The Equine Athlete Vol. 3, No. 6: 1990
Reprinted with permission

Gary M. Pusillo, Ph.D., P.A.S. Farmers Feed & Supply Company

Productive and proper feeding of horses is only one of the factors on which successful horse husbandry depends. Other factors include giving the horse the necessary care and attention and maintaining the animal's healthy condition. Each of these factors is fundamentally important to the horse owner and caregiver. If one factor does not receive proper attention, the results secured will not be satisfactory, regardless of how favorable the other conditions of the animal may be.

Horses are one of the most difficult livestock species for which to make general feeding recommendations because they are used for a wide variety of activities from backyard pleasure riding to racing or endurance work (Table 1). A horse requires nutrients for maintenance, growth, reproduction, and production. With horses, the production is for work - riding, racing and training. The work, however, usually is irregular and often very strenuous - characteristics that create a particular stress on the animal and make the job of feeding according to nutritive needs quite difficult.

The importance of vitamin and mineral supplementation for the performance horse is often overlooked because of the difficulty in determining proper levels. It is next to impossible to determine in a controlled, experimental environment whether the addition of a specific nutrient at a specific level will affect the athletic ability and health of a horse under all conditions.

Nutritive needs of horses do not necessarily remain the same from day to day or from period to period. A number of factors continually influence the determination of these needs, including:

• age and size of the animal; a climatic condition;
• the kind, quality, and amount of feed;
• the system of management; and
• the health, condition, and temperament of the animal.

When determining the amounts of a nutrient to add into a horse ration, it is essential to allow for margins of safety over the levels necessary to meet the bare requirements. Table 2 contains the nutrient levels that have been used with good results by some of the most successful Standardbred owners and trainers in New Jersey, Pennsylvania, and New York. These values should provide sufficient margins of safety when dealing with 900- to 1000-lb performance horses subjected to light to intense work.

To add these nutrients into the feed, estimate the average number of pounds of total feed the animals will consume daily. Multiply the amount of each feed consumed by the concentration of the particular nutrient in question. Subtracting these values from the recommended allowance will result in a value that will be the total shortage of the nutrient in question. Dividing this number by the average number of pounds of daily feed consumption will result in the nutrient amount needed to be added per pound of feed. It should be remembered that horses will often consume about half hay and/ or pasture. The vitamins and minerals, therefore, need to be added to the concentrate mix at twice the level indicated since the other half of the ration will be hay and/or pasture to which no vitamins are added.

The following is an example using the calcium requirements of a 1000-lb racehorse as the nutrient in question. The calcium allowance is recommended to be 70 g per head daily (see Table 2). Assume that this racehorse is consuming 10 Ibs of grain and 10 Ibs of hay on a daily basis for a total of 20 lbs of feed. For this example, assume that the grain contains 0.08% and the hay contains 138% calcium on an as-fed basis. Multiply the 10 Ibs of grain by 0.08% calcium and the 10 Ibs of hay by 1.38% calcium.

The resulting numbers, 0.008 lbs (3.6 g) and 0.138 lbs (62.6 g), for the grain and hay, respectively, need to be subtracted from the calcium allowance of 70 g. The resulting value 3.8 g (70 - 62.6- 3.6), divided by the 20 lbs of total feed consumption, results in the amount of calcium that needs to be added per pound of feed: 0.19 g. Since half of the ration is hay, calcium needs to be added to the grain mix at twice the level indicated (i.e., 2 x 0.19 g = 0.38 g). In 1 ton of grain, 760 g (2000 x 0.38 g), or 1.67 Ibs (760 )454 lbs) of calcium, needs to be added. Calcium carbonate, which contains approltimately 38% calcium, would need to be added at 4.39 lbs (1.67 lbs ) 38% ca), or 2000 g (760 g ) 38%), to meet the calcium need. This method can be used with any nutrient to be added to a ration.

The challenge is to deliver the correct amounts and proportions to each horse at the correct time (i.e., Nutritional Categories). While Table 2 represents the suggested requirements for a 900 to 1000-lb horse, general relationships and properties apply to each nutrient under each horse nutritional category (see Table 1). Highlights of several minerals and vitamins are outlined in the following paragraphs.

Vitamin E functions in at least two metabolic roles: as a fat-soluble antioxidant and in a more specific role interrelated with the metabolism of selenium¹. The increasing use of fats or the utilization of feeds with unsaturated fatty acids, which are susceptible to rancidity, warrants the use of higher levels of Vitamin E, since rancidity destroys vitamin E. Vitamin E also helps to maintain red blood cell levels and to act with selenium in prevention of exertional myopathy.²

Personal observations of race horses treated with levels of 2000 IU of Vitamin E 5 months before, and continued throughout the racing season, showed increased stamina in treated horses. These horses also showed a decreased recovery time between races with less problems of tying-up.^3,4 Vitamin E supplementation improves the breeding performance in both mares and stallions⁵ and has been reported to prevent anhidrosis in some cases.⁶

An important interrelationship exists between Vitamin E and selenium. Vitamin E can substitute to a certain extent for selenium, and selenium can likewise substitute for some of the Vitamin E; however, neither one can substitute entirely for the other. Both of these nutrients are needed by the horse and both have a metabolic or nutritional role in the body. In addition, both have an antioxidant effect.⁵ Vitamin E in cellular and subcellular membranes is the first line of defense against peroxidation of vital phospholipids. Selenium in glutathione peroxidase is a second line of defense that destroys these peroxides before they cause damage to the membranes.¹ Seleniun is necessary for maintenance of muscle, and it is suggested that the racehorse or performance horse obtain at least 2 mg of selenium daily. The Food and Drug Administration (FDA) allows the addition of selenium to horse rations at a level of 0.3 ppm. Selenium in excess can be harmful; thus, it should be used carefully. Selenium status can be estimated by measuring serum selenium or glutathione peroxidase levels.

Copper works in conjunction with iron in hemoglobin formation and is an integral part of several enzymes in the body. Cooper is closely associated with normal bone development in young growing animals. A deficiency of copper can cause failure of interchains' cross-linkages in both collagen and elastin, which results in abnormal mineralization. Horses with copper deficiency may also have osteo-chondrosis.⁷ A deficiency of iron and copper results in anemia and in severe cases, the horse's breathing becomes labored. Hemoglobin levels are important to people who race horses because racehorses have an extra need for oxygen during heavy activity. A hemoglobin level of at least 16 g per 100 ml of blood is a common level trainers try to achieve in their horses. Copper is not contained in hemoglobin, but a trace amount is necessary to serve as a catalyst before the body can utilize iron for hemoglobin formation.⁵

Interactions involving copper are among the more complex in animal nutrition. Ingestion of zinc-contaminated pasture and paint containing zinc has been reported to cause copper deficiency and osteochondrosis in foals.^8,9

Going beyond the question of total level of trace elements in a horse's diet to considerations of bioavailability, valency, and organic complexes is essential. Valence state is critical in some cases such as selenium. In general, the valence state that occurs naturally in the element in the earth's crust is the least toxic.¹⁰

Currently, the most common contaminants of concern in commercial mineral products usually are considered to be lead, cadmium, arsenic, vanadium, and fluorine. Toxic levels of elements are dependent upon the chemical form, the species involved, and the age of the animal. Arsenic and lead are cumulative poisons, therefore, excesses for long periods could be more damaging than feeding a single dose. For example, the lead content in some zinc oxide could cause such damage. The lead content of poor quality zinc oxide products can be dangerously high when used at high levels over extended periods of time, and can result in lead toxicity. One characteristic clinical sign of lead toxicity in horses is laryngeal paralysis.

It is questionable whether the macro- and micro- mineral bioavailability in most horse rations is sufficient to meet the horse's maintenance and productive needs. The intake of bioavailable vitamins and minerals is especially crucial in the performance horse. The bioavailability of minerals and vitamins may be the one factor standing between a poor performer and a peak performer. Bioavailability is that portion of a nutrient that can be utitized by the animal to fulfill the functions for which the nutrient is needed. The relative biological availability of mineral and vitamin sources is of both practical and economic concern to the horse producer and trainer. This is especially true when feed ingredients inherently low in a specific nutrient are being used.

Studies have shown that sources of nutrients are not equally available to horses. For example, a wide variation is found in the biological (nutritional) value of processed phosphates. Of all phosphates available, monocalcium phosphate has the best biological availability for the horse. It is the recommended phosphate source for young and fast-growing performance horses.

The bioavailability values vary greatly among different sources of iron with that from good, inorganic sources generally more readily available than in most feeds. Of all the iron compounds most frequently utilized, iron oxide (ferric oxide) has the lowest bioavailability. Absorption of iron by horses can be increased by dietary additions of ascorbic acid and the amino acids histidine, cysteine, and lysine.

Vitamins and minerals are extremely important to horse health and performance, yet it is the management of mineral and vitamin nutrition that is most often neglected in performance horse operations. Supplying minerals and vitamins, in addition to quality feedstuffs, is the best way to ensure the animal's nutritional requirements are being met. Optimum performance can be achieved through optimum nutrition.

Individual feeding and the study of each individual horse's needs are extremely important. At best, any recommendation can only serve as a guide; thus, the horse trainer, owner or caregiver must prepare to make adjustments in the feeding program when a situation arises. Proper attention to details, like mineral and vitamin supplementation, will mean the difference between developing a champion and just another horse.

1. Scott, ML: Vitamin E and Selenium Solution of a Long- Term Enigma. Animal Nutrition and Health, Aug/Sept, 1979.
2. Hintz, HF: Supplements for the Exercising Horse. Anim Health Nutr, April, 1987.
3. Darlington, FG, Chassels JB: Summary. 8(71), 1956.
4. Darlington, FG, Chassels JB: Summary. 9(64), 1957.
5. Cunha TJ: Horse Feeding and Nutrition. Orlando, Fl, The Academic Press, Inc., 1980, pp59-109.
6. Ensaminger ME, Oldfield, JE, Heinemann WW: Feeds and Nutrition. Clovis, Ca, The Ensminger publishing Co., 1990, pp 1065-1117.
7. Hintz HF: Lameness and Nutrition in Horses. Maryland Nutrition Conference, 1986.
8. Bridges CH, Womack JE, Harris ED, Scrutchfield WL: Considerations in Osteochondrosis of Suckling Foals. JAVMA 185-173.
9. Eamens GL, et al: Aust Vet J 61:205, 1984.
10. Scott JT: Probable Contaminates in Trace Mineral Ingredients. California Nutrition Conference, 1977.