Horses are not ruminant animals. Their digestive system is fundamentally different from that of a cow, sheep, or goat. Both ruminants and horses are herbivores that gain energy from plant matter. Horses are classified as non-ruminant herbivores, specifically known as hindgut fermenters. This distinction is based on where the primary breakdown of fiber occurs, which impacts how horses must eat and how they are managed.
Defining the Ruminant
A ruminant animal, such as cattle, deer, or goats, uses a process called foregut fermentation. Their stomach is divided into four distinct compartments: the rumen, reticulum, omasum, and abomasum. The largest compartment, the rumen, acts as a massive fermentation vat where specialized microbes break down plant cellulose before it reaches the true stomach.
This system allows the animal to rapidly ingest large amounts of forage and store it for later processing. Ruminants regurgitate partially digested food, called cud, and chew it a second time to further break down tough plant fibers. This prolonged microbial action allows the animal to efficiently extract nutrients from fibrous plants and absorb microbial protein and B vitamins produced by the rumen microbes.
The Horse’s Unique Digestive System
In contrast to ruminants, the horse has a simple, single-chambered stomach, classifying it as a monogastric animal. This relatively small stomach, with a capacity of only about 3 to 5 gallons, is followed by a short small intestine. These foregut structures are responsible for the enzymatic digestion of proteins, fats, and non-structural carbohydrates.
The horse’s ability to digest tough plant fiber comes from the massive structures located at the end of its digestive tract—the hindgut. The hindgut consists of the cecum, a large, comma-shaped organ that functions as a fermentation vat, and the large colon. This is where microbial fermentation of fibrous forage occurs, which is why horses are called hindgut fermenters.
The cecum alone can hold up to 8 to 9 gallons and is approximately four feet long in an average horse. Symbiotic microbes within the cecum and colon break down structural carbohydrates into volatile fatty acids (VFAs). These VFAs are absorbed through the intestinal wall and provide the horse with up to 70% of its energy. Because fermentation happens after the small intestine, the absorption of microbial protein and B vitamins is less efficient compared to ruminants.
Why This Matters for Horse Care
The horse’s reliance on hindgut fermentation dictates specific feeding and management needs. The continuous production of stomach acid and the small stomach size mean a horse is adapted to graze almost constantly, consuming small amounts of forage throughout the day. This constant trickle of food helps maintain a healthy, neutral pH in the hindgut, which is necessary for the fiber-digesting microbes to thrive.
Sudden changes in diet, especially excessive starch or sugar from grains or lush grass, can overwhelm the small intestine. When undigested starch reaches the hindgut, it is rapidly fermented by a different group of microbes, causing a quick drop in pH and an increase in lactic acid. This condition is known as hindgut acidosis.
This imbalance can kill off beneficial fiber-digesting bacteria, releasing toxins. This puts the horse at high risk for digestive issues like gas colic and laminitis. Therefore, all dietary transitions must be made very slowly to allow the hindgut microbial populations time to adapt.