Architecture of the Equine Digestive System

Learn about the equine digestive system to make the best choices in what and how to feed your horse.
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Learn about the equine digestive system to make the best choices in what and how to feed your horse.

Imagine looking down the length of a 100-foot garden hose. Now imagine gathering up that hose and fitting it inside your horse's belly. One end of the tube is at his mouth, and the other is at his tail, with the majority balled up in his abdominal cavity.

In a horse's stomach, enzymes break down digesta. It then moves through 60-70 feet of small intestine (not to scale here), where more enzymes continue the digestion process. After nutrients are extracted and absorbed into the bloodstream, the digesta empties into the cecum and then the large intestine for microbial fermentation and further nutrient absorption. | ? Kip Carter Illustration

In a horse's stomach, enzymes break down digesta. It then moves through 60-70 feet of small intestine (not to scale here), where more enzymes continue the digestion process. After nutrients are extracted and absorbed into the bloodstream, the digesta empties into the cecum and then the large intestine for microbial fermentation and further nutrient absorption. | ? Kip Carter Illustration

You've just pictured a rough image of your horse's digestive system. One hundred feet of tube through which ?everything you feed him travels, with ?digestion and absorption processes all along the path. That's a lot of tube. And, when things go right, the system is very efficient. However, so many things happen in those 100 feet, it's not too surprising that there are quite a few potential problems.

There are also many rules in feeding horses: feed small meals often; feed only high-quality hay; make any feeding changes gradually; never feed cattle feed to horses, etc. Why does feeding your horse seem so complicated, and why so many rules? The answer lies in the architecture of the horse's gut?how his unique digestive system is designed.

I've always thought understanding how your horse's digestive system works is more important than trying to memorize all those rules. If you understand the architecture of the gut and how digestion and absorption of nutrients works in horses, you don't need to memorize anything?it all just makes logical sense. Then, when you're faced with a new situation, you don't have to try to remember the ?appropriate rule, you can just think of what makes sense. That will help you make the best choices in what and how to feed your horse.

The horse's gut is fairly unique compared to other livestock species. The horse is classified as a nonruminant herbivore?an animal that eats plants and is not a ruminant. Several livestock species are ruminant herbivores, including cattle, sheep and goats. Ruminants have stomachs that are divided into compartments, whereas horses have simple stomachs with only one compartment. Animals with simple stomachs are classified as monogastrics, including horses, pigs, dogs, cats and humans.

With those basic differences defined, let's look at the horse's gut. We're going to start at the beginning, follow it through to the back end and examine what goes on in each section.

The Upper Gut
The gut starts at the mouth, which the horse uses to take in feedstuffs and chew. In horses, a unique aspect of the mouth is that the physical act of chewing stimulates the production of saliva, which is not necessarily the case in other species. To understand the importance of this, think of saliva as lubrication. If your horse doesn't chew adequately, there will be larger chunks of feed and less lubrication (saliva) to help the feed flow smoothly through the digestive tract.

Providing regular dental care is the first step horse owners can take to help ?ensure adequate chewing. This decreases the risk of digestive tract problems, such as choke, and helps ensure optimal digestion and absorption of nutrients.

The next part of the gut is the esophagus, or throat. The horse's esophagus is unique in how it attaches to the stomach. The attachment is at such an angle and the muscles are so firm that once the digesta passes that point, it's not coming back?it's a one-way trip. The horse normally cannot belch or regurgitate. In fact, if something makes it into the horse's stomach that should not be there, such as a toxic substance, his stomach would rupture before he could ever regurgitate.

This is different than in cattle. Cows can belch and "chew their cud" (or ?ruminate) when partially degraded food moves back up the esophagus from the stomach and is then chewed and swallowed again. This allows them to break down less digestible foods so nutrients are more available farther down the tract, which is one of the reasons cattle are better able than horses to utilize poor quality hay.

Now we enter the horse's stomach. As I mentioned before, the horse has a ?monogastric stomach, meaning a single compartment or a simple stomach. This single compartment contains primarily ?digestive enzymes and hydrochloric acid, so feed is degraded by enzymatic digestion.

This is also quite different from cattle, as a cow's stomach comprises four compartments, with the largest compartment being the rumen. The rumen is a very large bag?large enough to fill a typical wheelbarrow. It contains billions of microorganisms?bacteria and protozoa. When feed enters the cow's rumen, it is digested (fermented) by the microbes. This accounts for one of the reasons you should feed your horse only products designed specifically for horses and not cattle, because ?microbes are able to digest and utilize some feed components (and some potentially toxic substances) that digestive enzymes cannot. (For more information, see "Why Cattle Feeds Don't Work," below.)

Another function of microbial fermentation is the digestion of fiber carbohydrates in the diet. Fibers are made of sugars linked together by a bond that requires a microbial enzyme to break. In ruminants, microbes in the rumen break down fibers into volatile fatty acids (VFAs). The VFAs are then absorbed from the small intestine and are an important energy source for the animal.
In the horse, these fibers pass through the stomach and small intestine with very little breakdown. This is another reason to feed high-quality hay to your horse. The more fibrous the hay, the less digested it will be in the upper gut (stomach and small intestine) and the fewer nutrients your horse will get out of the hay. Cattle are quite efficient at ?retrieving nutrients even from fairly poor-quality roughages due to the microbial fermentation in the rumen.

One more interesting difference between the equine and bovine stomach is the rate of passage. In cattle, it can easily take 24 to 36 hours for feedstuffs to pass through the entire stomach. In horses, digesta usually passes through the stomach within two hours, though it can be as short as 15-20 minutes. The faster digesta moves, the less efficient digestion processes may be.

Moving on, the next part of the horse's gut is the small intestine. This is a tube that is about 3 inches in diameter and 60-70 feet long.

As digesta moves through the small intestine, more digestive enzymes are produced, and nutrients are degraded into components that can be absorbed into the bloodstream. In fact, the small intestine is the major site of nutrient absorption: Most if not all of the fat in the diet is digested and absorbed here, soluble carbohydrates (sugars and starch) are primarily digested and absorbed in the small intestine, and it is the only appreciable area of absorption of amino acids from dietary protein. The ?majority of vitamins and several minerals are also absorbed in the small intestine.
Here again, the rate of passage of digesta through the small intestine is fast?as short as 45 minutes, with a maximum rate of about eight hours. In 10 hours, feed has passed all the way through the stomach and small intestine in the horse.

Anything that we can do as horse owners to slow down the rate of passage in the stomach and small intestine can help increase the efficiency of digestion and nutrient absorption. About the only way to do that is to slow down your horse's rate of intake. Feeding management practices such as placing large, round stones in the feed tub can accomplish that goal?your horse has to pick around the stones, slowing down intake.

Why Cattle Feeds Don't Work

Feeding cattle feeds to horses is never a good idea for several reasons. First, horses have different nutritional requirements than cattle, so any feed that is designed for cattle will not specifically meet your horse's needs. Further, the differences in the animals' digestive systems set the scene for ingredient variations that can cause problems for your horse.

Remember, the horse's simple stomach contains primarily digestive enzymes and hydrochloric acid, so feed is degraded by enzymatic digestion rather than the microbial fermentation found in a cow's rumen. This means that cattle can utilize poor quality or highly fibrous feedstuffs much more efficiently than horses. Therefore, cattle feeds often contain ingredients that are good for cattle but provide few nutritional benefits to your horse due to poor digestibility. Further, cattle feeds sometimes include ingredients that can be detrimental to horses, such as ionophores.

Ionophores are antibiotics that have been shown to increase feed efficiency and growth rate in cattle. However, ingested ionophores can be toxic to horses, resulting in damage to the heart, skeletal muscle, kidneys and liver?possibly resulting in death. In fact, even feeding cattle feed that is not supposed to contain ionophores can be risky, because there is no guarantee that a feed labeled for cattle is completely free of ionophores.

Cattle feeds also often contain urea, a source of nonprotein nitrogen. In cattle, the rumen's microbes can take that nitrogen and use it to synthesize protein. The microbial protein is then available as an additional protein source to meet the amino acid requirements of the animal.

In horses, there is no appreciable microbial population in the stomach, so the urea is not utilized to form protein. It is converted to ammonia and absorbed in the small intestine. The amount of urea commonly found in sheep or cattle feed is not usually toxic to the horse, but it doesn't serve any function, and the horse must excrete the resulting ammonia through the urinary system. However, if large amounts of urea are ingested by a horse, the high levels of ammonia that are absorbed can be toxic, ultimately resulting in death.
The Horse's Unique Hindgut
At this point, you understand how the horse's upper gut functions and why horses are fed differently than cattle (and other ruminants). Now let's compare horses to other monogastrics, such as people.

Our stomachs and small intestines are similar to those in horses, but do we eat the same way? How much quality time did you spend grazing in the pasture today? I'm guessing none. (I certainly didn't!) So why is grazing not normal for people? Why is it that many horses can stay fat on good-quality hay or pasture alone, and we can't eat enough roughages, such as lettuce and celery, to maintain body weight? Well, we've only discussed half of the gut so far?the upper gut. The answers to these questions lie in the unique structure of the horse's hindgut when it is compared to almost all other monogastric digestive systems.

The horse's hindgut includes the ?cecum and the large intestine, or ?colon. The hindgut comprises more than 65 percent of the digestive tract's total capacity. The cecum is a large bag located at the junction of the small and large intestines. It can hold seven to eight gallons, and is full of microorganisms (bacteria and protozoa). When digesta passes into the cecum, it is subject to microbial digestion, or fermentation.

Sound familiar? The horse's cecum functions much like the rumen in cattle?a large fermentation vat. This is why the horse is able to derive a great deal of ?energy from pasture and hay?the microbes in the cecum and colon break down the fiber, and the resulting VFAs are absorbed from the hindgut. Humans and most other monogastrics don't have a functional cecum, and without a significant source of fermentation, little digestion of fiber can occur. In fact, we primarily eat fiber sources to help maintain our digestive tracts?the fiber mostly passes on through and helps keep us "regular."

But now, why again are horses different from cattle, if the cecum functions much like the rumen? Remember, the rumen is part of the stomach and falls before the small intestine, and the cecum lies at the junction of the small and large intestine. Now, where is the major site of nutrient absorption? The small intestine. Although the fermentation in the cecum is highly efficient, many of the nutrients can't be absorbed there. For instance, the microbes may liberate more nutrients such as protein and amino acids from hay that passed undigested through the upper gut along with the fiber. However, because there is little to no absorption of amino ?acids from the hindgut, that protein will not be used to help meet the horse's amino acid requirements. Again, feeding high-quality hay and feeds will help maximize digestion in your horse's upper gut as well as help ensure he'll receive adequate nutrients to meet requirements.

Although the microbial fermentation in the horse's hindgut does not yield the same nutritional benefits as in the cow's rumen, it does serve several important functions: VFAs from fermentation of ?fiber and other carbohydrates are ?absorbed and are an important source of energy for maintenance or low activity levels. The hindgut is also the major site of water absorption. Some minerals are absorbed from the hindgut, including phosphorus and some electrolytes. The microbes also synthesize several B vitamins, and those ?vitamins are absorbed from the hindgut.

Hindgut Problems
The hindgut can also be a source of ?problems for horses, especially when not managed properly. The microbial populations in the cecum and colon are fairly sensitive to pH, and changes in the acidity of the hindgut can have devastating results in the horse, such as colic. This explains why sudden changes in feed can result in colic in horses.

For example, when a horse gets into the feed room and eats a large quantity of grain, there will be a sudden influx of ?undigested sugars and starch from that grain into the hindgut. Under normal conditions of small meals of grain, most of the sugars and starch are digested and absorbed in the upper gut. But if a horse is allowed to overeat grain or other feedstuffs high in soluble carbohydrates, the sugars and starch can overflow from the upper gut into the hindgut. This causes the microbial population in the hindgut to shift from mostly fiber-fermenting microbes to more starch-fermenting ?microbes. The starch-fermenting microbes produce excess gas and lactic acid, resulting in a decrease in pH, which overall may lead to colic and possibly laminitis.

Another problem in the hindgut is simply due to the architecture of the tube. At one point?the pelvic flexure?the diameter of the colon drastically narrows, and, at the same time, the tube makes a hairpin turn. This area is at high risk for impaction of digesta, and many impaction colics originate at the pelvic flexure. Finally, unlike many other species, the horse's intestine is not held in place by membranes, so it can move about and actually twist around itself and possibly other organs, further increasing the risk of colic.

When horses are in their natural situation, wandering on thousands of acres, grazing throughout the day and moving freely, their digestive systems work fairly well with small amounts of forage moving through pretty much all the time. But with the demands and constraints placed on horses by people, good feeding management is required to keep our horses healthy and comfortable. And the farther we take them from their natural environment, the more management-intensive we have to be to keep them healthy.

Now that you understand how the gut is designed to work, the feeding management rules in the box below should make sense. There are many more feeding management practices and rules for horses than those listed, but again, now that you understand the fascinating equine gut, you will hopefully never have to memorize a rule again.

Feeding-Management Rules

1. Feed small meals often. This helps your horse's digestive tract work most efficiently, as well as reduces the risk of digestive disturbances, such as colic.

2. Feed no more than about 0.5 percent of your horse's body weight in grain per meal (5 pounds for a 1,000-pound horse). This helps reduce the risk of soluble carbohydrate overload to the hindgut. When using feeds lower in sugars and starch than grain, you can increase the amount fed in a meal.

3. Feed at least 0.1 percent of your horse's body weight per day (dry matter) in roughage (10 pounds of hay for a 1,000-pound horse). Adequate fiber is necessary to keep the microbial population healthy and maintain proper hindgut function.

4. Make feeding changes gradually. Any sudden change in feed and hay can cause a pH change and/or shift in microbial population in the hindgut, resulting in digestive disturbances. Minor changes can be made over three to four days, and major changes may need to be spread over a few weeks.

5. Only use feeds designed and labeled for horses. Feeds designed for other species will not meet horses' specific nutrient requirements and may contain substances that are toxic to horses. (See "Why Cattle Feeds Don't Work," on the previous page.)

6. Never feed moldy feed or hay to horses. Horses are more sensitive to many substances than most other species due to their inability to regurgitate. It is also important to maintain stability of the microbial population in the hindgut.
J. Kathleen "Katie" Young, Ph.D., is a consulting equine nutritionist working with Land O'Lakes Purina Feed. Prior to starting her ?consulting business, Sunrise Equine Services, in Lenexa, Kansas, Dr. Young worked at Farmland Industries, first as equine nutritionist and horse feed program manager, and later as a business consultant and professional development trainer for Farmland's local member cooperatives.

Dr. Young earned her bachelor's degree from Missouri State University and her doctorate in equine nutrition and exercise physiology from Texas A&M University. During her stay in Texas, Dr. Young also served as a faculty member in the Equine Science Section of the Animal Science Department, teaching courses in equitation, training and horse management. She also was supervisor and coach of the ?school's equestrian teams and a board member of the Intercollegiate Horse Show Association.

Dr. Young has more than 35 years of ?experience in the horse industry. She started riding as a child in southwest Missouri, first as a barrel racer and later moving into hunters and jumpers. After moving to Texas, Young continued participating in hunter/jumper shows, as well as dressage and eventing competitions, and she has played competitive polocrosse. Dr. Young has worked as a trainer and riding instructor for more than 30 years, and continues to do so in the Kansas City area.

This article originally appeared in the October 2009 issue of Practical Horseman magazine.SaveSave