Nutritional Requirements of Horses and Other Equids

For maintenance of body weight and to support normal daily activity, the digestible energy (DE) requirement of the nonworking adult horse in good body condition is estimated to be on average 0.03 Mcal/kg bodyweight (see related tables), with a minimum requirement of 0.03 Mcal/kg for easy keepers (ie, draft, warmblood, some Morgan, and Quarter horses and most ponies) and 0.04 Mcal/kg for hard keeper horses (ie, Thoroughbreds and related breeds). For obese or emaciated horses, the estimated ideal body weight in kg should be used rather than current body weight for calculating energy intake.

Caloric intake in obese equines should not be restricted for prolonged periods of time (24 or more hours fasting, less than 1% energy intake per day) for either weight loss or medical tests because of the risk of hyperlipidemia, especially in ponies, donkeys, and severely obese horses.

Cold weather increases the energy requirement, especially for equines that are not housed in stalls or that lack adequate shelter outside. The lower critical temperature (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.

See recommendations in the associated table. However intakes should be adjusted to maintain body condition scores of 5 or 6, which will differ with breed and regional feed availability. Warmblood, draft, draft-cross, pony, and easy keeper breeds may require 10%–20% less than recommended to sustain desired growth and avoid obesity and potential metabolic issues.

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 Mcal requirements by 1.11, 1.13, and 1.20, respectively. Voluntary intake of roughage decreases as the fetus gets larger (last 2 months of gestation), and it may be necessary to increase the energy density of the ration by using supplemental, higher-energy concentrates in late pregnancy in higher-maintenance mares.

To support lactation, the NRC (2007) has estimated that 792 kcal of DE/kg of milk produced per day should be added to the increased maintenance needs. Lactating light breed horses (eg, Thoroughbred, Quarter horses, Arabians) maintained body weight when fed 28–31 Mcal DE/day. According to the NRC (2007), lactating 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 earlier) throughout pregnancy and lactation. Mares should be maintaining or gaining in body weight to optimize reproductive success during the subsequent breeding season.

The energy requirements of work are influenced by many factors, including type of work, condition and training of the horse, 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^aand Desirable Body Condition Scores) should be adjusted to meet individual needs and to maintain body condition scores between 4 and 7 for optimal athletic performance, depending on performance type (racing versus long term such as show, distance competitions, or riding lessons/training).

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 years 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 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.

Salt (NaCl) requirements are markedly influenced by sweat losses in equine animals, unlike other domestic species. 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 can cause NaCl losses >30 g (1 oz) in only 1–2 hours of hard work. The upper limit for salt inclusion in the ration of even hard-working horses is recommended to be no more than 6% of the total ration, although at this level it might reduce voluntary feed intake.

However, NaCl is the only mineral for which horses are known to have true “nutritional wisdom.” Horses will voluntarily seek out and consume salt in amounts to meet their daily needs if given the opportunity and do not have acute sweat or lactation losses. Salt, either in block form or loose in containers, should be available free choice, but may not be used if the ration contains sufficient salt to meet dietary needs. Supplemental salt and electrolytes may be provided by oral dosing or added to feed or water to replace acute losses during hard work, but prolonged, forced supplementation, if not needed, will enhance excretion, which will reduce the homeostatic hormonal ability to adjust to acute losses.

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 (ie, electrolyte mixtures given orally 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 daily magnesium requirement for maintenance has been estimated at 0.015 g/kg body weight, based on limited studies. Working horses are estimated to require 0.02 to 0.03 g/kg body weight 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 commercial feeds used for horses contain 0.1%–0.3% magnesium. Although deficiencies are unlikely, hypomagnesemic tetany has been reported in lactating mares and acutely 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 weight. 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 have higher potassium needs because of sweat, milk, and urinary losses but if on high-forage rations should not need additional supplements unless there are acute, large losses (eg, prolonged competition, as in endurance). 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. This is easily supplied by most good quality forages and commercial concentrate feeds. 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.

In horses with the genetic defect hyperkalemic periodic paralysis, potassium intake needs to be restricted. Soaking hays in water will leach out potassium as well as the water-soluble sugars.

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 soils not deficient in the mineral. 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 adult horses is estimated to be 8–10 ppm in the total ration based on limited data. Many commercial concentrates formulated for horses contain >20 ppm. Excessive iron supplementation (fairly common, especially in performance horses [see below]) may inhibit adequate copper 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 also may cause hypochromic microcytic anemia and pigmentation loss.

Horses are extremely tolerant of copper intakes that would be fatal to sheep. Excessively high copper intakes (upper limit not established but estimated at 2–3 times the recommended level) 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 adult horses. 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, intestinal or tick parasitism) should be considered to be at risk of iron deficiency. Excess iron intake potentially interferes with copper absorption and utilization and may cause microcytic, microchromic anemia. Therefore, 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, due to contamination of forages from environmental pollution, have induced copper deficiency and developmental orthopedic disease in young horses.

This trace mineral is essential but has a very narrow range of safety, relative to others. The dietary requirement for selenium is estimated to be 0.1 mg/kg feed dry matter in the total ration. However, there are regions of the world (including the lower Great Lakes, the Pacific northwest, Atlantic coast, and Florida in the USA, as well as parts of New Zealand) where acidic soils are profoundly selenium-deficient and supplementation may be necessary. In other areas associated with alkaline soils, including parts of Colorado, Wyoming, and North and South Dakota in the USA, forages may contain 5–40 ppm of selenium, which is sufficient to produce clinical signs of toxicity.

Exercise increases glutathione peroxidase (selenium-containing enzyme) activity and may increase need for supplementation in heavily exercised horses, but detailed recommendations are not available. No more than 0.002 mg/kg body weight should be supplemented on a daily basis. Toxicity, such as mane and tail hair loss and horizontal hoof separation and loss, has been seen with as little as 5 mg selenium/kg feed dry matter. Acute death was associated with supplementation of 10 times the recommended amounts administered in feed.

The vitamin A requirement of horses usually can be easily met by beta-carotene, a naturally occurring retinol precursor, which is converted to the active form in the liver and stored there. Fresh green forages and good-quality hays are excellent sources of carotene, as are corn and carrots. It is estimated that 1 mg of beta-carotene is equivalent to ~400 IU of active vitamin A (retinol/retinyl compounds).

However, because of oxidation of beta-carotene during storage, the content of forages decreases with storage. Hays stored >1 year may not furnish sufficient vitamin A activity. Horses consuming fresh green forage for 3–4 months 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 months, but horses fed only conserved forages without access to fresh grazing may be at risk of deficiency. Rations for all classes of horses without access to fresh forages should provide at least 30 IU active vitamin A / kg body weight (whether as beta-carotene or an active synthetic form such as retinyl acetate).

However, prolonged feeding of excess active retinyl or retinol compounds (>10 times recommended amounts) may cause bone fragility, bone exostoses, skin lesions, and birth defects such as cleft palate and micro-ophthalmia (based on data from both horses and other species). The proposed upper safe concentration is 16,000 IU of the active forms of the vitamin per kg feed dry matter. There is no known toxicity associated with beta-carotene in horses.

Horses exposed to ≥4 hours 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. Deficits are rare but can cause bone abnormalities in rapidly growing young horses confined to stalls and fed only fresh cut forages.

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, equine degenerative myeloencephalopathy, and equine motor neuron disease. It is likely that 50 IU vitamin E/kg feed dry matter is adequate for most stages of the life cycle and moderate activity.

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 doing prolonged aerobic work (eg, endurance and distance riding, lesson horses used for multiple lessons per day, etc) and/or fed high-fat (>5%) rations may have increased needs for vitamin E. Supplementation with 500–1,000 IU vitamin E/day may be necessary for equine animals working hard aerobically and/or fed high-fat (>7%) rations. Excessive supplementation (>5,000 IU/day for an average adult horse) results in decreased vitamin A status 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. 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 weight/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, but human-grade ascorbic acid supplements were sufficient to increase or maintain plasma concentrations after prolonged (>24 hours) transportation stress.

Prolonged supplementation to nonstressed horses may reduce endogenous synthesis and/or enhance excretion, resulting in deficiencies if supplementation is abruptly discontinued.

Thiamine is synthesized in the cecum and colon by bacteria, and ~25% of this may be absorbed and meet normal needs. Thiamine deficiency rarely has been reported in horses, even those fed poor-quality hay and grain. Occasionally, horses are poisoned by consuming plants that contain thiaminases, which results in acute deficits. It has been reported that 3 mg thiamine/kg ration dry matter has maintained peak food consumption, normal gains, and normal blood thiamine concentrations 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. IV administration of thiamine solutions can cause adverse reactions, so oral supplementation is preferable.

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 the recommended daily intakes of 0.04 mg riboflavin/kg body weight might be appropriate for horses with compromised intestinal function.

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

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

Folacin, pantothenic acid, and vitamin B probably are synthesized in adequate quantities in the normal equine intestine. There are no known dietary requirements for these water-soluble vitamins, but they are generally recognized as safe to supplement.

Biotin supplementation (15–25 mg/day to adult horses) has been documented to improve hoof quality in horses with soft hoof walls, especially after a major ration change (eg, following importation to another country) or major GI disturbances (ie, intestinal resection prolonged, severe diarrhea, or drastic dietary changes associated with import/export from other countries).