Nutritional Requirements of Horses

For maintenance of body weight and to support normal activity, the daily digestible energy (DE) requirement (in Mcal) of the nonworking adult horse in good body condition is estimated to be on average 33.3 kcal/kg body wt, with a minimum requirement of 30.3 kcal/kg for easy keepers or draft/warmblood types of horses and 36.3 kcal/kg for hard keeper adult horses. For obese or emaciated horses, the estimated ideal body weight in kg should be used in the equation rather than current body weight. For weight gain, it is estimated that 1 unit of change in body condition score takes 16–20 kg body wt gain and that each kg of gain requires 20 Mcal DE above maintenance requirements. Caloric intake in obese horses should not be restricted severely for prolonged periods of time because of the risk of hyperlipidemia, especially in ponies and donkeys.

Cold weather increases the energy requirement by 0.00082 Mcal DE/kg body wt for each degree Celsius drop below the lower critical temperature (LCT) of the animal. However, the LCT of cold-adapted adult horses in Canada was estimated to be –15°C, whereas donkeys acclimatized to summer temperatures in Nevada had an LCT of 26°C. Wind, precipitation, and body condition also affect LCT. Therefore, LCT must be estimated based on regional average temperatures and conditions and perhaps type of horse. For example, draft breeds with thick hair coats would tolerate lower temperatures than a thin-haired, thin-skinned Thoroughbred.

For growth, the daily DE requirement of light horse breeds is estimated to be maintenance DE Mcal/day = (56.5X^–0.145)/1,000 times body wt in kg plus the caloric requirements for growth = (1.99 + 1.21X – (0.021X²) × ADG, using the above equation(s) for the maintenance DE, and X as the age in months and ADG as the desired average daily gain in kg. This equation will give an estimate of caloric requirements; intakes should be adjusted to maintain body condition scores of 5 or 6 in growing horses. Warmblood, draft, and draft-cross breeds may require 10%–20% less than calculated by the equations above to sustain rapid growth and avoid obesity.

During pregnancy, if the mare is not exercised or exposed to extreme weather conditions, maintenance DE intakes are usually adequate until the last 90 days of gestation. Energy requirements during months 9, 10, and 11 of gestation are estimated by multiplying estimated maintenance requirements by 1.11, 1.13, and 1.20, respectively. Voluntary intake of roughage decreases as the fetus gets larger, and it may be necessary to increase the energy density of the ration by using supplemental concentrates in late pregnancy.

To support lactation, the NRC has estimated that 792 kcal of DE/kg of milk produced per day (see Table: Estimated Average Daily Nutrient Requirements of Mature Horses and Ponies) should be added to the increased (36.3 kcal/kg body wt) maintenance needs. Lactating light horses (eg, Thoroughbred, Quarter horses) maintained body weight when fed 28–31 Mcal DE/day. Draft mares may require as much as 43 Mcal/day. However, this recommended level of energy intake has increased body weight gain in lactating pony mares, indicating that it may exceed the needs of some breeds or individuals. The mare's body condition should be evaluated on a regular basis and maintained in the range of 5 to 7 using the body condition scores of 1 to 9 (described above) throughout pregnancy and lactation. Mares should be maintaining or gaining in condition to optimize reproductive success during the breeding season, even if lactating at the same time.

The energy requirements of work are influenced by many factors, including type of work, condition and training of the horse, fatigue, environmental temperature, and skill of the rider or driver. As the duration of exercise increases and level of activity is maintained, the DE requirement per unit of time worked actually decreases. For these reasons, DE recommendations for various activities of light horses (see Table: Energy Requirements of Work for Light Horses^a and Desirable Body Condition Scores) should be adjusted to meet individual needs and to maintain body condition scores between 4 and 6 for optimal athletic performance.

Requirements for calcium and phosphorus are much greater during growth than for maintenance of mature animals. The last third of pregnancy and lactation also appreciably increase the requirement. Aged horses (>20 yr old) may require more phosphorus than is required for adult maintenance (0.3%–0.4% of total ration). Excess calcium intake (>1% of total ration) should be avoided in aged horses, especially if renal function is reduced.

For all horses, the calcium:phosphorus ratio should be maintained at >1:1. A desirable ratio is ~1.5:1, although if adequate phosphorus is fed, foals can apparently tolerate a ratio of up to 3:1 and young adult horses a ratio even higher. Work does not appreciably increase calcium or phosphorus requirements as a portion of diet.

Salt (NaCl) requirements are markedly influenced by sweat losses. It is recommended that horse rations contain 1.6–1.8 g salt/kg feed dry matter, although there are limited data on the precise requirements. Sweat losses may cause NaCl losses >30 g (1 oz) in only 1–2 hr of hard work, and feed concentrations of salt for working horses are recommended to be at least 3.6 g NaCl/kg feed dry matter. The upper limit for salt inclusion in the ration of even hard-working horses is recommended at 6% of the total ration. However, NaCl is the only mineral for which horses are known to have true “nutritional wisdom.” Horses voluntarily seek out and consume salt in amounts to meet their daily needs if given the opportunity. Salt or salt blocks should be available free choice. Supplemental salt may be provided by oral dosing or added to feed or water in addition to free-choice salt to replace acute losses during hard work, but prolonged, excessive, forced supplementation will enhance excretion, which will reduce the ability to adjust to acute losses in the future. Forced oral administration of concentrated salt pastes (electrolytes) to dehydrated horses can cause abdominal malaise. Some horses, usually those confined to stalls, will ingest excessive amounts of salt, possibly due to restricted feed intake and/or boredom. This will not cause health problems as long as adequate water is available, although it will increase water intake and urination. Salt poisoning is unlikely unless a deprived horse is suddenly allowed free access to salt, or if water is not available to horses force-fed salt (eg, electrolyte mixtures given PO during competitions). Excessive salt content of feed or water will limit voluntary intakes, precluding toxicity but putting the horse at risk of energy deficits.

The most satisfactory method to provide supplemental calcium, phosphorus, and salt is to furnish a mixture of one-third trace mineral or plain salt and two-thirds dicalcium phosphate free choice. Trace mineral salt blocks do not contain additional calcium or phosphorus.

The daily magnesium requirement for maintenance has been estimated at 0.015 g/kg body wt based on limited studies. Working horses are estimated to require 0.019 to 0.03 g/kg body wt for light to strenuous exercise, respectively, due to sweat losses. The requirements for growth have not been well established but have been estimated to be 0.07% of the total ration. Most feeds used for horses contain 0.1%–0.3% magnesium. Although deficiencies are unlikely, hypomagnesemic tetany has been reported in lactating mares and stressed horses. The upper limit of recommended intake is estimated to be 0.3% of ration dry matter based on data from other species, but adult horses have been fed rations with higher magnesium content without apparent adverse effects. Anecdotally, high magnesium intake has a pharmacologic calming effect on horses, but large doses of magnesium sulfate (ie, Epsom salts) are also laxative.

The recommended potassium intake for maintenance in adult horses is 0.05 g/kg body wt. Most roughages contain >1% potassium, and a ration containing ≥50% roughage provides more than sufficient potassium for maintenance animals. Working horses, lactating mares, and horses receiving diuretics need higher potassium intakes because of sweat, milk, and urinary losses. Hard work may increase intake needs by a factor of 1.8. It has been proposed that rations fed to hard working horses should provide 4.5 g potassium/Mcal DE. Potassium chloride is the most common salt used to supplement rations. However, upper safe limits have not been established, and although excesses are usually efficiently excreted by the kidneys in healthy horses, acute hyperkalemia caused by the rapid absorption of concentrated salt mixtures can induce potentially fatal cardiac arrhythmias. Forced oral supplementation with large doses of potassium salts should be avoided, even in hard working horses.

Iodized salts used in salt blocks or commercial feeds easily fulfill the dietary iodine requirement (estimated to be 0.35 mg/kg feed dry matter), as do forages grown in iodine replete soils. Late pregnant mares may require slightly higher intakes (0.4 mg/kg feed dry matter), but iodine toxicity has been noted in pregnant mares consuming as little as 40 mg of iodine/day. Goiter due to excess iodine intake has been well documented in both mares and their foals, and several cases were associated with large amounts of dried seaweed (kelp) in the diet. Except in regions where the soils are known to be severely iodine deficient, iodine supplementation should not be necessary for horses.

The dietary copper requirement for horses is probably 8–10 ppm, although many commercial concentrates formulated for horses contain >20 ppm. The presence of 1–3 ppm of molybdenum in forages, which interferes with copper utilization in ruminants, does not cause problems in horses. However, excessive iron supplementation (fairly common, especially in performance horses [see below]) may inhibit adequate absorption. Copper deficiency may cause osteochondritis dissecans in young, growing horses and is associated with a higher risk of aortic or uterine artery rupture in adults. Copper deficits may also cause hypochromic microcytic anemia and pigmentation loss. Horses are extremely tolerant of copper intakes that would be fatal to sheep. However, excessively high copper intakes potentially reduce the absorption and utilization of selenium and iron, and should be avoided.

The dietary maintenance requirement for iron is estimated to be 40 mg/kg feed dry matter. For rapidly growing foals and pregnant and lactating mares, the requirement is estimated to be 50 mg/kg feed dry matter. Virtually all commercial concentrates formulated for horses and most forages contain iron well in excess of the recommended concentrations. Only horses with chronic blood loss (eg, parasitism) should be considered to be at risk of iron deficiency. Excess iron intake potentially interferes with copper utilization. The presence of anemia (low PCV or red cell volume) alone is not sufficient indication for iron supplementation in horses.

The zinc requirement is estimated to be 40 mg/kg feed dry matter, although there is evidence that this recommendation may be as much as twice the actual requirement to prevent signs of deficiency in most horses. This mineral is relatively innocuous, and intakes several times the requirement are considered safe, although intakes >1,000 ppm have induced copper deficiency and developmental orthopedic disease in young horses.

The dietary requirement for selenium is estimated to be 0.1 mg/kg feed dry matter in most regions. However, there are regions of the world (including the lower Great Lakes, the Pacific northwest, the Atlantic coast, and Florida in the USA, as well as parts of New Zealand) where soils are profoundly deficient in this important but potentially very toxic trace mineral. In other areas (including parts of Colorado, Wyoming, and North and South Dakota), forages may contain 5–40 ppm of selenium, which is sufficient to produce clinical signs of toxicity (see Selenium Toxicosis). Exercise increases glutathione peroxidase (selenium-containing enzyme) activity and may increase need for supplementation in heavily exercised horses. No more than 0.002 mg/kg body wt should be supplemented on a daily basis; toxicity has been seen with as little as 5 mg selenium/kg feed dry matter.

The vitamin A requirement of horses can be met by β-carotene, a naturally occurring precursor, or by active forms of the vitamin (eg, retinol). Fresh green forages and good-quality hays are excellent sources of carotene, as are corn and carrots. It is estimated that 1 mg of β-carotene is equivalent to ~400 IU of active vitamin A. However, because of oxidation, the carotene content of forages decreases with storage, and hays stored >1 yr may not furnish sufficient vitamin A activity. Horses consuming fresh green forage for 3–4 mo of the year usually have sufficient stores of active forms of vitamin A in the liver to maintain adequate plasma concentrations for an additional 3–6 mo. Rations for all classes of horses should provide a minimum of 30 IU active vitamin A/kg body wt (whether as β-carotene or an active synthetic form such as retinyl acetate). Prolonged feeding of excess retinyl or retinol compounds (>10 times recommended amounts) may cause bone fragility, bone exostoses, skin lesions, and birth defects such as cleft palate and microophthalmia (based on data from both horses and other species). The proposed upper safe concentration for chronic administration is 16,000 IU of the active form of the vitamin per kg feed dry matter. There is no known toxicity associated with β-carotene in horses.

Horses exposed to ≥4 hr of sunlight per day or that consume sun-cured hay do not have dietary requirements for vitamin D. For horses deprived of sunlight, suggested dietary vitamin D₃ concentrations are 800–1,000 IU/kg feed dry matter for early growth and 500 IU/kg feed dry matter for later growth and other life stages. Vitamin D toxicity is characterized by general weakness; loss of body weight; calcification of the blood vessels, heart, and other soft tissues; and bone abnormalities. Dietary excesses as small as 10 times the recommended amounts may be toxic and are aggravated by excessive calcium intake.

No minimal requirement for vitamin E has been established. However, it has been established that selenium and vitamin E work together to prevent nutritional muscular dystrophy (white muscle disease, see White Muscle Disease in Goats), equine degenerative myeloencephalopathy, and equine motor neuron disease. Evidence of vitamin E deficiency is most likely to appear in foals nursing mares on dry winter pasture or horses fed only low-quality hay unsupplemented with commercial concentrates. Horses forced to exert great physical effort and/or fed high-fat (>5%) rations may have increased needs for vitamin E. However, if selenium intakes are adequate, it is likely that 50 IU vitamin E/kg feed dry matter is adequate for most stages of the life cycle and moderate activity. Supplementation with 500–1,000 IU vitamin E may be necessary for horses working hard and/or fed high-fat (>7%) rations. Excessive supplementation (>5,000 IU/day for an average adult horse) results in decreased vitamin A absorption and should be avoided.

Vitamin K is synthesized by the microorganisms of the cecum and colon in sufficient quantities to meet the normal requirements of horses. However, consumption of moldy sweet clover hay may induce vitamin K–dependent coagulation deficits (see Sweet Clover Poisoning). The synthetic form of vitamin K (menadione) is nephrotoxic if administered parenterally to dehydrated horses.

Mature horses synthesize adequate amounts of ascorbic acid for maintenance from glucose in the liver. Some horses may need supplemental ascorbic acid (0.01 g/kg body wt/day) during periods of severe physical or psychologic stress, eg, prolonged transportation or weaning. Oral availability is variable. Ascorbyl palmitate is reportedly more readily absorbed than ascorbic acid or ascorbyl stearate. Prolonged supplementation to nonstressed horses may reduce endogenous synthesis and/or enhance excretion, resulting in deficiencies if supplementation is abruptly discontinued.

Although thiamine is synthesized in the cecum and colon by bacteria and ~25% of this may be absorbed, thiamine deficiency has been reported in horses fed poor-quality hay and grain. Although not necessarily a minimum value, 3 mg thiamine/kg ration dry matter has maintained peak food consumption, normal gains, and normal blood thiamine concentrations in skeletal muscle in young horses. As much as 5 mg/kg feed dry matter may be necessary for horses exercising strenuously, although verifiable deficits have not been recorded. Occasionally, horses are poisoned by consuming plants that contain thiaminases, which results in acute deficits (see Bracken Fern Poisoning).

Riboflavin deficits have not been documented in horses. Previous correlations with low riboflavin intake and recurrent uveitis in horses have not been substantiated. However, there is no evidence of toxic effects as a result of supplementing this water-soluble vitamin, and daily intakes of 0.04 mg riboflavin/kg body wt are recommended.

Intestinal synthesis of vitamin B₁₂ is probably adequate to meet ordinary needs, provided there is sufficient cobalt in the diet. Deficiencies of cobalt in horses have not been reported. Vitamin B₁₂ is absorbed from the cecum, and feeding a ration essentially devoid of vitamin B₁₂ for 11 mo had no effect on the normal hematology or apparent health of adult horses. Vitamin B₁₂ injected parenterally into racehorses and foals is rapidly and nearly completely excreted via bile into the feces.

Niacin is synthesized by the bacterial flora of the cecum and colon and is synthesized in the liver from tryptophan. There is no known dietary requirement for niacin in healthy horses.

Folacin, biotin, pantothenic acid, and vitamin B₆ probably are synthesized in adequate quantities in the normal equine intestine. However, biotin supplementation (15–25 mg/day) has been documented to improve hoof quality in adult horses with soft, shelly hoofwalls.