The Contented Colon
by: Karen Briggs
January 2000

If you have a veterinary textbook somewhere on your shelves, chances are you've seen one of those photos of a sick horse's innards-miles and miles of wet, purplish loops of intestine, spilling out in all directions. The poor equine posing for that photo had no doubt expired by the time the flash went off, and chances are he only ended up being an intestinal poster boy because he'd been opened up by veterinary surgeons in a vain attempt to save him from a case of severe colic.

What's my point? Well, the very thought of the sheer size and volume of the equine gastrointestinal tract makes it tough to visualize, much less understand. It's huge, it's daunting, and it's persnickety to boot. But if you'd rather your horse didn't end up on the operating table, opened up from stem to stern, it's vital that you have an idea of how his digestive system functions, and what you can do to help keep it happy. Let's take a look inside.

Inside The Gut

At first glance, the design of the equine digestive tract looks like an evolutionary mistake. The stomach, for example, is surprisingly small for an animal the size of the horse, with a capacity of only about two to four gallons (7.5 to 15 liters). In contrast, the small intestine can reach an amazing 70 feet (about 22 meters) in length if uncoiled and stretched out, with a diameter of three to four inches and a capacity for 10 to 12 gallons of material. Compared with what we know about the GI tract of other animals, the equine gastrointestinal tract seems strangely out of proportion. But from Mother Nature's point of view, everything's just fine. In his wild state, the horse never expected to ingest large quantities of food at one time; his digestive system is optimally designed for his wandering, grazing lifestyle, which introduces small amounts of food to his system almost constantly.

If we tour through the equine digestive system the way a particle of food does, we can discover a number of other important links between his physiology and his diet.

When a horse tears off a mouthful of grass with his teeth, or uses his talented lips to pick up hay or grain from the ground or a feed tub, the food first is transferred to the back of his mouth by the tongue. There, it is ground by the horse's wide, flat molars and mixed with saliva (which almost immediately launches the digestive process by beginning to break down starches). A thoroughly chewed mouthful of oats absorbs its own weight in moisture; hay absorbs about four times its own weight.

From there, the base of the tongue pushes the food past the soft palate and into the pharynx, the opening to the esophagus. The esophagus, a flexible tube that leads down the neck to the stomach, pushes the food along by a series of muscular contractions. These contractions move in only one direction, meaning that what goes down, for better or worse, stays down. Horses have no capacity for reverse peristalsis (vomiting).

Food particles then hit the liquid acid of the stomach, but surprisingly little digestion goes on in the stomach itself. There is a small microbial population that initiates a bit of fermentation, and there also is some enzymatic action. Since food remains in the stomach for only 15 minutes on average before being pushed on to the small intestine, there isn't time for any major food breakdown. The stomach's main job is to liquefy everything, then pass it on to the small intestine as soon as it gets about two-thirds full. This process continues as long as the horse keeps eating.

Although food remains in the stomach only for a brief interval, its presence (or absence) has a direct bearing on your horse's health. The upper, inner portion of the stomach's lining is made up of a non-glandular, squamous cell layer, which is vulnerable to the hydrochloric acid the stomach secretes. When the stomach has food in it most of the time (as it would if the horse were grazing free-choice), the food tends to absorb the acid and keep it from splashing this upper layer. Horses which are fed infrequently (one or two large meals a day, rather than several smaller meals) are more at risk of stomach ulcers, which can result from exposure to stomach acids. Forage does the best job of absorbing these acids. Horses fed a hay-only diet typically have a zero incidence of ulcers, while those on a mixed diet are more at risk.

The next stop on the tour is the small intestine, a coiled and convoluted tube suspended from the loin region by a fan-shaped membrane called the mesentery. The first section of the small intestine, the duodenum, is shaped like a U-turn, presumably to prevent food from being forced back into the stomach if the small intestine should become distended. The small intestine can hold up to 30% of the GI tract's total capacity, and it is the primary site for protein digestion and the absorption of amino acids (although grains are processed more thoroughly here than is forage).

Inside the small intestine, enzymes go to work to break down food materials. Starch that has not already been digested by saliva is converted to maltose, and other complex sugars and carbohydrates are broken down to simple-sugar forms so that they can be absorbed through the intestinal walls. (They then are transported by capillaries into the blood and eventually carried to the liver, the horse's major chemical processing plant.)

The small intestine also is the primary site for the digestion and absorption of fats. Most animals use gall bladder secretions to break down fats, but horses have no gall bladder; nonetheless, they seem able to utilize diets containing 10-15% fat very efficiently for energy and weight gain. Finally, the fat-soluble vitamins A, D, E, and K are absorbed in the small intestine, as are calcium, some phosphorus, and B vitamins. On average, it takes 60 to 90 minutes for the liquefied food to pass through the whole length of the small intestine on its way to the hindgut.

The last portion of the small intestine, the ileum, leads to the final stop of the gastrointestinal tour, the hindgut, made up of the cecum, large (or ascending) colon, small colon, rectum, and anus. Here's where the bulk of the hard work of digestion is done. In the hindgut digestion is largely microbial, rather than enzymatic. In other words, it's performed by a population of billions of symbiotic bacteria which efficiently break down plant fibers into simpler compounds called volatile fatty acids (VFAs), which can be absorbed through the gut wall. Not only are these bacteria a natural part of the digestive process, they're essential to it.

The cecum, approximately four feet long and with a capacity of seven to nine gallons, is the site of the first stage of hindgut digestion. The cecum begins high in the horse's right flank area, and extends down and forward toward the diaphragm. Plant fibers, composed of cellulose and other hard-to-digest molecules, pass through the stomach and small intestine unaffected by enzymes, but when they hit the "fermentation vat" of the cecum, the population of bacteria makes short work of them, usually breaking them down within about five hours. The size and structure of the cecum (the physiological equivalent to our appendix, but far more useful) are such that the passage of food is slowed in order for the microbes to do their job.

From the cecum, the partially digested food moves on to the large colon, where fermentation continues. The large colon is almost 12 feet in length, on average, and holds an impressive volume of 14 to 16 gallons, or 50-60 liters, of food (about 38% of the GI tract's total capacity). The large colon also is where food dwells longest-between 36 and 48 hours. It has a "sacculated" construction that resembles a series of pouches. This can facilitate the breakdown of large quantities of fibrous material, but also can become a risk factor when the pouches become distended with gas during a bout of colic, as they seem custom-made for twisting and even strangulating their own tissues.

Once the food has been processed thoroughly in the large colon, it moves on to the small colon, another 10 to 12 feet long, but smaller in diameter (about four inches). The vast majority of the nutrients have been absorbed by this time, and what's left in the gut is whatever the horse cannot digest or use. The main function of this portion of the hindgut is to reclaim excess moisture from the remaining material. By the time feed leaves the small colon, it has become solid again and has been molded into fecal balls. The small colon empties into the rectum, and some 36 to 72 hours after it began its journey, the waste material from a horse's meal is expelled as manure through the anus.

Digestive Delicacy

The equine gastrointestinal tract functions very well under normal conditions. But as every horseman knows, it's also extremely sensitive and easy to upset. Any sudden change in diet, for example, can severely compromise the population of gut bacteria so essential for fiber digestion-and when these bacteria start dying off, the horse can be at risk of colic, or at the very least, not getting all the nutrients out of his feed. Thus, it's always best if feed changes are made gradually, over a period of a couple of weeks.

Keeping the gut microflora happy can be difficult if your horse is under stress. If he has been shipped a great distance, if he recently has suffered an illness or a surgery, if he is a high-performance racehorse or show horse, if he recently has been moved to a new herd situation, if he's a foal who's just been weaned, if he's been on antibiotics…any of these scenarios, and many more, can predispose his gut bacteria to dying off. Without a healthy population of microflora working in the cecum to process fiber, most horses will find their ability to digest their meals compromised. They might drop weight, look listless and dull, suffer from anorexia (loss of appetite), and run an increased risk of colic.

Another trigger for digestive upset can occur when the horse receives a large, carbohydrate-rich meal (typically, one that is light on forage and heavy on grain). Under those conditions, the small intestine might not be able to process and absorb all of the nutrients completely before the meal is moved on to the hindgut by involuntary muscle contractions. Carbohydrates processed by the large intestine spell trouble. When excess amounts of soluble carbohydrates reach the fermentation vat of the cecum, they are broken down to produce not only volatile fatty acids, but also lactic acid. An increase in lactic acid lowers the overall hindgut pH level, which in turn can make the environment hostile for the gut bacteria. The bacteria begin to die off, and in the process can release endotoxins (poisons). Between the endotoxins and the lactic acid, the stage is set for colic or laminitis. Suddenly the old horseman's rule of feeding small amounts, often, begins to make a lot of sense, particularly if your horse is on a high-grain diet.

What horses really need in their diets for good gastrointestinal health is fiber-and lots of it. Millions of years of evolution have optimized the equine digestive tract for extracting nutrition from gritty, fibrous grasses and weeds. When it's kept busy doing that, the system generally operates without a hitch. The further we deviate from the horse's natural pattern of grazing and foraging, the more likely that we'll present his digestive tract with a challenge it can't handle. That includes diets that contain more than 50% grain by weight. Case in point: in 1995, I spent a year managing a little riding school in Bermuda. I quickly learned that while chronic obstructive pulmonary disease (COPD or heaves) was practically non-existent on the island (thanks to the open-air lifestyle the horses enjoyed), the incidence of colic was alarmingly high. The reason? Because Bermuda is so small (only about 20 miles long and less than a mile wide), it has to import all of its horse feed by cargo ship. Hay is extremely expensive to import, running more than $20 U.S. a bale at the time I was there, while grain, being less bulky, was considerably less expensive to ship. As a result, most islanders fed their horses large amounts of grain and a negligible amount of fiber, in many cases amounting to only one or two flakes of hay per day. The shortage of fiber in the Bermudian horses' diets set them up for gastrointestinal disaster. A great many of them died because of it. (My solution, by the way, was to place the horses in my care on large quantities of soaked beet pulp, which provided them with highly digestible fiber at a reasonable price. I'm pleased to say not one of the 16 school horses in my care colicked while I was in residence.)

As a general rule of thumb, horses should take in between 1.5% and 3% of their own body weight in feed every day-and at least half of that (and often much more) should be forage of some kind. Whether it's pasture, hay, or some other form of roughage isn't as important as the quantity, because the horse's gut literally needs that amount in order to stay in good digestive health.

So why feed grain at all? In the wild, horses have no access to concentrated forms of carbohydrates-and little need for them, because they are not doing "work" in the sense that we humans demand. When we domesticated the horse, we began to ask him to expend energy over and above what he normally would do in the course of his wild day, and grains help provide the fuel he needs to perform for us. In addition, we bred horses to be larger, stronger, faster, more elegant-and often, less hardy, and more dependent on high-energy concentrates to maintain a healthy body weight.

Nonetheless, grain always should be considered an optional add-on to the diet. It should be fed as necessary only to supplement the nutrition provided by his forage, and in accordance with his condition, his metabolism, and the amount of work expected of him.

You can help make grain meals more digestible by feeding them in small amounts, and by choosing grains that are already partially processed, by rolling, cracking, or crimping the seedcoats. Pelleted and extruded feeds, which include grains ground finely and bound together with a binding agent, are also "pre-processed" and easier to digest. Feeding partially processed grains rather than whole ones is a particularly good idea if you're dealing with a horse whose digestive ability is compromised by age (very old or very young), poor dental health, recent illness, surgery, or any number of other stresses.

Likewise, if you suspect your horse's digestive talents are operating at less than optimum, offer him hay that is soft and leafy, not coarse, stemmy, and tough. Or consider adding beet pulp or soaked hay cubes to his diet to provide him with extra fiber.

Probiotics can be a useful addition to your horse's diet if you're trying to help him recover from a stressful episode. A probiotic is a culture of live microbes, and/or their fermentative metabolites that can help stimulate the growth of "good" gut microflora. In turn, this should help the horse combat stress, absorb nutrients from his diet, and fight disease. Improved digestive efficiency (as indicated by weight gain, less undigested matter in the manure, and a generally improved outlook) has been reported as a result of the use of probiotics. (This is much like your doctor recommending that you eat yogurt with live cultures when you are taking antibiotics.)

Consider adding a probiotic product to your horse's feed if he

    * is under stress from showing, racing, or shipping
    * is a hard keeper or has a poor appetite
    * has loose manure or chronic diarrhea
    * has recently undergone a dietary change (or has just been weaned)
    * has large amounts of undigested material in his manure
    * shows other signs of digestive distress, such as recurring colic
    * is recovering from surgery
    * has received recent treatment with antibiotics or anthelmintic (deworming) drugs
    * has recently undergone a change of environment, such as being introduced to a new herd
    * is struggling to adapt to extremes of temperature (a heat wave or cold snap)
    * is over 18 years old.

Depending on the format and dosage you choose, probiotics can be used preventatively, on a short-term basis, to treat specific problems, or at times when you expect increased stress levels (for example, before you ship your horse long distances). Some probiotics can be fed routinely as a prophylactic; simple brewer's yeast, for example, is an inexpensive probiotic addition to the diet that not only helps foster a healthy population of gut microflora, but also is a beneficial source of B vitamins.

Beware of Beaches

If you live in an area that has very sandy soil, you'll no doubt be familiar with the dreaded effects of sand colic (see next month's "Back to Basics" article). When horses ingest sand particles along with their feed (either while grazing or while eating hay or grain off the ground), the sand tends to accumulate in the gut, forming an interior "beach" that can't be budged by normal intestinal contractions. Eventually, the sand can form an impaction (blockage) in the large or small intestine-and that can mean big trouble for your horse. Preventing sand from building up inside your horse is a constant challenge. You have to be careful not to let him eat from the ground, and in some cases you might have to restrict his grazing and turn-out. Of the various products purported to help purge sand from the system, there's good documentation only behind psyllium, a natural plant mucilage that binds to the sand in the gut and moves it along toward the exits. (Bran, which is also sometimes recommended for sand removal, actually tends to float on top of the blockage and leaves it untouched.) Ask your veterinarian for recommendations on the best routines with which to feed psyllium. Some preparations are meant to be fed daily; others, on an off-again, on-again schedule.

Toxic Plants

In order to keep your horse's gastrointestinal tract in good shape, you'll also want to ensure he never comes into contact with any toxic plants or materials. This might seem painfully obvious, but it's doubly important when you're dealing with horses. Unlike your household pets, they can't vomit up a poison they've ingested. Read up on the toxic plants indigenous to your area and do a careful sweep of your pastures and barn area every so often, looking for offenders. Many an owner has been caught unaware by an ornamental yew or an oleander bush; don't depend on the bitter taste of these plants to warn away an inquisitive foal or bored pasture ornament.