Cecum & Hindgut Fermentation in Horses: How It Works

Horses are not cows. That sounds obvious, but it matters more than you think when it comes to digestion. Both animals ferment fiber to extract energy (see our full equine digestive system overview), but they do it in completely different parts of the digestive tract, and that difference shapes everything about how we feed horses and what goes wrong when we get it wrong.

Cows are foregut fermenters. They have a massive four-chambered stomach (the rumen being the star of the show) that breaks down fiber before it ever reaches the small intestine. Horses are hindgut fermenters. Their stomach and small intestine handle the easy stuff first, and the real fiber digestion happens downstream in the cecum and large colon. This arrangement has some serious advantages and some very real vulnerabilities.

The Cecum: A Fermentation Vat You Didn't Know About

The equine cecum is a comma-shaped, blind-ended pouch sitting at the junction where the small intestine meets the large intestine. In an average-sized horse, it holds about 25 to 35 liters. That is roughly the size of a large trash can. It sits on the right side of the abdomen, tucked against the body wall, and it's the first major fermentation chamber feed encounters after leaving the small intestine.

Here's what happens. The horse chews hay or pasture grass, swallows it, and the stomach begins protein digestion with acid and pepsin. The feed moves into the small intestine, where enzymes break down simple sugars, starches, fats, and proteins. Whatever the horse's own enzymes can handle gets absorbed there. But structural carbohydrates like cellulose, hemicellulose, and pectin pass through the small intestine mostly untouched because mammals simply do not produce the enzymes needed to break them apart.

That undigested fiber flows into the cecum, and this is where things get interesting. The cecum is packed with billions of microorganisms: bacteria, protozoa, and fungi that collectively form the hindgut microbiome. These microbes produce the cellulase enzymes that the horse cannot make on its own. They break the bonds in plant fiber that would otherwise be completely indigestible.

The Large Colon: Where Fermentation Continues

The cecum doesn't work alone. Material leaving the cecum passes into the large colon, which is itself a massive fermentation vessel. The equine large colon is roughly 3 to 3.5 meters long and can hold up to 80 liters. It folds back on itself in a distinctive pattern: right ventral colon to sternal flexure, sternal flexure to left ventral colon, left ventral colon to pelvic flexure, pelvic flexure to left dorsal colon, left dorsal colon to diaphragmatic flexure, and finally to the right dorsal colon.

Those flexures matter because they are narrowing points. The pelvic flexure in particular is notorious. The colon's diameter shrinks dramatically there, and it's where impactions most commonly lodge. A horse that isn't drinking enough, whose feed is too dry, or whose motility has slowed for any reason will feel the consequences at the pelvic flexure first. It's a bottleneck, quite literally, and the single most common site for impaction colic.

Fermentation continues throughout the large colon, with additional VFA absorption and water reabsorption happening along its length. By the time digesta reaches the small colon, most of the nutritional value has been extracted. The small colon forms the familiar fecal balls, reclaiming the last of the water before expulsion.

Volatile Fatty Acids: The Real Payoff

When cecal and colonic microbes ferment fiber, the end products are volatile fatty acids (VFAs), primarily acetate, propionate, and butyrate. These three compounds are absorbed directly through the gut wall and provide a significant portion of the horse's daily energy needs. Research from Texas A&M estimates that VFAs can supply 60 to 70 percent of a horse's total energy requirements when the diet is primarily forage-based.

Each VFA plays a slightly different metabolic role:

• Acetate is the most abundant, typically making up around 65 percent of total VFA production. It is used for energy throughout the body and is a precursor for fat synthesis.
• Propionate accounts for roughly 20 percent. It is converted to glucose in the liver, making it especially important for meeting the horse's blood sugar needs.
• Butyrate makes up about 15 percent. It is the primary energy source for the cells lining the large intestine itself, keeping the gut wall healthy and functioning.

The ratio of these VFAs shifts depending on what the microbes are fermenting. A forage-heavy diet produces more acetate. When excess starch or sugar reaches the hindgut (because the small intestine couldn't absorb it all), fermentation shifts toward lactate and propionate production, dropping the pH of the cecal environment. That pH drop is where trouble starts.

The Microbiome Is Not Optional

The microbial populations in the cecum and large colon are not random. They are highly specialized communities that develop over time in response to the horse's consistent diet. Fiber-fermenting bacteria (primarily from the Firmicutes and Fibrobacteres phyla) dominate in horses on forage-based diets. Starch-fermenting and lactic acid-producing bacteria (like Lactobacillus and Streptococcus species) increase when concentrate feeds are a larger part of the ration.

This specialization is exactly why abrupt diet changes are so dangerous. When you suddenly swap a horse from one hay type to another, dramatically increase the grain ration, or turn a horse out on lush spring pasture after months of dry hay, you are flooding the hindgut with substrates that the current microbial population is not equipped to handle efficiently.

Think of it like replacing the engine in your car while driving on the highway. The old microbial community can't process the new feed efficiently, the new community hasn't had time to establish itself, and in that gap, opportunistic bacteria take over and produce byproducts that wreck the neighborhood.

Why Rapid Diet Changes Can Kill a Horse

Here is the chain of events, and it can unfold in hours, not days.

A horse that has been eating primarily grass hay gets a sudden large meal of grain. The small intestine can only process a limited amount of starch per meal (roughly 2 to 4 grams per kilogram of body weight, depending on the starch source). Anything beyond that capacity overflows into the cecum and large colon undigested.

The fiber-fermenting bacteria in the hindgut are not great at processing this starch. But the starch-fermenting bacteria, which were present in small numbers, suddenly have an abundance of their preferred substrate. They multiply rapidly and produce large quantities of lactic acid. The pH in the cecum drops from its normal range of about 6.5 to 7.0 down toward 5.0 or lower.

At this lower pH, the fiber-fermenting bacteria start dying off. As they die, some release endotoxins (lipopolysaccharides from their cell walls) into the gut lumen. The acidic environment also damages the mucosal lining of the cecum and colon, making it more permeable. Endotoxins leak through the compromised gut wall into the bloodstream.

The result can be any combination of colic, diarrhea, endotoxemia, and laminitis. The laminitis connection is particularly devastating because the damage to the laminae in the hooves may not become apparent for 24 to 72 hours after the initial insult, long after the digestive upset seems to have resolved. By then, the structural damage inside the hoof is already done.

This exact sequence is one of the most common causes of laminitis in horses, and it is entirely preventable.

Fructans: The Pasture Problem

Starch isn't the only culprit. Fructans, a type of storage carbohydrate found in cool-season grasses, behave similarly in the hindgut. Unlike simple sugars, fructans pass through the small intestine undigested and are fermented by hindgut microbes. When fructan levels in pasture are high (cold nights followed by sunny days, spring growth, stressed grass during drought), a horse grazing normally can ingest enough to trigger the same acidosis cascade described above.

This is why laminitis-prone horses and those with metabolic syndrome need careful pasture management. Grazing muzzles, limited turnout during peak fructan hours (late afternoon on sunny days), and choosing warm-season grass species for paddocks can all reduce risk. The coronary band and the structures it supports depend on a stable hindgut environment more than most people realize.

Practical Feeding Rules That Protect the Hindgut

Understanding hindgut fermentation isn't just academic. It translates directly into how you should feed every horse in your care:

• Make all diet changes gradually over 7 to 14 days. This gives the microbial populations time to adjust. Swap hay types by mixing old and new in increasing ratios. Increase grain by no more than half a pound every few days.
• Keep forage as the foundation (see our horse nutrition guide). Horses should consume a minimum of 1.5 to 2 percent of their body weight in forage daily. For a 1,000-pound horse, that is 15 to 20 pounds of hay or pasture equivalent. The hindgut microbiome needs a steady supply of structural fiber to function properly.
• Limit starch per meal. If you feed grain or concentrate, split it into multiple small meals rather than one or two large ones. Keeping each meal under 4 to 5 pounds of commercial feed helps ensure the small intestine can process the starch before it reaches the cecum.
• Manage pasture transitions carefully. Spring grass is high in fructans that behave similarly to starch in the hindgut. Limit grazing time initially and increase gradually, especially for horses prone to metabolic issues.
• Provide constant access to water. Hydration is critical for normal gut motility. Dehydration slows the movement of digesta through the colon and increases impaction risk.
• Consider prebiotics and probiotics cautiously. Research on equine-specific probiotics is still evolving. Some products show promise for stabilizing hindgut pH during transitions, but the evidence is mixed. Consult your vet before adding supplements, and don't mistake a probiotic for a substitute for proper feeding practices.

The Bottom Line

The cecum and large colon are not just passive tubes that food passes through. They are active fermentation chambers housing a complex microbial ecosystem that provides the majority of a forage-fed horse's energy. Respecting the biology of hindgut fermentation, feeding adequate fiber, making changes slowly, and limiting starch overload, is one of the most important things any horse owner can do. The microbes keeping your horse alive need consistency. Give it to them.

Frequently Asked Questions

Sources

• Texas A&M AgriLife Extension. "The Equine Digestive System." animalscience.tamu.edu
• Julliand, V., and Grimm, P. "Horse Species Symposium: The Microbiome of the Horse Hindgut." Journal of Animal Science, 2017. academic.oup.com
• Milinovich, G.J., et al. "Microbial Ecology of the Equine Hindgut During Oligofructose-Induced Laminitis." The ISME Journal, 2008. nature.com
• NRC. "Nutrient Requirements of Horses, 6th Revised Edition." National Academies Press, 2007. nap.nationalacademies.org
• Al Jassim, R.A.M., and Andrews, F.M. "The Bacterial Community of the Horse Gastrointestinal Tract and Its Relation to Fermentative Acidosis, Laminitis, Colic, and Stomach Ulcers." Veterinary Clinics of North America: Equine Practice, 2009. sciencedirect.com
• UC Davis Center for Equine Health. "Nutrition and Colic." ceh.vetmed.ucdavis.edu