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Energy Deficiency
After an animal’s initial adjustment to the palatability of the feeds in its diet, the average amount consumed by the healthy animal will be an amount adequate to meet its energy needs if the feed is available and its gastrointestinal tract will hold that amount. The maximum daily amount of air dry feed that a horse can consume is equal to 3 to 3.5% of its body weight. If this amount of feed does not meet the horse’s energy needs, the energy concentration of the diet must be increased. For the horse, this is accomplished by feeding more grain, adding fat to the diet or feeding a more digestible, better-quality forage.
There are four reasons the horse may not consume enough dietary energy to meet its needs: (1) a sufficient amount of feed is not available, (2) its gastrointestinal tract won’t hold enough of the available feed because the digestible energy density of the feed is too low, (3) it can’t consume enough because of a physical problem (e.g., injury or bad teeth), or (4) it doesn’t want to consume enough because of illness, stress, inadequate water intake, or poorly palatable feed. Regardless of the reason for inadequate feed intake, the first and most noticeable effect is lassitude. This is because horses need 80 to 90% of all the feed ingested for energy, 8 to 14.5% for protein, 2 to 3% for minerals, and less than 1% for vitamins. With inadequate feed intake, the greatest deficit will be dietary energy, followed by protein. Unless there is a disease-related increase in the loss of minerals or vitamins, signs and effects of deficiencies of these nutrients, during periods of inadequate feed intake, occur much later, to a lesser degree, and are masked by signs of energy and protein deficiency.
Inadequate feed and, therefore energy, intake causes hormonal changes that decrease the body’s energy utilization by reducing physical activity, milk production, and growth rate. The hormonal changes increase utilization of the body’s stored and structural sources of energy (carbohydrates, fats, and proteins) resulting in weight loss. The utilization, or deposition, of excess body fat and protein alter the horse’s appearance.
Once body fat stores near depletion, utilization of the body’s only remaining source of energy, protein, accelerates. Body protein use is not random. Proteins providing structural support in the form of bones, ligaments, tendons, and cartilage are used after those in the blood, intestines, and muscle. During feed deprivation, a loss of function occurs earlier in the tissues and organs whose protein is used first. The order of occurrence of decreased organ function either as a result of feed and, therefore, energy deprivation or as a result of protein deprivation is as follows.
- Liver and plasma proteins decrease. If sufficiently severe, the decrease allows fluid to leave the plasma, resulting in edema and stocking.
- Gastrointestinal tract degeneration. With prolonged feed deprivation, intestinal mass, absorptive surface area, and enzyme activity decrease, which impairs nutrient digestion and absorption.
- Diminished defense against infectious organisms making the individual more susceptible to the occurrence and severity of infectious disease.
- Impaired respiratory and cardiac function.
- Skeletal muscle degeneration, which decreases muscle mass and strength. This change occurs more slowly than the changes previously described.
By the time muscle wasting or weakness is evident, feed deprivation- induced alterations of other body functions are well underway. To prevent and correct these alterations, the causes for inadequate feed intake should be corrected, if possible, and adequate dietary calories and protein given to meet the horse’s needs and correct the deficits present.
The thin, weak horse should be fed a good quality forage and up to an equal weight of grain, or, instead of forage and grain separately, a complete feed containing both. Water and salt should be easily available at all times or several ounces (60 to 120 g) of salt may be added to the grain to encourage water intake to decrease the risk of feed impaction. The amount of feed fed weak, thin horses (with a body condition score of 3 or less, should be increased gradually to prevent diarrhea, colic, feed impactions, other gastrointestinal disturbances, or founder. Begin by feeding one-third of the amount the idle mature horse would need if it were at its optimum (not current) body weight. This would be about 0.5 lb of total feed per 100 lbs of optimum body weight per day (0.5 kg/100 kg day). Divide the amount fed into at least four feedings daily. If a problem occurs, feed smaller amounts more frequently. If no problem occurs, gradually increase the amount fed over the following 1 to 2 weeks, up to twice that needed for maintenance. This is about 3 lbs of total feed per 100 lbs body weight per day (3 kg/ 100 kg/day). At this time, or initially for horses that are only moderately thin, the forage or complete feed may be available at all times for the horse to eat as much as it likes. When this is done, unless there is some reason the horse will not or cannot eat normally, it will consume daily an amount close to 3% of its body weight and gain about 2 lbs (0.9 kg) per day. If the thin, malnourished horse does not eat and gain these amounts, it should be thoroughly examined, the reason determined, and this reason corrected. Is the horse unable to eat normally because of a physical problem or pain, or is it unwilling because it is sick?
This article is from "Feeding and Care of the Horse", second edition, by Lon D. Lewis, Lippincott Williams & Wilkins, 1995. Reprinted with permission from the publisher.
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