Horses are hindgut fermenters, a digestive strategy that dictates much about their care and diet. This unique process allows them to thrive on a forage-based diet, extracting energy from plant material indigestible to many other mammals. Fermentation is the microbial breakdown of fiber, primarily cellulose, that occurs in the large intestine rather than the stomach. Understanding this specialized digestion is the foundation for maintaining a healthy horse.
Defining Hindgut Fermentation
Hindgut fermentation refers to the digestive process where microbial organisms break down plant fibers in the final sections of the digestive tract. Horses are classified as non-ruminant herbivores, meaning they possess a simple, single stomach, unlike ruminants such as cows or sheep. The horse’s foregut—the stomach and small intestine—utilizes enzymes to digest and absorb proteins, fats, and simple carbohydrates like starches.
Fiber largely bypasses the foregut’s enzymatic digestion due to the lack of necessary enzymes. This undigested fiber then passes into the hindgut, where a dense and diverse population of bacteria, protozoa, and fungi takes over. This delayed fermentation is a necessary adaptation that enables the utilization of structural carbohydrates.
The Anatomy of Equine Digestion
The horse’s hindgut organs are the cecum, the large colon, and the small colon, which together represent up to 70% of the entire digestive tract’s capacity. The cecum is a large, comma-shaped, blind-ended sac, often described as a fermentation vat, which can hold approximately 8 gallons of material. Digested material from the small intestine enters the cecum, where it can be retained for up to seven hours to allow sufficient time for microbial action.
After the cecum, ingesta moves into the large colon, which is long and convoluted, measuring about 10 to 12 feet in length. The large colon’s structure provides a vast surface area for continued fermentation and nutrient absorption. This structure houses the symbiotic microbial ecosystem responsible for breaking down the structural carbohydrates found in hay and grass.
Nutritional Byproducts of Fermentation
The microbial action within the cecum and large colon converts complex fiber, such as cellulose and hemicellulose, into usable energy sources for the horse. The primary products of this fermentation are Volatile Fatty Acids (VFAs), specifically acetate, propionate, and butyrate. These VFAs are absorbed directly through the hindgut lining and transported to the liver and other tissues, providing 50% to 70% of the horse’s daily energy requirement.
Acetate is the most common VFA produced and is used by the horse for fat synthesis and as a general energy source. Propionate is often converted into glucose, a necessary fuel for the brain and certain muscles. Beyond energy, the hindgut microbes also synthesize B vitamins, such as Biotin, and Vitamin K, which can be absorbed and utilized by the horse.
Dietary Management for Horses
Because the horse relies heavily on hindgut fermentation, a consistent, high-fiber diet is necessary for maintaining the health of the microbial population. Forage, such as hay or pasture, should form the foundation of the diet, ideally constituting at least 1.5% of the horse’s body weight daily. This constant supply of fiber keeps the fermentation process stable, which is necessary for the fiber-digesting bacteria to thrive.
The microbial ecosystem is highly sensitive to sudden changes in diet or an overload of undigested starch from grains. If the small intestine is overwhelmed by a large, high-starch meal, excess starch passes into the hindgut. This allows lactic acid-producing bacteria to multiply rapidly, leading to a drop in the hindgut’s pH, known as hindgut acidosis. This pH change can kill off beneficial fiber-digesting microbes and is a risk factor for health issues, including colic and laminitis. Feeding smaller, more frequent meals helps maintain the delicate microbial balance.