Horses are hindgut fermenters, a type of non-ruminant herbivore that digests plant fiber through microbial fermentation in the cecum and large colon rather than in a multi-chambered stomach like cattle. This distinction shapes everything about how horses eat, how they extract energy from grass and hay, and why their digestive health can be so fragile.
What “Hindgut Fermenter” Actually Means
The term describes where in the digestive tract the heavy lifting of fiber breakdown happens. Horses eat mainly plant material, and the tough, fibrous parts of that material (cellulose and hemicellulose) can’t be broken down by the horse’s own enzymes. Instead, billions of bacteria, protozoa, and fungi living in the cecum and colon do the work. These microbes ferment the fiber and produce volatile fatty acids, primarily acetate, propionate, and butyrate, which the horse absorbs and uses as fuel. When a horse’s diet is mostly hay and grass, up to 70% of its energy can come from these microbial byproducts.
This is fundamentally different from how cattle, sheep, and goats handle the same problem. Those animals are ruminants: food enters a massive four-chambered stomach where microbial fermentation happens first, before the material moves on to the true stomach and intestines. Ruminants can also regurgitate partially digested food and chew it again (“chewing cud”) to break it down further. Horses can’t do this. Their esophagus only moves food in one direction, which means they can’t vomit, burp, or pass gas through their mouth. That one-way valve is part of what makes digestive blockages so dangerous for them.
A Small Stomach for a Large Animal
A horse’s stomach is surprisingly small relative to its body size, holding roughly 3 to 5 gallons (about 18 liters). It makes up only 9 to 10% of the total digestive tract volume. Compare that to a ruminant, where the stomach accounts for about 70% of the tract, or even a pig at 29%. This small stomach is a direct reflection of how horses evolved: they’re designed to eat small amounts almost continuously rather than large meals at set times.
The stomach also produces hydrochloric acid around the clock, regardless of whether food is present. An adult horse secretes roughly 1.5 liters of gastric juice per hour. When the stomach is empty for extended periods, that acid has nothing to buffer it, which is why gastric ulcers are extraordinarily common in horses. Studies report ulcer prevalence ranging from 53 to 90%, depending on the population studied. Horses that go long stretches without forage, or those fed large grain meals with limited hay, are at the highest risk.
How Food Moves Through the Tract
After the stomach, food enters the small intestine, which is about 70 feet long. This is where sugars, starches, proteins, fats, and many vitamins and minerals get broken down and absorbed into the bloodstream. Passage through the stomach and small intestine is fast, averaging about 5 hours total, with most food spending only 2 to 6 hours in the stomach itself.
Whatever the small intestine doesn’t fully digest, mainly fibrous material and any undigested starch, moves into the hindgut. The cecum sits at the junction between the small and large intestine. It’s a large, blind-ended sac that acts as a mixing chamber, combining incoming material with its resident microbial population. Volatile fatty acid production in the cecum alone accounts for roughly 30% of the digestible energy in a fiber-based diet.
From the cecum, material passes into the large colon, which is about 12 feet long and divided into four main sections. Fermentation continues here, and water is reabsorbed. The total retention time in the cecum and colon averages around 35 hours, giving the microbes enough time to extract energy from tough plant fibers. From mouth to manure, the full transit time is roughly 36 to 72 hours depending on the diet.
Why This Design Creates Health Risks
The horse’s large colon doesn’t follow a straight path. It folds back on itself at several points, and at these turns the diameter changes dramatically. The most notable is the pelvic flexure, where the colon narrows sharply as it bends. The junction between the right dorsal colon and the transverse colon is another tight spot. These bottlenecks are the most common sites for impaction colic, a painful and potentially life-threatening condition where feed material or gas gets trapped.
Colic is broadly any abdominal pain in a horse, and the anatomy of the hindgut is a major reason it’s so prevalent. If a blockage forms at one of these narrow points, the colon behind it can distend with gas, causing severe pain. Because horses can’t vomit to relieve pressure, problems that would resolve easily in other animals can escalate quickly.
The microbial population itself is another vulnerability. The bacteria and fungi in the hindgut are specialized for fermenting fiber. When a horse’s diet changes suddenly, especially if large amounts of starch or sugar reach the hindgut undigested, it can cause a rapid shift in the microbial community. The wrong microbes proliferate, producing lactic acid and endotoxins that can trigger laminitis (a painful inflammatory condition in the hooves) or severe diarrhea.
Built for Continuous Grazing
The entire system makes more sense when you consider how horses evolved. Free-ranging horses graze for the majority of every 24-hour period, voluntarily stopping for a maximum of about 3 hours at a time. Their digestive tract is adapted to this near-constant intake of dry, fibrous material. The small stomach empties quickly and expects frequent refills. The hindgut microbes need a steady supply of fiber to function properly. The continuous acid production assumes food will always be arriving to buffer it.
This is why horses generally do best on diets built around hay or pasture, offered in a way that mimics trickle feeding. High-starch concentrates like grain are a relatively modern addition. They can provide useful calories for working horses, but the digestive system wasn’t designed around them. Grain is digested primarily in the small intestine, and when too much is fed at once, the overflow reaches the hindgut and disrupts the microbial balance.
One advantage of the hindgut fermenter design is speed. Because fermentation happens after the small intestine has already absorbed the easy-to-digest nutrients, horses can move a large volume of feed through their system and extract nutrients more rapidly than ruminants. This made evolutionary sense for an animal that needed to cover ground while fueling itself on relatively low-quality forage. It also means horses tend to be “easy keepers,” maintaining body weight efficiently compared to similarly sized animals.