Start to Finish: Journey Through the Equine Digestive System

For many horse owners, one of the most satisfying times of day is walking through the barn when all the horses are eating. The rattle of feed tubs and the chorus of peaceful chewing and munching signifies that all is well.

It’s a chore we do day after day, year after year, but most of the time we don’t stop to think about what’s involved.

The equine digestive system is a marvel of design comprised of both a foregut and hindgut. Let’s follow the journey of how food progresses through the gastrointestinal tract from the moment your horse drops his nose into that feed bucket or hay bin and starts chewing.

Foregut

Together, the mouth, esophagus, stomach, and small intestine are known as the foregut. This is where the digestive process begins, and enzymes start to break down feed material.

Mouth

Horses chew in both a lateral and vertical movement. Unlike humans and canines, horses must physically chew to produce saliva.

Mastication (chewing) kick off the digestive process. The saliva produced by chewing helps food move through the system, and also serves as a buffer against acid in the stomach.

When a horse has worn or missing teeth, as can happen in old age, or even when a younger horse has teeth in poor condition, this negatively impacts digestion because the animal can’t chew properly.

If the horse’s teeth can’t break down food and chew well—for whatever reason—this can put the horse at greater risk of esophageal obstruction (choking), gastric impaction, small or large intestinal impaction, and weight loss.

When the horse swallows, chewed feed leaves the mouth and enters the pharynx, the chamber where the nasal and oral cavities meet but are separated by the soft palate before the opening of the esophagus or trachea.

Esophagus

Muscles in the pharynx push food on into the esophagus, which is approximately 4 to 5 feet long, and empties into the stomach.

Feed material can only travel down the esophagus—not up. A ring of muscle ring known as the esophageal sphincter (or cardiac sphincter) lies between the end of the esophagus and the opening to the stomach. Horses cannot vomit or “burp up” feed or gas.

The horse’s esophagus is composed of skeletal muscle for the first two-thirds and smooth muscle the last third. (For comparison, a dog’s esophagus is skeletal muscle the entire length.)

Stomach

Compared to other livestock, the horse’s stomach is relatively small, holding only about 4 gallons.

The equine stomach is divided into two regions: squamous (upper) and glandular (lower). Hydrochloric acid (HCL) is constantly produced in the lower region and helps begin to digest feed material into smaller pieces.

Feed material mixes with HCL in the stomach before being released into the small intestine.

“In the wild, horses constantly graze, traveling over wide ranges daily to consume what they need while leaving a ball of forage in their stomach, which also buffers the stomach lining against gastric acid production,” notes Caitlyn Henderson, DVM, of Peterson Smith Equine Hospital in Ocala, one of Floria’s largest equine clinics.

Small intestine

If you could stretch out the small intestine, it would be 50 to 70 feet long. This is where much of the starch, protein and fat in the horse’s diet is digested and absorbed.

While exiting the stomach, feed material passes through the three portions of the small intestine. First is the duodenum, second is the jejunum (the longest portion), and third is the ileum, which has a thicker muscular layer than the duodenum and jejunum right before it meets the cecum.

The horse doesn’t have a gallbladder. To aid in the digestive process, the liver continuously secretes bile into the first portion of the small intestine (duodenum), which helps digest fat.

Feed material passes through the small intestine fairly quickly--about 1 foot per minute.

Hindgut

The cecum, large colon, small colon, and rectum all make up the equine hindgut. This is where fermentation (bacterial digestion) occurs, as the digestive process continues.

Structural carbohydrates (forage) make up the bulk of the horse’s diet. The foregut isn’t capable of effectively digesting this material, so it is fermented in the hind gut using the bacteria, fungi, and protozoa in the microbiome.

Cecum

Traveling from the small intestine, feed matter enters the cecum. Think of this portion of the digestive tract as a “mixing bowl” that can hold up to 10 gallons. The microbiome in the cecum works to digest fiber as it is moved through by muscular contractions.

Large Colon

Feed matter moves from the cecum into the large colon, which can hold up to 20 gallons.

The large colon is comprised of four regions: right ventral, left ventral, right dorsal and left dorsal.

When all is functioning normally, it takes two to three days for feed matter to move all the way through the large colon.

Between the ventral (bottom) and dorsal (top) sections, the large colon becomes narrower and doubles back at a point called the pelvic flexure. This is where rectal palpation can be used to feel for gas distension of the large colon, cecum, and small intestine.

Henderson explains that veterinarians also use rectal palpation to check for impaction of the small colon and limited areas of the large colon, cecum, and some areas of the small intestine.

Using rectal palpation, the veterinarian may also be able to determine if the colon is displaced, either to the right or left.

On occasion, the left ventral and dorsal regions of the large colon can float up because of gas distension and be caught between the spleen and the left kidney (a condition known as nephrosplenic entrapment). Rectal palpation can also be used to check for this condition.

Small colon

The last section of the equine GI tract is the small colon, which is about 10 to 12 feet long. Water absorption primarily occurs here.

Fecal balls form in the small colon and move on to the rectum, the final 12 inches of the digestive system, to pass out of the horse.

Smooth Functioning

Considering the complexity of the equine digestive system, it’s not surprising that there can be disruptions, especially in performance horses, who routinely face the challenge of travel, stress, training, and competition.

Although hard-working horses typically require more energy and calories than can be provided by forage alone, their digestive systems are still designed to eat mainly forage. Even when we must turn to concentrates to meet increased demand, it’s critical that we don’t cut the horse short on the fiber from roughage that his body requires.

For example, 1% of body weight in forage is the minimum daily intake to keep the hindgut functioning properly, but 1.5% is preferred.

Gradual changes

For performance horses that travel to compete, one of the most important things you can do to avoid digestive upset is to consistently feed the same forage. Bring along the hay/cubes/pellets your horse is used to eating so you never have to make abrupt changes.

Microbes in the hindgut need time to gradually adjust to a new source of forage. This is especially relevant if you change to a totally different type of forage—for example, from a grass hay to a legume hay or a mixed hay with alfalfa.

Many horse owners think about changes in grain/concentrate more than they do about changing hay, but changes in either should be made gradually over a week’s time.

Add one-quarter of the new hay to three-quarters of the “old.” Continue “swapping in” new to replace the old over a week’s time until you’ve switched over entirely.

If you’re switching a horse to a richer forage than he’s been eating, you might extend the “swap-over” time to 7 to 10 days to allow the microbes in the hindgut to adjust to the change.

Go easy on the grain

“High-starch concentrate feeds that are primarily grain or contain high amounts of molasses or other sugars are a ‘buffet’ for the microbes in the hindgut, causing massive production of gas,” cautions Henderson.

She notes that this can lead to colic, dysbiosis (microbial imbalance of the hindgut), diarrhea, and laminitis if the horse has a pre-existing metabolic disorder.

Some performance horses are fed significant amounts of grain/concentrate. Even if you use a low-starch feed, take care to avoid overwhelming the digestive system. Break down the total daily amount into smaller meals and feed at evenly spaced intervals.

A safe rule of thumb is not to feed more than ½ pound of grain/concentrate per 100 pounds of body weight in one meal, or no more than 0.5% of total body weight per day.

This doesn’t apply to forage, but to any type of concentrate or grain, including pellets and beet pulp.

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A healthy GI tract contributes to optimal performance. It’s in your hands to protect your horse and give his digestive system what it needs to function as intended.