The intestine is the tubular segment of the digestive tract that extends from the stomach to the anus and constitutes the main site of digestion, fermentation, and nutrient absorption in the horse. It is anatomically divided into two major sections: the small intestine (approximately 20 metres in an adult horse) and the large intestine (comprising the caecum, large colon, small colon, and rectum, totalling roughly 7 metres).
The small intestine digests proteins, soluble carbohydrates, and fats through enzymatic action; the large intestine houses trillions of bacteria and protozoa that ferment structural carbohydrates (cellulose, hemicellulose) in hay and pasture grass, producing volatile fatty acids that supply up to 70 percent of the horse's resting energy requirements. The anatomy of the large colon — four distinct sections with two sharp flexures — creates multiple sites where feed material can become impacted or displaced, which is why intestinal problems are the leading cause of equine why intestinal anatomy makes horses especially colic-prone. Gut motility, fluid balance, and microbial population are influenced by diet composition, feeding practices, hydration (especially important in winter — see winter water intake and its effect on gut motility), and deworming interval.