Courtesy of Kentucky Equine Research
Fiber (hay/pasture) is an energy source that is often overlooked in horse nutrition. Horses have a highly developed hindgut that houses billions of bacteria capable of fermenting large quantities of plant fiber. Volatile fatty acids (VFAs), the end product of fiber fermentation, are absorbed from the hindgut and transported to the liver. Once in the liver, VFAs can be converted to glucose and be stored as liver glycogen or be converted to fat, and be used to fortify the body’s fat stores. Fiber, therefore, can be used as an energy source for a horse being exercised for several hours since fermentation of fiber and absorption of VFAs continue long after a meal has been eaten.
A performance horse’s intestinal health is critical to success. Normally, the digestive system of the horse is active, moving feed ingredients through the length of the tract. Inactivity of the digestive system, due to dehydration or electrolyte imbalances, can cause severe colic and even death. Research conducted in Germany has underscored the importance of fiber in maintaining gut health for horses that exercise for several hours. Studies there have shown that a diet high in fiber resulted in an increased water intake. Further, animals supplemented with a simple hay and salt diet had 73% more water in their digestive tracts after exercise and approximately 33% more available electrolytes than animals on a low fiber diet. The additional water and electrolytes in the digestive tract of the high-fiber animals is probably due to the high water-holding capacity of plant fiber. More importantly, the water and electrolyte pool created by a high-fiber diet can be used to combat dehydration and electrolyte imbalances that derail so many endurance horses.
Another important attribute of a digestive system full of fiber is maintenance of blood flow to the digestive system during exercise. The physical presence of fiber in the digestive system will help insure that blood is not totally diverted away from the digestive system with the onset of exercise. In one study, the percentage of cardiac output (blood flow) distributed to the digestive system was higher in fed ponies compared to fasted ponies during exercise. For performance horses, maintenance of blood flow to the digestive system will aid in the ability of gut tissue to remain active and could prevent colic.
In addition to hay and grass as fiber sources, there are so-called “super fibers” that have the same beneficial aspects of forage fibers for maintaining gut health and fluid and electrolyte balance, but contain more energy. The additional energy is the result of both a high fiber content and a low lignin (nondigestible fiber) component. Therefore, these ingredients have more fiber available for microbial digestion. These super fibers (beet pulp, soybean hulls, almond hulls, oat hulls) contain energy equivalent to oats and barley, but they are safer to feed because they do not produce the symptoms of grain overload.
If the protein intake of an exercising horse exceeds its requirement, then the extra protein can be used as a source of energy. The amino acids associated with the extra protein are broken down by the liver, and the nitrogen is excreted as ammonia. The carbon skeletons that are left can be oxidized to produce ATP or used to make glucose or fat. Excessive protein intake should be avoided in heavily exercised horses for a number of reasons. First, water requirements increase with increased protein intake. This can be devastating for horses performing some types of exercise such as endurance racing, where they typically struggle to maintain proper hydration. Second, accumulation of nitrogen end-products (ammonia and urea) in the blood can lead to nerve irritability and disturbances in intestinal function and carbohydrate metabolism. Further, increased ammonia excretion in the urine may lead to respiratory problems associated with ammonia buildup in the stall.
Energy metabolism within the body is not 100% efficient. A certain amount of energy is lost from each chemical reaction in the form of heat. In order for the horse to remain healthy and continue to exercise, excess heat must be dissipated from the body. If the horse is unable to rid itself of this heat, body temperature can rise to the point where it becomes life-threatening. For horses, the main route of heat dissipation is through a form of evaporative cooling known as sweating. In evaporative cooling, the sweat gland takes fluid from the circulatory system and secretes it out to the surface of the skin. Once the hot fluid (sweat) is on the skin, it spreads out and evaporates. This takes heat away from the body. Unfortunately, sweating also takes water and electrolytes away from the body.
As water is lost from the blood, the remaining blood becomes thicker. This increased blood viscosity decreases perfusion potential and negatively influences tissue oxygenation. With intense exercise, water loss can become so extreme that blood volume is decreased and further sweating is not possible. If the horse is not rehydrated, death from heat stroke will occur. Idle horses may drink 10 to 12 gallons of water each day, and for horses performing in hot, humid environments, this requirement may be doubled or tripled.
Electrolytes are substances that dissociate in solution into electrically charged particles called ions. In horses, electrolytes play an important role in maintaining osmotic pressure, fluid balance and nerve and muscle activity. During exercise, sodium, potassium, chloride, calcium, and magnesium are lost in the feces, urine and sweat. Loss of these electrolytescauses fatigue and muscle weakness, and decreases the thirst response to dehydration. Therefore, it is vital to replenish electrolyte losses in performance horses that sweat heavily. Excellent commercial electrolyte supplements are available in both powder and paste forms.