absorption takes place.

The hindgut's purpose is to ferment the the plant fiber or complex carbohydrates the horse ingests. This helps generate fatty acids, Vitamin K, Vitamin B and some amino acids that the horse can use for energy or calories. The colon not only serves to absorb these nutrients but also a portion of water that accompanies the food as it travels through the digestive tract.

On a side note, horses cannot vomit or regurgitate. What goes in the mouth must pass through the entire digestive tract and be eliminated out through the rectum.

More Info: en.wikibooks.org