Food for Thought: Details of the Equine Digestive Tract

Grass, hay, and grain go in one end of the horse and what's left comes out the other...what more does anyone need to know about the digestive tract? For owners who want to keep their horses healthy, the answer is, "Plenty!"

Horses are herbivores, or plant eaters. Unlike cattle and many other cud-chewing herbivores, horses are not ruminants. The horse's digestive system is made up of the foregut (stomach and small intestine) and the hindgut (cecum and colon). Each part has an important function, and each can also be the site of problems ranging from the slightly troublesome to the deadly serious. An understanding of the structure and function of each section of the system can help horse owners keep their equine charges free of digestive upsets.

MOUTH Digestion begins in the mouth as horses chew their feed, grinding it into smaller pieces and moistening it with saliva. Amylase, an enzyme in the saliva, begins the process of breaking down carbohydrates. Saliva also helps food travel smoothly through the esophagus, the four-foot-long tube leading to the horse's stomach. Food that is too dry may stick in the esophagus, contributing to a condition called choke. While not the same as choking in humans, this is still a serious situation that might require veterinary treatment.

What owners can do: To be sure the horse can chew properly, schedule dental checkups once or twice a year to smooth any jagged edges and make sure the horse's teeth are properly aligned. Owners of choke prone horses can soak feeds, or add water to grain and hay before feeding. All horses should have a constant supply of clean water.

STOMACH Swallowed food moves down the esophagus to the stomach, a relatively small organ with a capacity of only two to four gallons. The organ's limited size is well suited to processing a continuous supply of food, such as when the horse is grazing or picking through hay. A large grain meal, however, may overfill the stomach, causing distention, discomfort, and signs of colic. Long periods with nothing to eat can also cause problems because the stomach continues to secrete gastric acid even when it is empty. Without the buffering function of saliva, which is produced only while the horse is chewing, digestive fluids can cause ulceration of stomach tissues. Although there is little absorption of nutrients at this location, digestion of protein begins in the stomach through the action of pepsin and hydrochloric acid. The stomach also regulates the rate at which feed passes into the small intestine.

What owners can do: Stay close to a natural feeding pattern by allowing the horse to graze or eat hay as continuously as possible. Rather than offering one large meal, split daily grain rations into two or more small feedings of less than five pounds each.

SMALL INTESTINE This tube-like organ can reach 70 feet in length, and food usually takes from one to eight hours to pass from one end to the other. Various digestive enzymes break down protein, fat, and carbohydrates, allowing nutrients to be absorbed by the blood. The makeup of this enzyme mixture changes in response to dietary modifications, with several days required to make the adjustment. Sudden variations in the type or amount of feed can result in less than optimum feed breakdown, keeping the horse from getting the maximum benefit from what has been eaten. Ideally, most of the starch portion of the diet is digested in the small intestine, leaving very little except fiber to pass into the large intestine. If the horse has eaten an extremely large grain meal or a great quantity of fresh grass, the digestive ability of the small intestine may be overwhelmed, resulting in a significant amount of starch being passed to the large intestine. Problems in the small intestine include hypermotility (spasmodiccolic); twisted sections that cut off circulation and passage of food; and intussusception, a condition in which part of the intestine becomes telescoped upon itself.

What owners can do: Feed small grain meals of no more than five pounds. Monitor consumption of fresh grass, especially for animals that are being turned out to pasture after long periods of stalling. Introduce new feeds gradually by mixing a handful of the new ingredient into regular feed and increasing by small quantities at each successive meal until the full amount is given. Make changes in hay or forage the same way. This method allows the intestine to adapt slowly to the modified diet, a process that should take about 7 to 10 days.

CECUM Bacteria, protozoa, and fungi in the cecum aid in the fermentation of dietary fiber, producing volatile fatty acids, an important source of energy. The process also gives off enough heat to keep the horse comfortably warm in chilly weather. Cecal microbes synthesize vitamin K and the complex of B vitamins. Excess starch that is not digested in the small intestine accelerates cecal fermentation. This causes overproduction of gas and lactic acid, and severe abdominal discomfort may follow. Changes in pH disturb the microbial balance within the cecum, leading to the production and absorption of toxins, and the result is often laminitis.

What owners can do: Make every effort to avoid upsetting the balance of microorganisms in the hindgut. Any change--moldy feed or hay, schedule variations, travel, stress, deworming, illness, use of antibiotics--can be a threat to digestive health, so these changes should be minimized or made slowly so that the organs of digestion have a chance to adapt. A course of probiotics, preparations designed to keep the microbial population of the cecum vigorous, may be given during stressful times. A veterinarian can advise on the use of probiotics.

COLON Mainly a site of fluid absorption, the colon can also be a source of colic pain if material stops moving freely. The colon makes two tight folds or turns where its contents sometimes become impacted, leading to a buildup of gas and possibly twisting. Food moves slowly through the hindgut, completing the transit in about two days. Under normal circumstances, indigestible portions of the feed are passed from the body as manure.

What owners can do: Build the horse's ration around high-quality roughage, adding concentrated feeds only as needed to meet the demands of reproduction, growth, or performance. Provide a constant supply of water. Follow a regular schedule of deworming, dental care, and exercise. Careful management will go a long way toward avoiding digestive problems.

Shivers in Horses

Shivers is a fairly uncommon equine condition characterized by tremors and exaggerated flexion of the hind limbs that is most noticeable when the horse is backed or its hind limbs are picked up (for example, for farriery). Often, the horse will also raise its tail head during an episode. The forelimbs are not usually affected, but if they are, the horse will hold its fore leg extended with the hoof just off the ground. Affected horses usually also have muscle loss and weakness. 

Shivers is most common in draft breeds, but has also been observed in Quarter Horses, Warmbloods, and rarely in Thoroughbreds. Although it is unproven, shivers is believed to be an inherited trait in draft horses. There is no known effective treatment for shivers and the prognosis for affected horses is guarded because many continue to worsen and may eventually require euthanasia.

The exact cause of shivers remains unknown. Some researchers have speculated that shivers is caused by an abnormal response in the nerves that sense position and tension in muscles and joints. When microscopically examining muscle from horses with shivers, researchers have seen muscle abnormalities similar to those observed with polysaccharide storage myopathy (PSSM). A study of 103 Belgian draft horses at least one year of age found shivers in 18% of the horses examined. Researchers took muscle biopsies from all 103 horses and observed the lesions of PSSM in 36% of horses. Only 6% of horses had both PSSM and shivers. Serum concentrations of selenium and vitamin E were not significantly different between normal horses and horses with shivers or PSSM. There was no statistically significant association between a horse having both PSSM and shivers. 

Several researchers have stated that since microscopic changes consistent with PSSM are so common in draft breeds (up to 66% of horses in some studies), that it is difficult to definitively link shivers with PSSM. However, it is possible that some clinical signs of PSSM, such as gait abnormalities, could be interpreted as the horse having shivers.

Muscle biopsy is a useful diagnostic test for horses with shivers to rule out PSSM. If PSSM is present, then dietary change to a higher fat, lower carbohydrate diet may help improve clinical signs in some horses. Most researchers recommend that horses with shivers have supplemental vitamin E and selenium because both have some protective and anti-inflammatory effects for nerves and muscle.

Vitamin E and Selenium Status in Horses

Vitamin E and selenium both function as antioxidants in the cell. Selenium is part of the enzyme glutathione peroxidase, which breaks down peroxides. Vitamin E is a fat-soluble vitamin that inserts itself into the cell membrane and neutralizes free radicals. Both selenium and vitamin E can offset a deficiency of the other, as long as both are not deficient.

Selenium deficiency or selenium-responsive conditions have been associated with several problems in horses, including white muscle disease, sporadic tying-up, retained placenta,
reduced fertility, masseter (jaw) muscle myopathy, and heart failure or difficulty swallowing in foals.

The best sample to assess selenium adequacy is whole blood. Because erythrocytes cannot produce more glutathione peroxidase, whole blood selenium concentration reflects long-
term selenium status over the lifespan of the sampled red blood cells (120 to 150 days). It is recommended to use royal blue top (trace mineral) tubes when measuring blood selenium concentration. Horses require 0.1 ppm of selenium per kg of dry matter intake (mg/kg); this equates to approximately 1 mg of selenium per day for a 1,000-lb (454-kg) horse.

Serum is the most commonly used sample for measuring vitamin E. The vitamin E requirement is approximately 1 IU/kg body weight/day, or 500 IU/day for an 1,100-lb (500-kg) horse. The source of vitamin E is very important, especially when treating horses for equine motor neuron disease, equine degenerative myelopathy, or other vitamin E responsive conditions. Natural (d-α-tocopherol), water-soluble sources of vitamin E are better absorbed and preferentially used by the horse. Synthetic vitamin E sources (d,l-α-tocopherol) contain a mixture of eight chemically different sources of vitamin E, only one of which is d-α-tocopherol. Studies have shown that natural vitamin E is more bioavailable than synthetic vitamin E and significantly raises both serum and cerebrospinal fluid concentrations of α-tocopherol compared to synthetic vitamin E.

Vitamin E deficient myopathy is a newly recognized cause of muscle loss and weakness. It is unknown if this is a unique problem or an early, reversible stage of equine motor neuron
disease. Affected horses have low serum (<2 µg/ml) and muscle concentrations of vitamin E. Supplementation with 5,000 IU natural vitamin E/day for six weeks has been shown to restore serum and muscle vitamin E concentrations to normal and most horses make a gradual return to normal exercise.

Providing Dietary Protein to Horses

Owners of dogs and cats know that their pets are carnivores—meat eaters—and commercially available food for these animals contains high protein levels similar to what would be found in the diets of wild felines and canines. Horses, on the other hand, are herbivores—plant eaters—and at first glance, it would not seem that grass, the most natural equine diet, would contain a great deal of protein.

However, grasses and legumes, either fresh or dried, usually do contain enough protein for maintenance of idle or lightly worked horses. Grass has a low to medium level of protein and does not contain a large amount of lysine, an essential amino acid. Alfalfa or lucerne, a legume, is an excellent source of protein, with levels that can approach 18 to 20 percent in the best-quality hay. Since horses need only about 10 or 11 percent crude protein as adults, and about 12 to 14 percent during growth, alfalfa could easily supply most or all protein requirements, though a straight alfalfa diet is not balanced for other nutrients.

In general, a mature horse doing little work needs only enough protein to maintain body tissues. Requirements for protein go up as horses are put into an exercise program, and the need increases with the workload because these active horses must build lean muscle tissue. Breeding stallions and late-pregnant mares need protein levels similar to those of horses in moderate to heavy exercise. Lactating mares have the highest protein requirement, up to twice as much as a mature horse in
moderate exercise. Finally, growing horses need high-quality protein as they increase in size and weight.

The National Research Council’s Nutrient Requirements of Horses contains guidelines for the amount of crude protein needed by horses of every size, age, stage of growth, reproductive status, and exercise level. Horse owners don’t need to pick up a copy of this publication, however, if they purchase horse feed that has been formulated and produced by a reputable manufacturer. Feed bags and labels indicate the type and amount of feed necessary to meet the needs of many classes of horses, from mature/idle to growing, lightly worked, heavily exercised, breeding, or retired equines.

Supplementing a forage-based diet by feeding a concentrate according to the bag’s directions will usually provide sufficient protein. Knowing that their equines need more protein than idle animals, owners of hard-working sport horses or lactating mares may wonder whether they should boost dietary protein levels through the use of special protein or amino acid supplements. Studies in both horses and human athletes have shown that this type of supplementation has little or no positive effect.

However, the intensity of the actual training program in that study is somewhat difficult to interpret. Several research experiments conducted at Kentucky Equine Research (KER) and other locations have examined the effects of dietary protein on metabolic responses to exercise. In two of these studies, lactate accumulation during exercise was lower when horses received a diet containing a high level of protein, but in a third study, lactate accumulation was unaffected by dietary protein level. In the KER study, it was found that muscle glycogen was somewhat lower in horses receiving a high-protein diet, while another study showed no effect of protein level on muscle glycogen concentration. At this point, there are no solid research data to support the theory that protein supplementation may be beneficial to equine athletes. Even in human athletes, benefits have only been sporadically reported. Because the branched-chain amino acids are the primary amino acids catabolized during exercise, researchers have also evaluated supplementation of these amino acids (especially leucine). Horses receiving a branched-chain amino acid supplement had lower lactate levels than unsupplemented horses in one study, but this study involved only mild exercise, so results might not apply to heavily worked horses. Further research in this area may turn up some positive benefits of protein supplementation, but at present, there is no recommendation for this practice.

Effect of Adding Soybean Oil to a Horse’s Ration

When fat is substituted for carbohydrate isocalorically (calorie for calorie) in a horse’s ration, blood glucose and insulin responses to feeding are reduced. It was unclear, however, whether this response was simply due to reduced glucose in the diet or if fat affects glycemic response in some other manner. An experiment carried out at Kentucky Equine Research (KER) was designed to evaluate whether adding fat to a grain meal would affect glucose and insulin response to feeding when the level of grain intake remained the same.

Nine Thoroughbred horses were used in this two-period switchback design experiment. Five of the horses were in training and were physically fit, and four were untrained. During period one, each horse was fed 2.27 kg (5 lb) of a grain mix, which consisted of 72% oats, 20% corn, and 8% molasses at 7:00 a.m. Five of the horses were also fed 200 ml (170 g) of soybean oil mixed into the grain. At 8:00 a.m. each horse was given 2.72 kg (6 lb) of mature bluegrass hay. Blood samples were taken from each horse by jugular catheter before feeding and at 1, 2, 3, 4, 6, 8, 10, and 12 hours post feeding. Water was available to the horses at all times. The same procedure was followed two weeks later with the soybean oil added to the grain of the four horses that served as controls during the first period.

Blood samples were collected and analyzed for lactate, glucose, and insulin at the conclusion of the study. Blood glucose was significantly lower one hour after feeding when soybean oil was added to the diet. Glucose remained lower for 3 hours post feeding. After 6 and 10 hours, blood glucose was higher in the fat-supplemented group. Insulin was lower in the fat-supplemented group 1 hour after feeding. After 8 and 10 hours, insulin was higher in the fat-supplemented group. Plasma L-lactate tended to be higher in the control group 4 hours after feeding and higher in the fat-supplemented group 6 hours after feeding.

These data suggest that the addition of fat (soybean oil) to a grain meal will affect glucose and insulin response to feeding. These effects are independent of the amount of carbohydrate in the diet and may be due to differences in the rate of gastric emptying when fat is included in the diet.

This article was based on information in a paper titled “Responses of Blood Glucose, Lactate and Insulin in Horses Fed Equal Amounts of Grain With or Without Added Soybean Meal” by J. Pagan, T. Rotmensen, and S. Jackson.