Dietary Energy for Horses

Horses get energy for exercise, growth, and body maintenance from the forage and concentrates they consume. Dietary energy is usually expressed in terms of kilocalories (kcal) or megacalories (Mcal) of digestible energy. Digestible energy (DE) refers to the amount of energy in the diet that is absorbed by the horse. Digestible energy requirements are calculated based on the horse’s maintenance DE requirement plus the additional energy expended during exercise. Basically, DE can be provided to horses by four different dietary energy sources: starch, fat, protein, and fiber.

Starch, a carbohydrate composed of a large number of glucose molecules, is the primary component of cereal grains, making up 50 to 70% of the grain’s dry matter. Of the grains
commonly fed to horses, corn has the highest starch content. Starch is a versatile energy source for the performance horse. Horses break down starch into glucose units in the small intestine, where it is absorbed into the blood. Once in the blood, these glucose units can be oxidized directly to fuel muscle activity or they can be used to make muscle glycogen, liver glycogen, or body fat.

Muscle glycogen is an important fuel for energy generation during exercise. In addition, glycogen is stored in the liver where it is available for the production and release of glucose into the blood during exercise. Maintaining blood glucose levels during exercise is of prime importance because glucose is the only fuel that is available to the central nervous system, and hypoglycemia (low blood sugar) is a potential cause of fatigue in exercised horses.

Starch is the dietary energy source of choice for glycogen synthesis because starch digestion results in a direct rise in blood glucose and insulin, two of the most important factors involved in glycogen synthesis. There is a limit, however, to the amount of starch a performance horse’s ration should contain. If large amounts of starch are fed in a single meal, the small intestine’s ability to digest and absorb the starch may be overwhelmed, and a substantial amount of the starch may pass into the large intestine where it will be rapidly fermented to lactic acid by bacteria. An increase in acid will lower the pH of the hindgut. This may kill other bacteria and lead to the release of endotoxins into the blood. The combination of these two factors may lead to colic or laminitis.

Fat is a less versatile energy source than starch because it can only be oxidized aerobically to produce energy or stored as body fat. Fatty acids cannot be converted to glucose or used to synthesize glycogen. There is reason to believe, however, that feeding fat may have a sparing effect on muscle glycogen in endurance horses. Research conducted in Sweden with Standardbred horses indicated that feeding fat to horses resulted in a greater mobilization and utilization of fat during long distance exercise. In this experiment, horses fed high levels of soybean oil (15% of the concentrate) had higher concentrations of free fatty acids in their blood than horses fed high-carbohydrate or high-protein diets. Respiratory quotients (RQ) were also lower in the fat fed horses during these exercise tests, indicating that a greater proportion of their energy was being generated from fat than in the control group of horses. Unfortunately, the horses fed the high-fat diets had lower muscle and liver glycogen storage than the control group. Therefore, it is important to supply adequate starch in the horse’s diet as well.

If the protein intake of a performance horse exceeds its requirement, then the extra protein can be used as a source of energy. The amino acids from this extra protein are broken down by the liver, and the nitrogen from the protein is excreted as ammonia. The carbon “skeletons” that are left can be oxidized to produce ATP, the main fuel of muscular activity, or used to make glucose or fat. Excessive protein intake should be avoided in the exercised horse for a number of reasons: (1) water requirements increase with elevated protein intake; (2) urea levels increase in the blood, leading to greater urea excretion into the gut, which may increase the risk of intestinal disturbances such as enterotoxemia; (3) blood ammonia increases causing a number of problems such as nerve irritability and disturbances in carbohydrate metabolism; and (4) increased ammonia excretion in the urine may also lead to respiratory problems because of ammonia buildup in the stall.

Fiber is an energy source that is often overlooked in horse nutrition. Horses have a highly developed hindgut that houses billions of bacteria and protozoa capable of fermenting large quantities of fiber. The end products of fiber fermentation can be used as energy sources throughout the day since fermentation continues long after a meal has been eaten. Proper gut function is essential to the health and well-being of the horse, so fiber should be considered an essential nutrient. Hay should be fed at a rate of intake equal to at least 1% of body weight per day. Grasses that are less mature at harvest make the best horse hay. An interesting alternative energy source for performance horses is beet pulp, a by-product of the sugar beet industry, made by drying the residual pulp after the sugar has been extracted. It contains a high percent of fermentable fiber and its DE content is similar to oats.

The rations of performance horses should include a mixture of energy sources. In this regard, moderation is the key. Excessive amounts of starch should be avoided as this may lead to colic, founder, or tying-up in horses. Too much fat may compromise glycogen storage and excessive protein may lead to problems associated with ammonia production. Fiber must be included in the diet to maintain proper hindgut function. Including the correct mixture of these energy sources in the performance horse’s ration should reduce problems associated with feeding and allow the horse to utilize energy-generating substrates most efficiently during exercise.