Energy Excess

Just as the first and major effect of inadequate feed intake is inadequate energy, the only clinically significant effect of excess feed intake is a surplus of dietary energy. If the feed excess is large and occurs at a single feeding —such as the horse suddenly having access to a large amount of a palatable, high-energy dense feed, such as a cereal grain—it may result in diarrhea, colic, or acute laminitis. However, if excess energy intake occurs over a more prolonged period, most of the surplus is stored in the body as fat. Some of the excess energy is given off as heat and used for increased physical activity. Increased body-heat production is used by many species of animals, including people, to compensate for excess dietary energy intake. The horse is unique in that it also compensates by increasing its physical activity. As a result, the horse that receives excess dietary energy is more apt to be excessively high spirited and buck, shy, and run away. Conversely, one way to calm a horse is to reduce its dietary energy intake. In addition to increased body fat deposition, heat production, and physical activity, excess dietary energy intake by the growing horse increases its growth rate and is thought to be a factor contributing to the occurrence of developmental orthopedic diseases.

The horse, like people and other animals, will eat the amount of feed needed to meet its energy needs if it is physically capable of doing so and the feed is available. But, if the feed is sufficiently palatable and high in energy density, some horses, like some individuals of nearly all species, will eat more than is needed. Although the excess over time averages only slightly above what is needed, if it continues, it will result in obesity.

Being overweight, like being underweight, is detrimental to health and performance ability. One of the first noticeable effects of excess body fat in the horse is decreased physical performance ability and increased sweating with physical activity. Sweating is due primarily to a decreased ability to cool the body because excess fat provides increased insulation. Twenty minutes of trotting by a horse that cannot cool itself properly produces sufficient heat to cause hyperthermia-induced fatigue and even death. Respiratory difficulties probably also contribute to decreased physical activity and performance in the overweight horse. Obesity increases respiratory difficulties because excess body mass increases oxygen needs, but decreases oxygen intake ability. The additional mass against the chest wall increases respiratory effort, reduces respiratory efficiency, and may lead to alveolar hypoventilation. These effects are reversed by weight loss.

Being either overweight or underweight is known to increase joint and locomotion problems in dogs; it may in horses. These problems are due to carrying excessive weight or, in the underweight animal, having decreased muscle mass. Obesity in other species is known to increase heart, circulatory, digestive and skin diseases, and cancer, to decrease resistance to infectious disease, and, as a result, shorten life span. Mortality at any age is 9, 25, 65, 230, and 1200% higher in people that are 15, 25, 40, 55, or 100% overweight, respectively. Whether obesity causes these effects in horses isn’t known, but it probably does to varying degrees.

There is only one way to correct obesity: dietary energy intake must be less than energy utilization. There are two ways to produce a body energy deficit: decrease feed intake or increase exercise. The use of both together is best. Exercise in conjunction with reduced caloric intake has been shown in other species to be beneficial for weight reduction for many reasons, including:

1. Increased energy expenditure.
2. Prevention of a decrease in resting energy expenditure that would otherwise occur when caloric intake is reduced.
3. Reduction in appetite, which may occur with a moderate increase in exercise by the relatively inactive individual.
4. Prevention of muscle and bone mineral losses that occur when caloric intake is reduced without increased physical activity.

The following exercise is recommended: walk, then trot, the horse long enough to make the horse begin sweating; then walk to cool down. Do this once and preferably twice daily.

Decreasing the horse’s caloric intake sufficiently to cause it to lose weight requires that the horse be confined to a dry lot or paddock, or to a stall without straw bedding. Coarse, high-fiber, low-calorie, long-stem hay free of dust, mold, and weeds is preferred. The amount of hay recommended is that which provides 50 to 75% of the horse’s caloric needs at rest and at optimum body weight. For example:

A horse’s obese weight is 1100 lbs (500 kg) and it is estimated that it should weigh about 880 lbs (400 kg). The 880-lb horse for maintenance needs 13.4 Mcal daily, and therefore for weight reduction needs (50 to 75%) 13.4, or 6.7 to 10 Mcal daily. Most mature grass hay provides 0.7 to 0.9 Mcal/lb. For weight reduction, you would feed the horse about 6.7 to 10 Mcal/day 0.7 to 0.9 Mcal/lb of hay, or about 8 to 14 lbs of hay daily. Ideally, divide the amount fed into at least two feedings daily. Water and salt should be always available, and no grain or protein supplement containing feed should be offered. The amount of hay fed should be adjusted as needed for each individual horse to obtain the decrease in body weight desired.

Weight reduction should be continued until the horse has a body condition score of 5 to 6, at which time the amount fed should be increased sufficiently to maintain that weight. Following weight reduction, the horse may be able to be put on pasture without regaining excessively if exercise is continued, but if ample pasture forage is available, or if exercise is discontinued regain will occur.

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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