The Gastrointestinal (GI) Tract

Then there is the matter of the large colon, with its sacculated construction that seems made to order for twisting or strangulating when the pouches become distended by gas during a bout with colic.

There is also the matter of length. If the horse's entire digestive tract were stretched out end to end, it would measure nearly 100 feet. Despite this length, however, food travels through a horse's digestive system quite quickly.

Harold Hintz, PhD, a member of the research staff within the Department of Animal Science at Cornell University in Ithaca, N.Y., says that when indicators (that show the speed of digestion) such as chromic oxide or colored particles are added to hay-grain diets, about 10% of the indicator is excreted within 24 hours; 50% within 36 hours, and 95% within 65 hours. The rate of passage is influenced by the type of feed ingested. Pelleted diets, for example, have a faster rate of passage than hay. Fresh grass also moves more rapidly through the tract than does hay.

In Mother Nature's defense, she never planned for the horse to be domesticated and fed concentrates. Her plan was for the horse to be a grazing animal which would roam over the grasslands, consuming relatively small amounts of forage on a more or less steady basis. Thus, there was no real need for a stomach that could hold large quantities of food.

With that as perspective, it also makes sense to have a capacious hindgut (primarily the cecum and colon) that is the final staging area to break down roughage components and extract nutrients that had passed through the stomach and small intestine without being absorbed. It even makes sense for the cecum and large colon to have a sacculated structure because this tends to slow the passage of food through the area, thus providing more time for a breakdown of fibrous material through microbial activity.

It was man and his molding of the horse to fit a whole new order of life that got in the way of nature's master plan. Instead of allowing the horse to graze at its leisure, man sets rigid feeding schedules in which the horse is asked to consume a specific amount of food in a specified time limit. In addition, man has added concentrated feeds into the equation. These concentrates, if not fed in appropriate amounts and intervals, can cause serious digestive problems for the equine.

In some cases, man's use of the horse calls for a great deal of exercise. In other cases, there is very little exercise. The amount of exercise has an effect on the horse's nutrient needs and, to some extent at least, on the way in which food is digested. Some researchers believe that mild exercise aids in the digestive process, while stressful exercise might impede it. At the same time, the non-exercising horse or the one that is only mildly exercised needs less food than does one that is involved in a heavy exercise regimen.

Unfortunately, man is not always the best custodian of the horse. There are idle animals which ingest far more food than they should, and heavily exercised horses which receive too little. Others are fed only once a day, no matter what their needs might be.

To understand just how the horse's gastrointestinal tract functions and the way in which it responds to various types of food, one needs to begin a journey at the mouth and follow the food on through the entire GI tract, with the trip ending as the waste product that hasn't been absorbed during the trip being excreted along the rectum and through the anus.

Helping to guide us on the journey will be information obtained through research, compiled and shared by Hintz, D. Douglas Householder, PhD, of Texas A&M University, and others.

Let the Journey Begin

The first stages of this trip will involve the foregut, which includes the mouth, esophagus, stomach, and small intestine. The journey begins when the horse uses its teeth to tear grass from the earth or utilizes its lips to pick up pieces of hay or grain from the ground, manger, or feed tub. The food is transferred to the rear of the mouth by the tongue for chewing by the molars and is mixed with saliva. The mixing of the food with saliva is the first step in the digestive process.

The horse's chewing action is primarily up and down, although there is side-to-side movement. The chewing breaks down the food by cracking the outer shell of grain and reducing long-stemmed hay to smaller particles.

When thoroughly masticated, hay will have absorbed about four times its own weight in saliva. Oats will absorb their own weight.

Once the food is masticated, the base of the tongue pushes it past the soft palate and into the pharynx.

The pharynx connects the mouth to the esophagus, which leads directly into the stomach. While the esophagus is efficient at moving food from mouth to stomach, it does have a drawback. Its contractions work in only one direction, which means it has no capability of regurgitating food if the stomach becomes upset or overloaded.

When the stomach reaches about two-thirds of its capacity, food begins passing into the small intestine. This filling and emptying process continues until the horse finishes eating.

Some digestion takes place in the stomach, but not much. There is a small microbial population in the stomach that will initiate some fermentation, and there is also limited enzymatic digestion. Food will remain in the stomach for only about 15 minutes before being passed on through to the small intestine.

Though the food remains in the stomach only a short period of time, its very presence has a direct bearing on the stomach's good health or lack of it.

There are two types of lining in the stomach. On top is the non-glandular or squamous lining, and on the bottom is the glandular lining. The upper portion is where problems can often occur with ulcers because it has little protection against the hydrochloric acid that is produced in the stomach. Research has shown that when horses are fed frequently, with the stomach containing food much of the time, the risk of ulcers is diminished. Frequent feeding means there is food within the stomach to absorb the acid. (See cover story on Ulcers in The Horse of March 1996.)

The type of feed provided for the horse also can have an effect on ulcers. In one study with two groups of ponies, Hintz reports, one group was fed a diet comprised of mixed feed and the other group was fed only hay. The group receiving the mixed feed had a 37% incidence of ulcers, while the group fed hay had zero occurrence.

The next stop on our journey through the GI tract is the small intestine. This is the longest single section of road to traverse. In an adult horse, the small intestine is about 70 feet long and is capable of holding 68 quarts of food or 30% of the capacity of the entire digestive tract. The small intestine is only about two inches in diameter.

The small intestine is arranged in a series of folds and coils suspended from the loin region by a fan-shaped membrane called the mesentery. The first section of the small intestine lies in a U-shaped curve, apparently nature's design to prevent food from being forced back into the stomach if the small intestine becomes distended.

The small intestine is the primary site for protein digestion and amino acid absorption. At least 60-70% of the dietary protein in grain-based diets might be digested by amino acids and absorbed before reaching the large intestine. However, less than one-third of hay protein is absorbed in the upper tract.

The small intestine can be compared to a chemical workshop. Starch that has not already been digested by saliva is changed into maltose by an enzyme in the pancreatic juice. Compound sugars are broken down into simple glucose-like sugar through the action of other enzymes. These sugars then are absorbed through the intestinal walls. They are carried out by capillaries that pass them along to the veins, which in turn convey them to the liver, the equine's largest gland and its principal chemical processing plant.

Most of the soluble carbohydrates are digested in the small intestine. The absorbed end-products of the carbohydrate digestion are glucose and other similar sugars.

The efficient digestion and absorption of carbohydrates in the small intestine are very important. If an inefficient job is done and large amounts pass on to the large intestine (hindgut), there is potential for colic.

The small intestine also appears to be the primary site for fat digestion and absorption. Even though the horse has no gall bladder, diets containing 10-15% fat can be utilized for energy.

Researchers at Texas A&M have conducted a series of studies involving fat in the diets of weanlings and adult performance horses. They have concluded that weanlings fed 10% added fat had higher average daily gain with less total feed intake than a group fed the same ration and amount without fat.

Their studies with performance horses indicated that fat can play a key role in staving off fatigue.

There was a time when it was believed that horses could not utilize fat. However, says Hintz, research has shown that they can utilize it just as efficiently as any other species. In fact, he says, studies at Cornell and other research facilities have indicated that mature horses can tolerate diets containing up to 18% or more of fat.

"For example," he says, "we found that the fat in diets containing 15% beef tallow and 85% alfalfa pellets was highly digestible."

The fat-soluble vitamins A, D, E, and K are also absorbed in the small intestine, as are B vitamins, calcium, and a small amount of phosphorus.

Passage of food through the small intestine takes approximately 60 to 90 minutes. During its passage through the small intestine, the food is liquid, which accounts in part for the relatively rapid passage through the long, folded tunnel.

The Hindgut

Time to move on to the next stop in this journey. We now come to the hindgut. Here the digestive action changes. In the foregut, digestion is primarily the result of direct enzymatic action. In the hindgut, there is, by contrast, a high rate of microbial activity.

The horse's hindgut is comprised of the cecum, large colon, small colon, rectum, and anus. The hindgut is the "work horse" of the equine's digestive tract. It is here that billions of microbes and protozoa produce enzymes that break down plant fiber. The fiber is converted into short-chained, volatile fatty acids that are absorbed and provide the horse with an energy source.

First stop on our journey through the hindgut or large intestine is the cecum. It is approximately four feet in length and can hold 28 to 36 quarts, about 15% of the equine digestive tract's total capacity.

The cecum starts high in the right flank area and extends down and forward to the region of the diaphragm. It is here that the fermentation process begins in earnest. The cell walls of plants are largely composed of cellulose, which is pretty much unaffected by the action of enzymes in the stomach and small intestine.

Because so much of a horse's normal diet is roughage in the form of grass and hay, its intake of cellulose is considerable; thus the need for large fermentation vats such as the cecum and large colon.

The cecum is sometimes referred to as the water gut as its contents are always fluid.

Connecting to the cecum, and our next stop on the journey, is the large colon. It is between 10 and 12 feet in length and has the greatest capacity of any of our "stops" along the GI tract--a total of 86 quarts, or 38% of the tract's total capacity. Continued microbial activity takes place here.

The size and structure of the cecum and large colon are designed to slow the passage of food through this portion of the GI tract so there is time for the microbes to do their work in breaking down plant fiber. Rate of passage through the cecum and large colon is 36 to 72 hours.

Next is the small colon, which is 10 to 12 feet long and four inches in diameter. It has a capacity of about 16 quarts, or 9% of the tract's total capacity. It is in the small colon that remaining moisture is resorbed (the cecum is the primary site of net water absorption) and the contents become solid, normally molded into fecal balls.

The small colon empties into the rectum, which is about one foot in length, and it, in turn, conveys its contents to the anus for excretion.

The billions of bacteria and protozoa in the hindgut are absolutely essential to the horse, says Joe Pagan, PhD, of Kentucky Equine Research. The by-products of the microbial activity provide the horse with a source of energy and micro-nutrients.

The microflora that reside in a horse's hindgut, while being capable of breaking down plant fiber in efficient fashion, nevertheless have a chink in their armor. They are quite susceptible to changes in diet, which means that any dietary changes should be made gradually rather than abruptly.

Earlier in our trip, we mentioned the importance of efficient absorption of carbohydrates by the small intestine. When excess amounts of soluble carbohydrates reach the hindgut, the fermentation process produces not only volatile fatty acids that are beneficial, but also lactic acid, which is not beneficial. An excess of lactic acid can set the stage for a bout with colic.

The problem can have its origin when large quantities of starch are fed to the horse in a single meal, reports Pagan. When that occurs, the small intestine's ability to digest and absorb the starch is overwhelmed and a substantial amount passes into the large intestine. At that location it is rapidly fermented to lactic acid by bacteria. An increase in lactic acid lowers the pH of the hindgut, which can result in the killing of other bacteria and lead to the release of endotoxins into the blood.

"The combination of these two factors," reports Pagan, "may lead to colic or laminitis. Therefore any means available to stabilize the fermentation of the hindgut should be explored, particularly in horses fed high-grain diets."

Pagan is of the opinion that this stabilization, along with increased fiber digestion and phosphorus absorption, can be improved by adding live yeast culture to the equine diet. Part of the basis for the theory is that yeast culture will stimulate cellulytic bacteria in the cecum and colon.

In one of his experiments, Pagan fed two horses a diet that included live yeast culture, while two were fed an identical diet, minus the yeast culture. The feces and urine from the horses were collected and analyzed. The collection periods were for five days after a preliminary feeding period of at least three weeks. His conclusion was that yeast culture supplementation increased nutrient digestibility in the hindgut.

In a second experiment, this one aimed at determining whether live yeast culture had an effect on phosphorus digestibility, he found that estimated true phosphorus digestibility was increased by 22% in the diets supplemented with yeast culture.

Pagan explains what happens with phosphorus in the hindgut this way:

"Research indicates that the major site of phosphorus absorption in the horse is in the large intestine. This is different from most other species, which absorb the majority of their dietary phosphorus in the small intestine. Because of his anatomical difference, phosphorus digestion in the horse is dependent on the microbial activity in the hindgut since much of the phosphorus in the horse's diet is in the form of phytin phosphorus, which must be exposed to phytase before it is available for absorption by the horse.

"Phytin phosphorus is largely unavailable to monogastric animals such as poultry and swine, whereas ruminants are capable of utilizing it well. The species difference is explained by the presence of the enzyme phytase produced by rumen microorganisms, which hydrolyzes the organically bound phosphorus and renders it available for absorption. Apparently, microorganisms in the horse's hindgut also produce this enzyme, allowing horses to utilize a good deal of the phytin phosphorus. Because phytase production and ultimately phytin phosphorus digestion depends on these microorganisms, it seems reasonable that increasing the fermentative activity in the horse's hindgut would result in an increase in phosphorus digestibility.

"Yeast culture supplementation may also be beneficial in stabilizing disturbances in fermentation resulting from carbohydrate overload in much the same manner as it affects the ruminant environment."

Where Nutrients Are Absorbed

Throughout our journey, we have discussed at what points certain nutrients are digested and absorbed. The following information from Hintz summarizes the basic nutrients and the sites of digestion and net absorption in the equine:

  • Protein--60-70% absorbed in the small intestine and 30-40% in the hindgut.
  • Soluble carbohydrates--65-75% in the small intestine and 25-35% in the hindgut.
  • Fiber--15-25% in the small intestine and 75-85% in the hindgut.
  • Fats--Though no specific percentages were listed as being available, the primary absorption site is the small intestine.
  • Calcium--95-99% in the small intestine and 1-5% in the hindgut.
  • Magnesium--90-95% in the small intestine and 5-10% in the hindgut.
  • Phosphorus--20-50% in the small intestine and 50-80% in the hindgut.
  • Vitamins--Though no specific percentages are listed as being available, the primary absorption site is the small intestine.

While many of the B vitamins are absorbed by the small intestine, considerable quantities of B vitamins are also synthesized by the microbes in the hindgut and absorbed from it.

Householder of Texas A&M believes, however, that certain B vitamins, such as thiamine, are not absorbed in sufficient quantities to meet the requirements of hard-working horses and that supplemental thiamine should be added to the diets of these animals.

Feeding Horses

After concluding our trip through the horse's digestive tract, one is left with the feeling that it is both complicated and efficient. The key for horsemen is to recognize the complexity, to avoid problems, and to plan feeding programs that take advantage of its efficiency. After all, a great many factors can influence the digestibility of nutrients by the equine.

An approach advocated by Householder is to feed horses according to class.

He uses the following approach in determining what percentage of a horse's body weight should be fed in the form of forage and concentrate for various classes:

  • Mature, but idle--2.5-2% of body weight in the form of forage and 0 to 0.5% in the form of concentrate.
  • Late gestation mares--1.0-1.5% of body weight for forage and 0.5-1.0% in the form of concentrate.
  • Lactating mares--1.0-2.0% of body weight for forage and the same, 1.0-2.0%, in concentrate.
  • Working horses--0.8-2.0% of body weight in forage and 0.5-2.0% in concentrate.
  • Weanlings--0.5-1.0% of body weight in forage and 1.5-3.0% in concentrate.
  • Yearlings--1.0-1.5% of body weight in forage and 1.0-2.0% in concentrate.

There is another important factor that must be considered when deciding how much food to launch on its journey through the horse's digestive system--the animal's weight.

Obviously, the most accurate way to determine a horse's body weight is to weigh it on a scale. However, a scale is not available on every farm. In those cases, one must depend on visual evaluation, weight tapes, or body measurement formulas.

One should use care in eyeballing a horse and estimating its weight. In one study, 88% of horsemen, many of them professionals, underestimated the weight of five horses on average by 180 pounds.

Unfortunately, many horsemen still rely on the old three-pound coffee can to determine how much concentrate is fed at each feeding and the "flake" approach in determining the amount of hay. The problem with the three-pound coffee can is that it cannot determine how much the contents truly weigh.

For example, if the can is filled with oats that weighed in at 32 pounds per bushel, the three-pound coffee can will hold two and one-half pounds of oats. However, if the oats weighed in at 38 pounds per bushel, the oats in the can will weigh four pounds. A three-pound coffee can filled with corn might also weigh four pounds.

The same can be true of hay. To simply feed two flakes from a bale two or three times a day does not give an indication of actual poundage. Two small flakes of second cutting alfalfa, for example, might weigh more than two large flakes of timothy.

Horsemen must also always bear in mind that feeding a large quantity of concentrate at one feeding can have serious repercussions when the feed makes its journey through the digestive tract. A stomach overloaded with concentrate that is high in carbohydrates pushes it quickly on to the small intestine. The small intestine can't properly absorb the contents because more keeps coming, so an excess amount winds up in the hindgut, where lactic acid is formed. Therefore, a variety of problems might follow.

The rule of thumb advocated by Householder is to never feed more than .75% of a horse's body weight in concentrate at any one feeding. This would mean that a 1,000-pound horse should never be fed more than 7.5 pounds of concentrate at a single feeding.

After completing our journey through the digestive tract, it also becomes obvious that one should space out feedings over a 24-hour period to give the tract equal time to digest each meal, rather than, for example, feeding the horse at eight each morning and four each evening, or, worse yet, once each day.

Then, there is the matter of changes in diet. All changes should be over a period of time in order to give the digestive tract an opportunity to adjust.

For example, if a horse is low in weight for some reason, one should increase food intake over a 10-day to two-week period. Changes in the type of concentrate and hay also should be made over a period of time rather than abruptly.

Horse owners should always remember that the horse is a non-ruminant herbivore which needs forage to maintain a healthy digestive tract and body. Householder recommends that a minimum of .75 to 1% of a horse's body weight be fed in the form of long-stemmed roughage. This is about what a horse will eat if allowed to graze at will. It is up to man to duplicate that when the horse is confined to a stall or non-grass lot.

Finally, the digestive tract cannot function well without adequate water. The mature, idle, 1,000-pound horse will drink approximately 10 to 12 gallons of water per day. Lactating mares and working and performance horses will consume even more.

Combining a common sense feeding approach based on knowledge of the route food travels on its journey through the GI tract can stave off many problems for the horse and its owner and result in an equine which is healthy and vigorous.

About the Author

Les Sellnow

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.

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