Nutritional Requirements for Horses
Although horses can use hay and other roughage much more efficiently than do other nonruminants such as poultry or pigs, this ability is limited by the anatomy of the equine GI tract and is less efficient than that of ruminants. The site of fermentation in horses is the cecum and large intestine, where large numbers of microorganisms digest hemicellulose and cellulose, utilize protein and nonprotein nitrogen, and synthesize certain vitamins. Some of the products of fermentation, such as volatile fatty acids and vitamins, are absorbed and used. Microbial protein synthesized from nitrogen entering the cecum and colon undergoes only limited proteolysis, and the supply of essential amino acids from an unbalanced dietary nitrogen source is not satisfactorily balanced by microbial amino acids for optimal growth. Horses, therefore, depend more on the quality of the protein in the ration than do ruminants.
Water requirements depend largely on environment, amount of work or physical activity being performed, nature of the feed, and physiologic status of the horse. The minimal daily water requirement of an adult horse is 33 mL/kg body wt/day (0.4 gal./100 lb/day) with average intakes of ~50 mL/kg body wt/day (0.65 gal./100 lb/day). Lactation or sweat losses, however, may increase the needs by 50-200%. Unlimited free access to clean water is usually recommended, although horses can easily adapt to only periodic access throughout the day if the amounts offered during the watering sessions are not limited.
Energy requirements may be classified into those needed for maintenance, growth, pregnancy, lactation, and work. Equations to estimate energy requirements at any state of performance or production have been derived primarily from studies of light horses (Table: Daily Nutrient Requirements of Growing Horses and Ponies and Table: Daily Nutrient Requirements of Mature Horses and Ponies). However, the need for energy differs considerably among individuals; some horses require much greater amounts of feed than others under similar conditions, and digestibility of feeds often differs greatly from published values. Therefore the caloric recommendations these formulas provide should be considered only a starting point to determine the actual energy needs of a given horse. Amounts fed should be adjusted to maintain a body condition score between 4 and 6 (see Table: Body Condition Scores for Horses). Emaciated and very thin horses have decreased stress tolerance and increased susceptibility to infections. Obese horses have decreased tolerance of exercise and heat, increased risk of laminitis and lipoma strangulation colic, and, if fasted, hyperlipidemia and hypertriglyceridemia. Obesity is also associated with insulin resistance and glucose intolerance.
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 weighing <600 kg (1,320 lb) is estimated to be 1.4 + (0.03 × BW), in which BW is the body weight in kg. The daily DE Mcal requirement for horses weighing >600 kg (1,320 lb) or draft and warmblood types of horses is estimated to be 1.82 + (0.0383 × BW) - (0.000015 × BW2). For obese or emaciated horses, the estimated ideal body weight in kg should be used in the equation rather than current body weight.
Cold weather increases the energy requirement by 0.00082 Mcal DE/kg BW 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) + (4.81 + 1.17X - 0.023X2) × 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. For foals <6 mo old in ambient temperatures <10°C, the equation 1.1 × (1.4 + 0.047 × BW), in which BW is the body weight in kg, should be used instead of the one(s) listed above to calculate the maintenance DE requirement. Draft or draft-cross breeds may require less.
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 National Research Council (NRC) has estimated that 792 kcal of DE/kg of milk produced per day (Table: Average Milk Production of Mares ) should be added to maintenance needs. However, this recommended level of energy intake has increased body weight gain in lactating ponies, indicating that it may exceed the needs of some breeds.
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 (Table: Energy Requirements of Work for Light Horses) should be adjusted to meet individual needs and to maintain desirable body condition.
Protein and Amino Acids:
Although some amino acid synthesis occurs in the cecum and large intestine, it is not sufficient to meet the amino acid needs of growing, working, or lactating horses; therefore, the protein quality of the feed is important. Weanlings require 2.1 g, and yearlings 1.9 g, of lysine/Mcal DE/day. Requirements for other dietary amino acids have not been established; however, the crude protein recommendations given in Table: Daily Nutrient Requirements of Growing Horses and Ponies, and Table: Daily Nutrient Requirements of Mature Horses and Ponies, should be adequate if good quality forages and concentrates are used in the ration.
Growing horses have a higher need for protein (14-16% of total ration) than mature horses (8-10% of total ration). Aged horses (>20 yr old) may require levels of protein equivalent to young, growing horses to maintain body condition; however, hepatic and renal function should be assessed before increasing the protein intake of old horses. Fetal growth during the last third of pregnancy increases protein requirements somewhat (10-11% of total ration), and lactation increases requirements still further (12-14% of total ration). Work apparently does not significantly increase the protein requirement, provided that the ratio of crude protein to DE in the diet remains constant and the increased energy requirements are met.
Because the skeleton is of such fundamental importance to performance of the horse, macromineral requirements deserve careful attention.
Excessive intakes of certain minerals may be as harmful as deficiencies; therefore, mineral supplements should complement the composition of the basic ration. For example, if the horse is consuming mostly roughage with little grain, phosphorus is more likely to be in short supply than calcium. However, if little roughage and large amounts of grain are being consumed, a deficit of calcium is more common. The total mineral contribution and availability from all parts of the ration (forages and roughages, concentrates and all supplements) should be considered when evaluating the mineral intake. However, aside from actual feeding trials, no suitable test for availability of minerals exists.
Requirements for calcium and phosphorusare 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 (>1.0% 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 tolerate a ratio of 3:1 and young adult horses a ratio of 6:1. 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 0.5-1.0% salt, 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. 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 blocks should be available free choice at all times. 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. Some horses, usually those confined to stalls, ingest excessive amounts of salt, possibly due to restricted feed intake and/or boredom. However, salt poisoning is unlikely unless a deprived animal is suddenly allowed free access to salt, or if water is not available to horses force-fed salt (electrolyte mixtures dosed PO). Excessive salt content of feed or water will limit voluntary intakes, precluding toxicity but putting the horse at risk of dehydration or energy deficits.
The most satisfactory method of providing 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 6.8 mg/lb (15 mg/kg) body wt based on limited equine studies. Working horses require 10-25% more magnesium for light to moderate exercise, respectively, due to sweat losses. The requirements for growth have not been well established but are estimated to be 0.07% of the total ration. Most feeds used for horses contain 0.1-0.3% magnesium. Though deficiencies are unlikely, hypomagnesemic tetany has been reported in lactating mares and stressed horses. Adding 5% magnesium oxide to the free-choice salt mixture has been reported to be protective in such cases. The upper limit of recommended intake is estimated to be 0.3% based on data from other species, but adult horses have been fed rations with higher magnesium content without apparent adverse affects.
Potassium requirements of foals fed a purified ration were determined to be 1% of the total ration. The requirement for sedentary adult horses is estimated to be 0.4% of the ration. Because most roughages contain >1.0% potassium, a ration containing =50% roughage provides sufficient potassium for maintenance animals. Working horses and horses receiving diuretics need more potassium due to sweat and urinary losses. Potassium may be supplemented by adding potassium salts such as KCl to the ration. Although upper safe limits have not been established, and excesses are usually efficiently excreted by the kidneys, acute hyperkalemia can induce potentially fatal cardiac arrhythmias. Therefore, excessive supplementation with potassium salts should be avoided.
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 ~0.15% dry-matter intake—a level that is apparently adequate for most individuals.
Most iodized salts fulfill the dietary iodine requirement (estimated to be 0.6 ppm). Iodine toxicity has been noted in pregnant mares consuming as little as 40 mg of iodine/day. Goiter due to excess iodine was noted in both mares and their foals, and several cases were associated with large amounts of dried seaweed (kelp) in the diet.
The dietary requirement for cobalt is apparently <0.05 ppm. It is incorporated into vitamin B12 by the 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 dietary copper requirement or horses is probably 8-10 ppm, though many commercial concentrates formulated for horses contain >20 ppm. The presence of 1-3 ppm of molybdenum in forages, which would interfere with copper utilization in ruminants, does not cause problems in horses. 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, high copper intakes potentially reduce the absorption and utilization of selenium.
The dietary maintenance requirement for iron is estimated to be 40 ppm. For rapidly growing foals and pregnant and lactating mares, the requirement is estimated to be 50 ppm. Virtually all commercial concentrates formulated for horses and most forages contain iron well in excess of the recommended concentrations. Only horses suffering chronic blood loss (eg, parasitism) should be considered to be at risk of iron deficiency. Excess iron intake potentially interferes with copper utilization.
Manganese requirements for horses have not been well established; amounts found in the usual forages (40-140 ppm) are considered sufficient. The zinc requirement is estimated to be 40 ppm of the ration. 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.
Rock phosphates, when used as mineral supplements for horses, should contain <0.1% fluorine. Fluorine intake should not exceed 50 ppm in the diet or 0.45 mg/lb (1 mg/kg) body wt. Excessive ingestion can result in fluorosis ( Fluoride Poisoning: Introduction).
Although molybdenum is an essential cofactor for xanthine oxidase activity, no quantitative requirement for horses has been demonstrated. Excessive levels (>15 ppm) may interfere with copper utilization.
The dietary requirement for selenium is probably not >0.2 ppm, but there are regions of the world (including the lower Great Lakes states, the Pacific northwest, the Atlantic Coast, Florida, and part of New Zealand) where soils are deficient. In other areas (including parts of Colorado, Wyoming, and North and South Dakota), forages may contain 5-40 ppm of selenium and produce toxicity ( Selenium Toxicosis: Introduction). Exercise increases glutathione peroxidase (selenium-containing enzyme) activity and may indicate an increased 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 ppm selenium intake.
The vitamin A requirement of horses can be met by ß-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 ß-carotene is equivalent to ~400 IU of active vitamin A. However, because of oxidation, the carotene content of forages decreases with storage, and hays that are stored >1 yr may not furnish sufficient vitamin A activity. Horses that have been consuming fresh green forage usually have sufficient stores of active forms of vitamin A in the liver to maintain adequate plasma levels for 3-6 mo. The NRC has suggested that diets for all horses should provide 30-60 IU vitamin A/kg body wt (13.6-27.2 IU/lb).
Prolonged feeding of excess active vitamin A (>10 times recommended amounts) may cause bone fragility, hyperostosis, epithelial exfoliation, and teratogenesis. The proposed upper safe concentration for chronic administration is 16,000 IU vitamin A/kg of dry ration. There is no known toxicity associated with ß-carotene.
Horses that are 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 D concentrations are 365-455 IU/lb (800-1,000 IU/kg) for early growth and 227 IU/lb (500 IU/kg) 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 requirement may be toxic and are aggravated by excessive calcium intake.
No minimal requirement for vitamin E has been established. Selenium and vitamin E work together to prevent nutritional muscular dystrophy (white muscle disease, White Muscle Disease). Evidence of vitamin E deficiency is most likely to appear in foals that are nursing mares on dry winter pasture or given 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 40-60 IU of vitamin E/kg (80-120 IU/lb) of ration is adequate for most stages of the life cycle and moderate activity. Supplementation with 500-1,000 IU vitamin E may, however, be necessary for horses that are working hard and/or are fed high-fat (>7%) rations.
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: Introduction). 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. Some horses may need supplemental ascorbic acid (5-20 g/day) during periods of stress. 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, resulting in deficiencies if supplementation is abruptly discontinued.
Although thiamine is synthesized in the cecum and colon by bacterial action and ~25% of this may be absorbed, thiamine deficiency has been seen in horses fed poor-quality hay and grain. While not necessarily a minimum value, 3 mg thiamine/kg ration dry matter has maintained peak food consumption, normal gains, and normal thiamine levels in skeletal muscle in young horses. As much as 5 mg/kg ration dry matter may be necessary for horses that are exercising strenuously. Occasionally, horses are poisoned by consuming certain plants that contain thiamine or antithiamines.
Under certain conditions, riboflavin may be required in the diet. Although some reports implicated riboflavin deficiency in equine recurrent uveitis ( Equine Recurrent Uveitis: Introduction), this has not been substantiated. Apparently, the dietary riboflavin requirement is not likely to be >2 mg/kg ration dry matter.
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 horses.
Folacin, biotin, pantothenic acid, and vitamin B6 probably are synthesized in adequate quantities in the normal equine intestine. Biotin supplementation (15-25 mg/day), however, has been documented to improve hoof quality in adult horses with soft, shelly hoofwalls.