When compared to the other herbivores, horses are quite unique when it comes to their digestive anatomy. Horses are known as “hind-gut” fermenters. This means that much of the fibrous material of their forage (hay/pasture) is digested in the later portions of their digestive tract. Furthermore, the horse’s digestive system is designed to process small meals throughout the day. Because horses’ digestive physiology is so distinctive, it is important for any equine enthusiast to understand how it works. It also explains why horses require special management to maintain optimal health.
Mouth & Esophagus
Digestions starts in the mouth. Horses will masticate (chew) their feed to reduce the particle size. This, along with the addition of saliva, aids in swallowing. Other important aspects of saliva include:
- Contains bicarbonate, which helps buffer the highly acidic stomach.
- Contains the enzyme amylase that helps break down carbohydrates.
- Horses may secrete anywhere from 10 to 20 gallons of saliva per day depending on moisture content of feed.
Horses should have 36 teeth. This includes 12 incisors (biting), 12 premolars (chewing), and 12 molars (chewing). Additionally, almost all male horses and about 28% of female horses will have one to four canine teeth. Finally, a horse may have wolf teeth in front of their upper and sometimes lower premolars. This means some horses may have up to 44 total teeth. Additionally, care for a horse’s teeth is critical to their digestive and overall health. We detail this further in our Managing a Horse’s Dental Health article.
The horse’s esophagus can be anywhere from 50 to 80 inches long. This long tube carries the feed from the mouth to the stomach. An interesting aspect of the horse’s esophagus is the strong sphincter muscle at the entrance to the stomach that allows feed to enter, but not exit the stomach. This means horses cannot vomit. Thus, if horses are given poor quality or moldy feed, they cannot regurgitate it. This can lead to serious health issues for the horse.
The role of the horse’s stomach is to quickly mix in acids and enzymes to aid in digestion. Interestingly, a horse’s stomach is surprisingly small. In fact, a horse’s stomach is only about 10% of the total volume of the horse’s digestive tract. Thus, on average, an adult horse’s stomach can hold anywhere from 2 to 3 gallons of feed before it is quickly passed on to the small intestine.
A horse’s stomach is small because they have evolved to eat small meals all day long. Thus, they do not need a large stomach to store or consume large meals. Passage rates of feed from the stomach into the small intestine can be as short as 15 minutes, with an average of 30 to 45 minutes for most feedstuffs. Other important aspects of the horse’s stomach include:
- Forage passes quicker through the stomach as compared to concentrates (horse feeds).
- Hydrochloric acid helps further break down feed and can eliminate harmful bacteria.
- It secretes pepsin, which helps break down proteins.
- The lower glandular region makes up the lower 2/3 of stomach and has protection against stomach acid.
- The upper non-glandular region is the upper 1/3 of stomach and does not have protection against stomach acid.
A real concern for many horse owners is the formation of ulcers. More can be read in our What Are Gastric Ulcers article on how best to reduce or manage gastric ulcers.
Most of the digestion and nutrient absorption happens in the small intestine, which makes up about 28% of the total digestive tract. Additionally, the horse’s small intestine is extremely long and can range in length from 50 to 70 feet and can hold up to 18 gallons. It is here that enzymes, primarily secreted by the pancreas, work to break down proteins, non-structural carbohydrates (starch), and fats for absorption.
The passage rate of feed in the small intestine is surprisingly fast. Like the stomach, the type of feedstuff will dictate how quickly it will move through. Feed can spend as little as 1 hour in the stomach and small intestine before entering the hind gut.
Another interesting aspect of the horse’s small intestine is the absence of a gall bladder. The purpose of a gall bladder in other species is to store bile produced by the liver. Bile is released after eating a meal. However, because horses have evolved to eat small meals throughout the day, they never had a need for it. Thus, bile is secreted constantly into the small intestine of the horse to help digest fat.
When the digested feed leaves the small intestine, it enters the horse’s hindgut. The hindgut consists of the cecum, large and small colon, and the rectum. This is the largest portion of the horse’s digestive tract (~62%) and can be up to 24 feet long and hold up to 40 gallons.
The purpose of the horse’s hindgut is to digest the fibrous material from their diet. This is done by microbial digestion, with the primary purpose of breaking down structural carbohydrates that have passed through the stomach and small intestine. Here, the microbes ferment the remaining digesta into volatile fatty acids, which can be absorbed and used as an energy source for the horse. Other products produced during this process include B-vitamins, Vitamin K, some amino acids, methane, and water.
The cecum can be described as a large muscular vat in the horse. It is like the rumen of cattle. In that, the cecum holds and ferments feed, which takes at least 7 or 8 hours. Additionally, the horse’s cecum is larger than the stomach, making up about 15% of the total digestive tract. It can be as long as 4 feet and hold up to 9 gallons of feed.
Unfortunately, the hindgut can be particularly prone to digestive disorders. This can be even more true for the cecum because digesta enters and leaves the cecum from the top of its structure. Because of this, it can be prone to impaction when the horse consumes large, dry meals. Furthermore, the microbial population in the cecum (and colon) can be sensitive to rapid dietary changes and lead to digestive upset. Two articles worth reviewing to help maintain hindgut and cecum health include:
Once digesta leaves the cecum, it enters the largest compartment (~40%) in the horse’s digestive tract, the large colon. While not as long as the small intestine, at only 10 to 12 feet, the large colon can hold up to 22 gallons of digesta. Here, microbial digestion is still ongoing with further absorption of nutrients.
Like the cecum, the large colon can be sensitive to dietary changes, stress and other digestive disorders. The large colon is also at risk of colic with its series of twists and turns. Studies have shown that nearly 40% of colic cases are due to either large colon displacement or impaction. Thus, horses need to be carefully managed and fed high quality feeds to maintain gut health. Another article worth reviewing in supporting overall digestive health in the horse is Feeding Your Horse Often.
Small Colon & Rectum
While the small colon is nearly as long as the large colon (around 10 feet), it is much smaller in diameter. Thus, it does not hold nearly as much volume. Once digesta is passed on from the large colon, the role of the small colon is to absorb water from the digesta and form fecal balls. These are then passed through the rectum.
All in all, by the time feed enters the mouth of the horse and passes through the digestive system, it can take as short as 36 hours or can be as long as 72 hours before being expelled as feces. The type of feed consumed will dictate just how fast it can be processed. For example, horse feeds are more easily broken down, and thus, digested quicker as compared to a more fibrous forage that will spend a longer time in the hind gut of the horse. If you have any questions or concerns about your horse’s nutritional or digestive health, please feel free to contact us for a free, personalized feeding plan!