The Equine GI Tract: Down the Hatch
- Aug 1, 2009
If the horse's gastrointestinal (GI) tract were a highway, no self-respecting engineer would take credit for its design. With its twists and turns and complicated passageways, it is a road that appears designed for accidents to happen. And happen they do.
We'll take a trip along this rather convoluted roadway with the help of sources who have studied the structure and the complicated workings of the equine digestive system and have reported on them in various presentations and interviews.
The sources on which we have relied most strongly are Judy Marteniuk, DVM, MS, of Michigan State University; Erin Malone, DVM, Dipl ACVS, of the University of Minnesota; a research team from the University of California, Davis; Jay Altman, DVM, who researched, developed, and is marketing a product designed to facilitate the equine digestive process; and a group of specialists from The Ohio State University Extension.
To provide an unimpeded journey, we will simply announce in advance that the information presented is an amalgamation of facts from the above sources, combined with some basic textbook outlines.
All trips should get under way only after studying a road map for an overview, so that we know where we are headed and how long it will take to get there. So it is with the GI tract. Our map tells us that our trip will begin in the lips and mouth and travel along the esophagus, which is normally about 4.5 feet in length. From there we travel through the stomach to the small intestine, which is a complicated route that is some 70 feet in length with twists and turns along the way. Then we arrive at a point where the road broadens. This is the cecum, which is about 4 feet long. The road narrows somewhat when we reach the large colon, which we'll travel along for 10 to 12 feet before arriving at the small colon, which is also 10 to 12 feet in length. At this point we approach the final stage of our journey--the rectum--which is about a foot long. Finally we arrive at our end destination, the anal opening. The roadway from the cecum to the anus is described under the umbrella road sign large intestine. In total, it is about 29 feet in length.
During the early stages of the journey, we will travel--relatively speaking--at high speed, and during the latter stages we will go at a more leisurely pace.
Let's begin the journey and investigate the thoroughfare and scenery.
It is in the mouth where food begins a trip that can last for a couple of days. The horse uses its teeth to chew the food and mix it with saliva until it reaches a moist bolus (soft chewed mass) stage that is easily swallowed. Essential to the production of saliva is water. A horse produces up to 10 gallons (85 pounds) of saliva in a day. Without an appropriate intake of water, the food could cause a "wreck" from the start, especially if it is a dry feed (low in moisture content).
When on pasture, the horse uses a combination of the lips, tongue, and teeth to grasp and maneuver the grass into the mouth, then he masticates (chews) the grass with a sweeping, grinding motion. To facilitate this motion, the horse's upper jaw is wider than the lower jaw.
The average horse makes about 60,000 jaw sweeps (chews) per day when grazing. This number diminishes when the horse is eating hay or other dry feed. In either case, chewing can produce uneven tooth wear. When that is the case, the horse cannot properly masticate the food, and it can cause problems during its trip through the digestive system. Proper dental care is essential to good digestion.
Once the horse has finished chewing, the food moves into the esophagus, and it travels directly into the stomach. The esophagus is something of an enigma. It is a simple muscular tube, but peristalsis--waves of muscle contractions that move food through the GI tract--send the food powerfully in only one direction. Between peristalsis and the muscles of the cardiac sphincter, the opening leading into the stomach, it is almost impossible for the horse to force anything back toward the mouth. In other words, the horse's GI tract is not designed to allow the horse to vomit.
In the stomach we run into another problem. Compared to the size of the horse, the stomach is very small. It comprises only about 10% of the horse's digestive system and has a capacity of between 8 and 17 quarts. It is designed to take in small quantities of food over a sustained period of time. This was all well and good when horses ran free and had little to do but graze all day.
However, under domesticated circumstances, this rarely happens. Instead, many horses are fed on a regular--and sometimes irregular--basis once or twice each day. Many horses eat large quantities of forage or concentrates at each feeding. This poses problems for the stomach, which must do a good job in the initial stages of digestion if the food's trip through the GI tract is to be smooth and unimpeded.
Food travels this part of the GI tract trip at speed. Sometimes food leaves the stomach 15 minutes after arriving when the horse consumes a large meal. While in the stomach, the food is mixed with pepsin--an enzyme utilized in the digestion of protein --and hydrochloric acid, which helps break down solid particles.
A key reason for feeding the horse small quantities of food frequently instead of one or two large meals is that the stomach does not do well when it is empty. There are strong acids at work in the stomach, but when there is a regular flow of food, the stomach acid is put to positive use in the digestion of fats and amino acids. However, when the stomach is empty, the acid sometimes attacks the unprotected nongladular, or squamous, cells in the area where the esophagus enters the stomach (the upper portion of the stomach). This irritation can result in the horse developing gastric ulcers.
At about 70 feet long and with a capacity of approximately 12 gallons, the small intestine comprises 28% of the horse's digestive system. It is in the small intestine of the horse that serious digestive processes take place. The intestine itself secretes some enzymes to facilitate the process, but the prime supplier is the pancreas. It provides enzymes to the small intestine that break down proteins into amino acids. At the same time, the liver adds lipases and bile to emulsify fats and to suspend the fats in water. Because the horse does not have a gall bladder to store it, bile from the liver travels directly into the small intestine.
Once the digestive process in the small intestine reaches a state of completion, the nutrients are absorbed through the walls of the small intestine and carried off by the bloodstream. Between 50% and 70% of carbohydrate digestion and absorption--and almost all amino acid absorption--occurs in the small intestine. In addition, vitamins A, D, E, and K are absorbed in the small intestine, as are some minerals, such as calcium and phosphorus.
The trip through the small intestine is also quite rapid--somewhere between 30 to 90 minutes. Traveling at speed, however, is not an advantage. The faster the food moves through the small intestine, the less time there is for the enzymes to perform their digestive tasks.
It is in the five-part large intestine that the GI tract trip becomes more leisurely. As mentioned above, the large intestine is comprised of the cecum, large colon, small colon, rectum, and anus.
After leaving the small intestine, the food is on a broader highway. While the cecum is only about 4 feet long, it has a capacity of 26 to 40 quarts of food and water. It is in the cecum where hay and grass that weren't digested in the small intestine are broken down.
Again we run into a design problem. First, the cecum is an odd-shaped organ, with the entrance and exit being one and the same. Food enters through the top entrance, is processed, then exits through the same opening.
The leisurely trip through the cecum can take as long as seven hours, as the bacteria and microbes that set up shop there do their job via fermentation. Vitamins and fatty acids that result from the fermentation process are absorbed in the cecum.
The microbes that break down the food are specific for the process. That means that if you change the diet of a horse, it might take up to three weeks or more for the cecum to develop a microbial population to handle the new feed.
To prevent problems when you have to change feed, extension specialists at The Ohio State University suggest:
- Week 1 Feed a mix of three-fourths of the old ration and one-fourth of the new ration.
- Week 2 Feed a mix of one-half of the old ration and one-half of the new ration.
- Week 3 Feed a mix of one-fourth of the old ration and three-fourths of the new ration.
- Week 4 Feed all new ration.
It is important the horse has access to free-choice water to prevent impaction from occurring in the lower part of the cecum. It is estimated that a 1,000-pound idle horse should consume a minimum of 10-12 gallons of water per day. A working horse should consume even more.
The trip's pace becomes even more leisurely when food reaches the large colon, where it might remain for 48-65 hours. Microbial digestion continues, and most of the nutrients from that digestive process are absorbed in the large colon.
We run into yet another design problem in the large colon: It consists of right and left ventral colons and the dorsal colon. The ventral colons have saccular construction, which means that a series of pouches are involved. The design is aimed at efficient digestion of large amounts of fibrous matter, but the pouches or sacs can become twisted or filled with gas during the fermentation process. The result in either case can be a serious case of colic. In gas colic, the horse exhibits pain as the gas stretches the gut wall. A twist might require surgery.
By the time food reaches the small colon the trip is nearly over. The horse has taken up almost all of the absorbable nutrients. What remains is matter that is not digestible. The prime function of the small colon is to reclaim excess moisture and return it to the body. In the process, fecal balls are formed, which can be passed through the rectum and are expelled out the anus.
The entire trip that starts with ingesting food and ends with waste material being expelled can take from 36 to 72 hours for the average horse.
The horse's GI tract may appear to have been designed by committee, but owners can take steps to negate some of the inherent engineering problems, our sources point out. Horses should have access to an unlimited water supply. The owner should offer food frequently throughout the day, and he or she should change diets gradually. And finally, the owner should observe the horse frequently to detect any signs of digestive problems when they are in the very early stages.
About the Author
Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.
POLL: University Equine Hospitals