Nutritional Requirements of Horses and Other Equids

Maintenance Requirements of Horses and Other Equids

For maintenance of body weight and to support normal daily activity, the digestible energy (DE) requirement of nonworking adult horses in good body condition is estimated to be on average 0.03 Mcal/kg body weight (see the table Estimated Energy Requirements of Work for Light Horses and Desirable Body Condition Scores), with a minimum requirement of 0.03 Mcal/kg for easy keepers (ie, draft horses, warmblood breeds, some Morgans, Quarter Horses, and most ponies) and 0.04 Mcal/kg for hard keeper horses (ie, Thoroughbreds and related breeds). See also information about feeding obese or emaciated horses and the table Definitions of Workload and Associated Guidelines for NRC definitions of light, medium, heavy, and very heavy work.

Cold weather increases the energy requirement, especially for equids with thin or clipped coats and those not housed in stalls or lacking adequate outdoor shelter. The lower critical temperature (LCT) is the temperature below which an increase in metabolic heat is needed to maintain body temperature.The LCT of cold-adapted adult horses in northern Canada was estimated to be –15°C (5°F) (12), whereas donkeys acclimatized to summer temperatures in Nevada had an LCT of 26°C (79°F). However, the actual thermoneutral zone for donkeys has not been calculated (13). Horses in more temperate climates had an LCT of approximately 5°C (41°F) (14).

Wind, precipitation, and body condition also affect LCT. Therefore, LCT must be estimated based on regional average temperatures and conditions as well as the individual horse. For example, draft breeds with thick coats would tolerate lower temperatures than thin-haired, thin-skinned Thoroughbreds. Age can also affect LCT, particularly in neonates and senior animals. Neonates may need supplemental warmth in the form of a blanket and/or dry shelter. Older horses with a BCS ≤ 4 may require an increase in digestible energy when temperatures fall below LCT, especially if the coat is not thick. The addition of forage is the best way to increase digestible energy, because forage fermentation in the cecum will provide an internal source of heat for the horse.

The upper critical temperature, or UCT, is at the high end of the thermoneutral zone, above which the horse must increase evaporative heat loss to maintain body temperature. The UCT averages approximately 25°C (77°F) for adult horses (14) and 38°C (100°F) for foals (8). When temperatures exceed UCT, additional water and salt should be readily available. Shaded areas with good ventilation (air movement) and coat clipping can also facilitate heat dissipation. It can take up to 21 days for a horse to acclimate to a change in climate—eg, when horses residing in northern continental areas travel south for winter competition. Maintenance energy needs are thought to increase when temperature exceeds the UCT; however, the degree to which this is so has not been quantified.

Growth Requirements of Horses and Other Equids

See recommendations in the table Estimated Minimum Daily Major Nutrient Requirements of Growing Horses and Ponies.

Intakes should be adjusted to maintain a body condition score of 5 and will differ with breed and regional feed availability. Warmblood breeds, 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. For a more precise estimate of energy intake for growth, the NRC's Nutrient Requirements for Horses includes an equation to more closely calculate digestible energy requirements for growth based on age in months (x), average daily gain (ADG; kg/day) and body weight (kg):

DE (Mcal/d) = {[(56.5x^−0.145) × BW] + (1.99 + 1.21x − 0.021x²)} × ADG.

Pregnancy and Lactation Requirements of Horses and Other Equids

During pregnancy, weight gain is expected to be 12–16% of the mare's normal, healthy body weight (15). Although most of the weight gain will occur during the third trimester of gestation, nutrient needs begin to increase around the fifth month of pregnancy (see the tables Approximate Minimum Energy Requirements Over Maintenance for the Nonexercising Pregnant Mare and Estimated Increases in Crude Protein and Lysine Requirements for the Nonexercising Pregnant Mare).

Digestible energy, crude protein, and lysine needs are estimated to increase approximately 3–4% over maintenance each month, beginning in month 5 (8), but will vary by individual. Voluntary intake of roughage may decrease 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 or fat/oil in late pregnancy in higher-maintenance mares.

In general, a concentrate specifically formulated for broodmares and growing foals is recommended to supplement forage; an alternative is adding an appropriate amount of a ration balancer to the diet when a concentrate is not being used, is being fed below manufacturer recommendations, or is not specifically formulated for pregnant mares and growing foals.

To support lactation, the NRC publication estimates that 792 kcal of DE/kg of milk produced per day should be added to the increased maintenance needs. Lactating light breed horses (eg, Thoroughbreds, Quarter Horses, Arabians) maintained body weight when fed 28–31 Mcal DE/day. According to the NRC, 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. See the table Estimated Nutrient Requirements for the Lactating Mare.

The NRC recommends 12.7 Mcal/day for the first 2 months of lactation in pony mares with a baseline body weight of 200 kg (440 pounds); however, because ponies tend to gain weight easily, a close eye should be kept on condition. The mare's body condition should be evaluated on a regular basis and maintained in the range of 5 to 6 using the body condition scores of 1 to 9 throughout pregnancy and lactation.

Mares should be maintaining or gaining body weight to optimize reproductive success during the subsequent breeding season. Broodmares should be maintained around a BCS of 5–6. A higher BCS increases the risk of metabolic complications and/or dystocia, whereas a BCS below 5 can increase the difficulty of conceiving. Note that there is no scientific evidence to support the practice of withholding feed to decrease or stop milk production.

Energy Requirements for Work of Horses and Other Equids

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. For these reasons, DE recommendations for various activities of light horses should be adjusted to meet individual needs and to maintain body condition scores between 4 and 6 for optimal athletic performance, depending on performance type (racing versus longer-term performance, such as show, distance competitions, or riding lessons/training). See the table Definitions of Workload and Associated Guidelines for NRC definitions of light, medium, heavy, and very heavy work.

Calcium and Phosphorus Requirements of Horses

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. Excessive calcium intake (> 1% of total ration) should be avoided in horses if renal function is decreased or if there is a history of urolithiasis.

For all horses, the calcium:phosphorus ratio should be maintained at > 1:1. A desirable ratio is approximately 1.5:1; however, 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 moderately increases calcium and phosphorus requirements. If the calcium:phosphorous ratio is reversed or if excessive dietary phosphorous is consumed, calcium absorption can be inhibited, resulting in skeletal abnormalities and nutritional secondary hyperparathyroidism.

Salt Requirements of Horses

Salt (NaCl), among other electrolytes, is crucial for a number of physiological processes, including water balance, muscle contraction, and acid-base balance. Salt requirements are markedly influenced by sweat losses in equine animals, unlike other domestic species that do not sweat. (See the table Estimated Sweat Production in Horses.) Horses are able to sweat at higher rates than any other mammal, and equine sweat is hypertonic. As such, horses can lose high quantities of electrolytes, particularly sodium, chloride, and potassium, when they sweat (25).

The NRC recommends that horse rations contain 1.6–1.8 g salt/kg feed dry matter; however, 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 hardworking horses is recommended to be no more than 6% of the total ration, although at this level, it might decrease voluntary feed intake. (See the table Estimated Sweat Composition and Electrolyte Replacement in Horses.)

Research has shown that a properly balanced electrolyte supplement is quickly absorbed from the GI tract to replace depleted electrolytes and should be given with water if the horse is dehydrated or sweating heavily (26). Note that adding dextrose to electrolyte mixtures has not been shown to increase the rate of electrolyte absorption in horses, so sources of salt and other electrolytes ought not to include high concentrations of dextrose or other sugars (27).

A hyponatremic state stimulates an appetite for salt (28). Horses voluntarily seek out and consume salt if given the opportunity, but consumption varies. Salt, either in block form or loose in containers, should be available free-choice.

Most commercial concentrate feeds do not provide sufficient salt, and sodium is not found in notable concentrations in forages, so a free-choice salt source is recommended. In addition, consumption of salt varies widely by individual—in one study, ranging from 19 to 134 g/day (29). Supplemental salt and electrolytes can be provided by oral dosing or added to feed or water to replace acute losses during hard work; however, prolonged, forced supplementation, if not needed, will enhance excretion, which will decrease the homeostatic hormonal ability to adjust to acute losses. If supplementing salt or electrolyte mixes in water, be sure to concurrently provide plain water in case the horse objects to the mixed solution.

Forced oral administration of concentrated salt pastes (electrolytes) to dehydrated horses can cause abdominal malaise and can potentially irritate equine gastric ulcer syndrome in any horse if not given concurrently with food or diluted in water. Some horses, usually those confined to stalls, may ingest excessive amounts of salt, possibly because of restricted feed intake and/or boredom. This generally will not cause health problems as long as adequate forage and water are available; however, 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 may decrease palatability and limit voluntary intakes, precluding toxicity but putting the horse at risk of energy deficits.

Magnesium Requirements of Horses

Magnesium is an important blood ion and enzyme activator and plays a role in muscle contraction. Most magnesium in the body is contained in the skeleton (60%) and muscle (30%) (8). The daily magnesium requirement for maintenance of horses has been estimated at 0.015 g/kg body weight, according to limited studies (8). Working horses are estimated to require 0.02 to 0.03 g/kg body weight for light to strenuous exercise, respectively, mostly because of sweat losses. Magnesium 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. Mixed, mostly grass hay samples analyzed at a commercial forage laboratory contained a range of 0.15–0.31% magnesium (30). 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 according to data from other species; however, adult horses have been fed rations with higher magnesium content without apparent adverse effects. Anecdotally, high magnesium intake has a pharmacological calming effect on horses; however, no data in horses have been reported to this effect, and oversupplementation could disrupt mineral balance. A large dose of magnesium sulfate (ie, Epsom salts) can be used as a saline laxative for treatment of intestinal impactions but can cause magnesium toxicosis if overdosed, so caution should be used with this approach (8).

Potassium Requirements of Horses

Potassium is critical for proper neuromuscular function and, as an intracellular cation, for acid-base balance. Most potassium (approximately 75%) in the body is found in the skeletal muscle (8). The recommended potassium intake for maintenance in adult horses is 0.05 g/kg body weight. Mixed, mostly grass hay samples analyzed at a commercial forage laboratory contained a range of 1.2–2.5% potassium, and a ration containing ≥ 50% roughage provides more than sufficient potassium for maintenance animals (30).

Working horses, lactating mares, and horses receiving diuretics have higher potassium needs because of sweat, milk, and urinary losses; however, if on high-forage rations, they should not need additional supplements unless they experience acute, large losses (eg, prolonged competition, as in endurance). Hard work may increase intake needs by a factor of 2; however, high-quality forage comprising ≥ 50% of the diet should accommodate the increased demand.

It has been proposed that rations fed to hardworking horses should provide 4.5 g potassium/Mcal DE (8). 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 hardworking horses.

In horses with the genetic defect hyperkalemic periodic paralysis (HYPP), potassium intake needs to be restricted. Soaking forages in water will leach out potassium as well as water-soluble sugars. Some commercial electrolyte preparations may need to be avoided in affected animals. Potassium in the total daily diet of HYPP horses should not exceed 1%.

Iodine Requirements of Horses

Iodized salts used in salt blocks and 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 might require slightly higher intakes (0.4 mg/kg feed dry matter); however, 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.

Copper Requirements of Horses

Copper is an important mineral for enzymes that affect connective tissue, mobilization of iron stores, mitochondrial integrity, melanin, and detoxification of superoxide (8). The dietary copper requirement for adult horses weighing 500 kg (1,100 pounds) at maintenance is estimated to be 100 mg/day in the total ration based on limited data. Many commercial concentrates formulated for horses contain > 20 ppm. Mixed, mostly grass hay samples analyzed at a commercial forage laboratory had an average copper concentration of 7.9 ppm, which, assuming an intake of 2.5% BW as fed, would provide roughly 99% of the horse's minimum daily needs (30). Excessive iron supplementation (fairly common, especially in performance horses [see below]) can 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 (8). 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 decrease the absorption and utilization of selenium and iron and should be avoided.

Iron Requirements of Horses

Iron is predominantly contained in hemoglobin, myoglobin, cytochromes, and a number of enzyme systems. Iron plays a key role in oxygen transport in the blood and cellular respiration (8). 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, with no ill effects.

Only horses with chronic blood loss (eg, intestinal or tick parasitism) should be considered to be at risk of iron deficiency. Excess iron intake may interfere with copper absorption and utilization and cause microcytic, microchromic anemia. Therefore, anemia alone is not a sufficient indication for iron supplementation in horses. Iron is absorbed more efficiently in newborn foals, which are at particular risk for toxicity. Excess iron in the form of ferrous fumarate has been reported to cause death in neonates. In adult nonruminants, dietary iron absorption is estimated to be 15% or less. Ponies fed 50 mg/kg BW/day of ferrous sulfate for 8 weeks did not show clinical signs of toxicosis; however, this far exceeds the amount fed in a typical diet (8).

Zinc Requirements of Horses

Zinc is part of over 100 different biological enzymes, with the highest concentrations of the mineral found in the iris and prostate gland. Other tissues with moderate zinc concentrations include skin, liver, bone, and muscle (8). The zinc requirement is estimated to be 40 mg/kg feed dry matter to prevent clinical signs of deficiency in most horses. This mineral is relatively innocuous, and intakes several times the requirement are considered safe; however, intakes > 1,000 ppm, due to contamination of forages from environmental pollution, have induced copper deficiency and developmental orthopedic disease in young horses. In practice, nutritionists commonly formulate rations to include a ratio of 3:1 to 4:1 of zinc:copper to maintain balance between these minerals.

Selenium Requirements of Horses

Selenium is an essential part of the antioxidant glutathione peroxidase and also affects thyroid hormone metabolism. Notably, this trace mineral has a very narrow range of safety, relative to other minerals. The dietary requirement for selenium is estimated to be 0.1 mg/kg feed dry matter in the total ration. Adult horses weighing 500 kg (1,100 pounds) require a minimum of 1 mg Se per day, with only modest increases needed to support heavy exercise and lactation (8). However, there are regions of the world (including the lower Great Lakes, the Pacific Northwest, the Atlantic coast, and Florida in the US, as well as parts of New Zealand) where acidic soils are profoundly selenium-deficient and supplementation can be necessary. In other areas associated with alkaline soils, including parts of Colorado, Wyoming, and North and South Dakota in the US, forages can contain 5–40 ppm of selenium, which is sufficient to produce clinical signs of toxicity.

Commercial feed concentrates are typically fortified with selenium. Laboratory analysis of mixed, mostly grass forage samples over 21 years reported an average of 0.194 ppm selenium, with a range from 0 to 1.2 ppm selenium, the middle to upper concentrations of which would provide sufficient daily selenium (30).

Exercise increases glutathione peroxidase activity and can increase need for supplementation in heavily exercised horses, but detailed recommendations are not available. There appears to be no advantage to supplementing the mature, idle horse with more than 0.1 mg selenium/kg ration (8). Clinical signs of acute toxicity may manifest as apparent blindness, head pressing, sweating, increased heart and respiration rates, and lethargy (8). Chronic toxicity can include mane and tail hair loss and cracking of hooves around the coronary band (8). Acute death was reported in polo ponies who received 2 g of selenium as sodium selenite via IV injection, which corresponds to 5 mg/kg body weight for a 500 kg horse, a lethal dose for most animals (31).

Other Mineral Requirements of Horses

The requirement for sulfur in horses is not established. However, sulfur-containing amino acids (methionine) and vitamins (biotin) are essential for healthy hoof growth. If the protein requirement is met, the sulfur intake of horses is usually approximately 0.15% dry-matter intake—a concentration apparently adequate for most individuals. Sulfur deficits may contribute to poor hoof quality. The NRC recommends approximately 15 g of sulfur per day for a 500-kg (1,100-pound) horse at maintenance, most of which will come from high-quality dietary protein containing cystine and methionine.

The dietary requirement for cobalt is apparently < 0.05 ppm (8). It is incorporated into vitamin B₁₂ by microorganisms in the cecum and colon and, therefore, is an essential nutrient per se only if exogenous sources of B₁₂ are not incorporated into the ration. The upper limit of intake is estimated to be 25 mg/kg feed dry matter according to data from other species (8).

Manganese is essential for carbohydrate and lipid metabolism. Requirements for horses have not been well established; amounts found in the usual forages (40–140 mg/kg dry matter) are usually sufficient. Laboratory analysis of mostly mixed grass forage samples show an average manganese concentration of 90.8 ppm, enough to meet daily requirements (30).

Fluorine requirements for horses have not been established; however, it is known to be involved in the proper development of bone and teeth. Forages may contain 2–16 mg/kg fluorine (DM basis), in contrast with cereal grains with 1–3 mg fluoride/kg dry matter. Intake should not exceed 40 mg/kg feed dry matter according to data from other species (32). Rock phosphates, when used as mineral supplements for horses, should contain < 0.1% fluorine. Excessive ingestion can result in toxicity; however, horses apparently are more resistant to fluorine excesses than are ruminants.

Although molybdenum is an essential cofactor for xanthine oxidase activity (which plays a key role in purine metabolism), no quantitative requirement for horses has been demonstrated. Excessive levels (> 15 mg/kg feed dry matter) may interfere with copper utilization. However, 1–3 ppm of molybdenum in forages, which interferes with copper utilization in ruminants, reportedly does not cause problems in horses (8).

Vitamin A Requirements of Horses

Vitamin A plays a key role in proper vision, particularly night vision, as well as in maintaining both adaptive and innate immune response and cell differentiation. Horses' vitamin A requirement usually can be easily met by consumption of beta carotene, a naturally occurring retinol precursor, which is converted to the active form in the mucosa of the small intestine and the liver, where it can be stored. Fresh green forages and good-quality hays are excellent sources of beta carotene, as are corn and carrots. It is estimated that 1 mg of beta carotene is equivalent to approximately 400 IU of active vitamin A (retinol/retinyl compounds) (8).

However, because of oxidation of beta carotene during storage, its content in 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; however, horses fed only conserved forages without access to fresh grazing could 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 (as either beta carotene or an active synthetic form such as retinyl acetate) (8). Commercial concentrate feeds and ration balancers for horses are typically fortified with vitamin A.

Prolonged feeding of excess (> 10 times recommended amounts) active retinyl or retinol compounds can cause bone fragility, bone exostoses, skin lesions, and birth defects such as cleft palate and micro-ophthalmia (according to data from both horses and other species) (8). Excessively high doses of vitamin A should be avoided in pregnant mares because of the risk of teratogenesis. The proposed upper safe concentration is 16,000 IU/kg feed DM of the active forms of the vitamin. No known toxicity is associated with beta carotene in horses.

Vitamin A deficiency classically manifests as night blindness, as well as impaired growth and hematopoiesis. While not unheard of, vitamin A deficiency is not common.

Vitamin A requirements for both pregnant and lactating mares are suggested to be 60 IU/kg BW, and 45 IU/kg BW for growth (8). Although no firmly established value has been elucidated, it is suggested that working horses need approximately 45 IU/kg BW, which is between the requirements at maintenance (30 IU/kg BW) and gestation/lactation.

Vitamin D Requirements of Horses

Vitamin D is required for the absorption of calcium in the small intestine and therefore has a key role in calcium homeostasis. Horses exposed to ≥ 4 hours of sunlight per day or that consume sun-cured hay do not have additional 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, resulting in calcification of soft tissue, 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.

Vitamin E Requirements of Horses

The main role of vitamin E is as a biological antioxidant. Vitamin E is lipophilic, incorporating itself into cell membranes to provide protection from oxidative damage. 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, or roughly 1,000 IU/day for a 500-kg (1,100-pound) horse.

However, it should be noted that the NRC recommendations do not differentiate between synthetic (dl-alpha-tocopherol) and natural (d-alpha-tocopherol) forms of vitamin E. The natural form of vitamin E is known to be more bioavailable per international unit compared to the synthetic form. It is estimated that 1 IU of natural vitamin E is equivalent to 0.67 mg of alpha-tocopherol, whereas 1 IU of synthetic vitamin E equals 0.45 mg of alpha-tocopherol (33). Hence, if supplementing synthetic vitamin E (dl-alpha-tocopherol), it is suggested that an additional 34% be included to make up for the difference. In other words, 1,000 IU of natural vitamin E is estimated to be roughly equivalent to 1,340 IU of synthetic vitamin E.

Evidence of vitamin E deficiency is most likely to appear in foals nursing mares on dry winter pasture or in horses fed only low-quality hay unsupplemented with commercial concentrates or straight vitamin E. Horses doing prolonged aerobic work (eg, endurance and distance riding, multiple lessons per day) and/or fed high-fat (> 5%) rations could have increased needs for vitamin E due to the higher production of free radicals from exercise and lipid metabolism. Supplementation with 500–1,000 IU vitamin E/day might be necessary for these horses and for those that do not have access to fresh pasture. Excessive supplementation (> 5,000 IU/day for an average adult horse) in healthy horses should be avoided.

Vitamin K Requirements of Horses

Vitamin K is important in the process of blood clotting. Dietary vitamin K requirements for horses have not been established. It is thought that vitamin K is synthesized by the microorganisms of the cecum and colon in sufficient quantities to meet the normal requirements of horses, with contributions from dietary sources (phylloquinone, the form found in plants/forage) as well. However, consumption of moldy sweet clover hay can induce vitamin K–dependent coagulation deficits. The synthetic form of vitamin K (menadione) is nephrotoxic if administered parenterally to dehydrated horses.

Ascorbic Acid Requirements of Horses

Ascorbic acid, or vitamin C, is a water-soluble vitamin that functions as a biological antioxidant that the horse can produce endogenously in the liver. Mature horses synthesize adequate amounts of ascorbic acid for maintenance from glucose in the liver. Some horses could need supplemental ascorbic acid during periods of severe physical or psychological stress (eg, illness, hospitalization, prolonged transportation, or weaning). Equids with decreased liver function or illness may also need supplementation.

Oral availability is variable. Ascorbyl palmitate is reportedly more readily absorbed than ascorbic acid or ascorbyl stearate; however, ascorbic acid supplements were sufficient to increase or maintain plasma concentrations after prolonged (> 24 hours) transportation stress (34). Information about oral supplementation dosage varies in the literature, but a few studies provide insight. Horses affected by asthma have shown decreased ascorbic acid in pulmonary epithelial lining fluid (35). Supplementation with 10 mg/kg/day ascorbic acid (along with 6 mg/kg/day alpha-tocopherol and 5.1 mcg/kg/day selenium) could help improve exercise tolerance in horses with asthma (36). A group of healthy horses experimentally receiving IV ascorbic acid at 100 mg/kg, but not at doses of 0, 25, or 50 mg/kg, showed decreased determinants of reactive oxygen metabolites, although plasma ascorbic acid did increase in a dose-dependent manner. It is unknown how different doses have affected sick or stressed animals (37).

Prolonged supplementation to nonstressed horses could decrease endogenous synthesis and/or enhance excretion, resulting in deficiencies if supplementation is abruptly discontinued. Accordingly, vitamin C supplementation should be tapered off over the course of a week or more.

Thiamine Requirements of Horses

Thiamine, also known as vitamin B₁, is water-soluble and is necessary for carbohydrate metabolism. Dietary sources include cereal grains, grain by-products and brewer's yeast. Thiamine is synthesized in the anterior part of the large colon; however, production and absorption might not be sufficient to meet an animal's needs under some circumstances. The NRC has therefore set minimum dietary requirements of 0.06 mg/kg BW/day, which is typically easily achieved with a balanced diet. 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 (8). Up to double the daily maintenance intake of thiamine is recommended for horses exercising strenuously; however, verifiable deficits have not been recorded.

IV administration of thiamine solutions can cause adverse reactions, so oral supplementation is preferable.

Riboflavin Requirements of Horses

Riboflavin, or vitamin B₂, is a precursor for nucleotides necessary for ATP synthesis, drug metabolism, lipid metabolism, and antioxidant defense mechanisms (8). Riboflavin synthesis occurs in the small and large intestines, with higher production in the latter, which is indicative of microbial riboflavin production. Dietary sources include legumes, such as alfalfa, as well as grass hays, with very little found in cereal grains. Horses fed forage-based diets of approximately 2–2.5% body weight DM typically consume more than enough riboflavin (8). 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.

Vitamin B Requirements of Horses

Vitamin B₁₂ (cyanocobalamin) is necessary for formation of RBCs and is a component of multiple enzyme systems, including protein synthesis and metabolism of fat and carbohydrates. 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 (8). Vitamin B₁₂ injected parenterally is rapidly and nearly completely excreted via bile into the feces and is not recommended.

Cobalt (Co), an essential component of vitamin B₁₂, stimulates the synthesis of RBCs and has therefore gained attention for the potential of performance enhancement via increased hematopoiesis. In the US, the Association of Racing Commissioners International measures plasma thresholds as the maximum allowable concentration of a substance in plasma. For cobalt, the primary and secondary plasma thresholds describe the difference between the normal nutritional level of this element and the prohibited, performance-enhancing level. Specifically, they have defined a primary plasma threshold of 25 ng/mL and secondary threshold of 50 ng/mL (38). Research has shown no performance-enhancing effects (ie, changes in aerobic capacity, plasma erythropoietin, hematocrit) in exercising Standardbreds given 50 mg of elemental cobalt as CoCl₂ in 10 mL of saline, IV, over 3 days before exercise (39). It has been suggested that improper cobalt intake can put horses at risk of laminitis and soft tissue damage; however, more research is needed (40).

Niacin Requirements of Horses

Niacin (also known as nicotinic acid or vitamin B₃) is involved in a number of oxidation-reduction reactions and has been reported to provide substrate for enzymes that are involved in DNA processing, cell differentiation, and cellular calcium mobilization (8). Niacin is synthesized by the bacterial flora of the equine hindgut and is synthesized in the liver from tryptophan. There is no known dietary requirement for niacin in healthy horses.

Other Vitamin Requirements of Horses

Folacin, pantothenic acid, and vitamin B₆ probably are synthesized in adequate quantities in the normal equine intestine. There are no known dietary requirements for these water-soluble vitamins; however, 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, after 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).