Vitamins & Minerals
Vitamins & Minerals
Vitamins are necessary for all body functions. However, horses and other living things need vitamins in only very small amounts. In general, vitamin requirements are expressed in International Units per kilogram of body weight, or IU/kg. An interesting fact about vitamins is that many of them can be produced by the horse, in its digestive tract, as it needs them.
Vitamins belong to one of two types:
- Fat-soluble vitamins — a group that includes vitamins A, D, E, and K.
- Water-soluble vitamins — a group that includes the B complex vitamins and vitamin C.
Bacteria in the horse’s digestive tract can produce all vitamins except vitamins A and E, which must be obtained from the diet.
One of the biggest misconceptions about vitamins is that “if a little bit is good, a little more is even better.” In fact, because daily vitamin requirements are so small, overdosing is very easy to do—and many vitamins, if fed in excess, can be toxic to horses.
Each species of animal has its own unique vitamin requirements, and the vitamin requirements of one species do not necessarily apply to another. The amount of research done on equine nutrition so far is comparatively limited. Much more is known about the vitamin requirements of humans, and even of other livestock species. One thing that is known is that most horses grazing on high-quality pastures are not likely to have vitamin deficiencies. Pastures are an excellent source of both fat- and water-soluble vitamins.
Function — needed for vision, especially night vision; needed for healthy skin, muscles, and bone (especially in growing animals); involved in reproductive function; an “antioxidant” that helps to prevent damage to cells
Source — beta-carotene is the “precursor” that horses need for microbes to produce vitamin A in the small intestine; green forages, pasture, hay, and yellow vegetables (such as carrots) are all excellent sources of beta-carotene; remember that hay stored for more than one year can lose beta-carotene content Deficiency — causes loss of appetite, weight loss, dull hair coat, poor vision (especially night vision), excessive tearing of the eyes, and anemia; can cause poor reproductive performance in both mares and stallions
Toxicity — causes stunted growth in young animals, poor skin quality, bone abnormalities, decreased blood clotting.
Function — called the “sunshine vitamin”; needed for mineral utilization and bone formation; involved in the absorption and use of calcium and phosphorous
Source — synthesized in the horse’s skin from ultraviolet rays of the sun; another source is cut and “sun-cured” plants — hays; however, vitamin D is not produced until the plant is cut and then sun-cured Deficiency — causes rickets; a condition in which bones soften, bend, and bow out; rickets occurs in most mammal species, including humans
Toxicity — seen in horses, a cumulative effect over time; excess vitamin D stimulates abnormally high absorption of calcium and phosphorous; the excess calcium deposits in the heart, blood vessels, salivary gland, diaphragm, and other organs; signs include inability to exercise, increased resting heart rate, sensitivity in leg tendons, increased water intake and urination.
Function — enhances the immune system; essential for normal functioning of all cells in the body; involved in DNA production; enhances absorption and storage of vitamin A by the body; known as an “antioxidant” that helps to prevent damage to cells
Source — green, growing pastures are a good source of vitamin E; since many horses lack access to pasture, they must get vitamin E from their foodstuffs; microbes in the equine digestive tract can produce some vitamin E, but generally not enough to meet nutrient needs; therefore, most horses should receive vitamin E supplements in their diet
Deficiency — may be seen in combination with a selenium deficiency; causes “white-muscle disease” in foals; muscle wasting, infertility, decreased immunity, slowed growth rate in young animals; current research has determined an association with EDM (equine degenerative myeloencephalopathy) and EMND (equine motor-neuron disease); more research is in progress, and vitamin E supplementation may have a positive effect on these conditions
Function — needed for production of factors that ensure proper clotting of the blood
Source — bacteria in the horse’s cecum produce large amounts of vitamin K; also available from green, leafy plants—in either pasture or hay
Deficiency — rare in horses, because the cecum produces large quantities of vitamin K; however, moldy sweet clover or hay containing moldy sweet clover can induce a deficiency, impairing blood-clotting and increasing risk of abortion in broodmares; disruption of bacteria in the intestine — from such causes as colic, diarrhea, or antibacterial drugs — can impair vitamin K production; signs of deficiency include nosebleeds and hematomas
Toxicity — rare in horses; vitamin K is an interesting vitamin: in nature, vitamin K is a fat-soluble vitamin supplied from plants, but the horse’s body converts fat-soluble vitamin K to water-soluble vitamin K, which it stores in the liver and can excrete in the urine; for this reason, large amounts of vitamin K are not stored by the body; an injectable form of water-soluble vitamin K can be toxic to horses, causing kidney failure and death; however, the oral/fat soluble form of vitamin K does not seem to be toxic to horses.
Water-Soluble Vitamins Overview
Function — the “partner” mineral to phosphorus (see below); needed for bone and tooth health; involved in bone structure and strength; important for bone growth in young/growing animals; needed for muscle contraction and relaxation; needed for blood clotting; plays a role in temperature regulation
Source — plants, ground limestone, oyster shells; grass and hay diets generally have ample calcium; alfalfa and other legume hays are rich in calcium; grains are very low in calcium content; absorbed in the horse’s small intestine
Deficiency — calcium deficiencies can result if a horse is deficient in vitamin D, which is needed for the proper utilization, absorption, and metabolism of calcium; calcium also interacts in the proper utilization of other minerals; older horses are less able to absorb calcium; deficiencies can result in decreased bone density, fractures, other bone problems, stiff movement, lameness, weight loss, and tooth problems; because cereal grains supply very little calcium, diets low in forage and high in grain can create deficiency problems; calcium deficiency can also result from an excess of phosphorus in the horse’s diet, which causes a decrease in calcium absorption and can result in “bighead disease” (osteodystrophia fibrosa)
Toxicity — maintaining the correct calcium:phosphorus ratio is extremely important in the equine diet; for young/growing animals, it should be between 1:1 and 2:1; for adult animals, it should be between 1:1 and 3:1; because wheat bran has a high phosphorus content with an inverted calcium:phosphorus ratio, diets high in wheat bran can cause imbalances by binding up the available calcium in the horse’s body.
Function — chloride and sodium go together; sodium chloride, or salt, is needed to regulate body fluids/water metabolism; it is also critical to the process of sweating, conduction of electrical impulses for nerves and muscles, and maintenance of electrolyte balance; chloride is needed for the production of bile and digestive-tract secretions
Source — horses’ rations should contain 0.5% to 1% sodium chloride; many commercial feeds do not contain adequate amounts of salt; as mentioned above, equine-nutrition specialists in the U.S. recommend adding a salt/trace mineral mixture to the horse’s daily diet (in feed or free-choice), or at least making a trace-mineralized salt block available to the horse at all times
Deficiency — rare; horses in a regular exercise or conditioning program typically require more salt; horses tend to crave salt and will usually lick enough from a trace-mineralized salt block to meet their sodium requirements; although chloride requirements have yet to be scientifically established for the horse, it appears that if a horse’s sodium requirements are met, its chloride needs are also met
Toxicity — horses tolerate high levels of sodium chloride — and if they have adequate salt and water available on a daily basis, they are not likely to over-consume; salt toxicity has been seen in horses drinking salt water (brine), horses fed salt when “salt hungry” (from having had no access to salt/salt lick for a prolonged time), and horses that have had more than 2% salt added to their diets without access to adequate water; signs of salt toxicity include colic, diarrhea, frequent urination, weakness, staggering, paralysis of the hind limbs, going down and being unable to stand; death can result.
Function — important for bone health; involved in enzyme function
Source — most commercial feeds contain adequate amounts, as do most good-quality hays; supplements; trace-mineralized salt blocks; green, rapidly growing pastures are a poor source of magnesium
Deficiency — rare, but has been seen in lactating mares, foals, animals in transport or otherwise under stress; the condition known as lactating, transit, grass, or winter “tetany” can result—with low blood magnesium levels causing nervousness, muscle tremors, staggering, increased respiration rate, sweating, convulsions, or the animal going down and paddling — and can result in death; an intravenous solution of magnesium and calcium, given promptly, can save the animal; necropsy of dead animals shows evidence of mineralization of arteries, some organs, and muscle
Toxicity — none documented in the horse.
Function — the “partner” mineral to calcium (see above); important for healthy bones and teeth; needed by the body to metabolize and use energy
Source — mare’s milk is a fairly good source; cereal grains are higher in phosphorus but contain almost no calcium and (as mentioned previously) can cause imbalances in the calcium: phosphorous ratio; absorbed in the large intestine of the horse; the phosphorus content of forage is related to the soil on which the forage is grown; soils in some parts of the United States are deficient in phosphorus
Deficiency — can lead to decreased bone density, stiff movement, lameness, weight loss, and bone fractures
Toxicity — excess phosphorus in the equine diet binds calcium and decreases calcium absorption; a calcuim:phosphorus ratio of less than 1:1 causes calcium deficiencies; can result in “big-head disease” or hyperparathyroidism (an overactive parathyroid gland).
Function — critical for regulating osmotic pressure of the body’s cells; helps maintain the acid/base balance of cells; needed for nerve-impulse transmission, proper muscle function, and carbohydrate metabolism
Source — forages, both pasture and hays, are good sources; oilseed meals; molasses also has high content
Deficiency — usually seen only in horses on high-grain/low-forage diets; also seen in horses in heavy exercise regimes, which tend to lose large amounts of potassium in sweat and urine; signs include fatigue, muscle weakness, lethargy, exercise intolerance, and decreased food and water consumption; deficiencies have been documented in young, growing foals fed high-grain diets; deficient animals go off feed, lose weight, and become unthrifty; feed a high potassium substance, such as potassium carbonate or potassium chloride, until normal feed consumption resumes
Toxicity — potassium is not generally toxic to horses, as it is readily excreted in the urine; the exception: horses of Quarter Horse breeding that trace back to the stallion Impressive and suffer from HYPP (hyperkalemic periodic paralysis) need low-potassium diets because potassium accumulates in their bodies.
Function — an essential element of several amino acids, some B vitamins, insulin, other body constituents, and chondroitin sulfate (a component of cartilage, hoof, and joint lubricants)
Source — water, commercial feed
Deficiency — none known in the horse
Toxicity — little is known of toxicity in the horse; an incident of accidental poisoning of adult horses fed a concentrated sulfur compound known as “flower of sulfur” resulted in lethargy, colic, a foaming yellow discharge from the nose, labored breathing—and eventual death of 2 of the 12 horses affected.
Function — part of vitamin B12 molecules; needed for synthesis of vitamin B12 in the cecum/large intestine of the horse, for production of red blood cells, and for carbohydrate and fat metabolism
Source — pasture with adequate cobalt in the soil
Deficiency — cobalt is an integral part of vitamin B12; if adequate cobalt is supplied to the horse, no vitamin B12 deficiency should occur, since microbes in the digestive tract synthesize B12; no known cobalt deficiency documented in the horse, either naturally or in research studies
Toxicity — none documented in the horse.
Function — needed for synthesis of red blood cells, healthy bone formation, proper absorption, utilization, and transport of iron, synthesis of melanin for pigment in the hair, and proper synthesis of collagen; important for healthy connective tissue and cartilage; aids in production of “elastin” — which is needed for flexibility and strength of blood vessels; the liver regulates copper metabolism
Source — best source is trace mineralized salt containing copper carbonate or copper sulfate; also in commercial horse feeds
Deficiency — has been seen in the U.S. in areas where pasture and hays are grown on sandy soils or in swampy, mucky, or peat-type soils; diets high in zinc, sulfur, and iron may decrease proper utilization of copper; deficiency is seen more in foals than in adult horses; results in abnormal bone and cartilage growth, causing stilted gait, lameness, and swelling of the fetlock joints; some incidence of bony growths above and below the fetlock joints, known as “DOD,” or developmental orthopedic disease; copper injections have been used to reduce or eliminate symptoms
Toxicity — in the horse, seen only at very high levels in research studies.
Function — needed for synthesis of thyroid hormones, which are responsible for regulation of body metabolism and body heat; the thyroid gland, placenta, and lactating mammary gland can accumulate iodine from the bloodstream; iodine is passed to the foal through mare’s milk; interestingly, horses with either too much or too little iodine in the diet may show enlargement of the thyroid gland (known as goiter); microscopic examination of a sample of thyroid tissue may help with diagnosis
Source — iodized salt is a good source
Deficiency — enlargement of the thyroid gland; low blood-plasma levels of T3 and T4 (thyroid hormones: T3 is triiodothyronine and T4 is thyroxine) are helpful for determining deficiency in adult horses but not reliable in foals; foals with a deficiency can be stillborn or born weak and unable to stand or nurse, though their dams may show no visible signs of deficiency; deficiency most seen in the Midwest, referred to as the “goiter belt” of the United States; certain plants interfere with iodine use in the horse’s body: kale, white clover, rutabaga, turnips, cabbage, broccoli, and other “goitrogenic” plants.
Toxicity — goiter results; feedstuffs and supplements containing very high levels of iodine, such as kelp (a seaweed), can cause dietary problems for horses; toxicity can also result from the use of multiple supplements that contain iodine.
Function — needed for oxygen storage and transport in the cells of the body; 60% of the body’s iron is contained in hemoglobin in the red blood cells and 20% in myoglobin in muscle tissue; absorbed primarily in the small intestine; copper is essential for the absorption, transport, and utilization of iron; blood tests are available to determine hematocrit and blood-hemoglobin levels to establish the horse’s iron status; however, these tests are not sensitive until blood iron levels are quite low; determining blood ferritin levels is another and more accurate test for a horse’s iron status
Source — pasture, hay, and grains; most common feedstuffs provide adequate levels of iron in the diet
Deficiency — anemia results; iron deficiency in a horse’s blood is usually due to blood loss, either excessive (from injury, for example) or prolonged (possibly caused by parasite overload); dietary deficiency not usually seen in the horse; the horse has large stores of red blood cells in the spleen, which it draws on during exercise or excitement; this ability to draw on reserves can mask iron deficiency for quite some time; ferritin tests are a very good indicator of deficiency and are not affected by the release of red blood cells from the spleen
Toxicity — usually not from dietary sources but from iron injections, to which some horses have demonstrated allergic reactions (fatal in some cases); foals appear more sensitive to iron toxicity; ferritin testing is the most accurate way to determine excessive levels of iron; signs of iron toxicity may include depression, dehydration, diarrhea, and liver failure, as well as death; excess iron can deplete the body of zinc and increase susceptibility to bacterial infections; the horse’s body has no means of excreting excessive iron.
Function — essential for fat and carbohydrate metabolism and for growth and reproduction; needed for the synthesis of chondroitin sulfate, which promotes healthy cartilage; needed for bone formation, especially of the limbs, skull, and inner ear; acts as a “scavenger” that ties up free radicals, thus helping to prevent oxidative damage to cells
Source — pasture, hay, and grains (except corn)
Deficiency — not known in the horse
Toxicity — not known in the horse.
Function — a very important micro-mineral; absorbed in the small and large intestine; acts as a “partner” with vitamin E in helping to prevent cell damage due to oxidation; selenium is needed to produce an enzyme that protects the water-based portion of cells, and vitamin E protects the fat-based portion of cells, so together the two act as “antioxidants” or cell protectors; also involved in DNA and vitamin C synthesis; enhances immune-system function, vitamin A utilization, and many other functions; a diagnostic blood test for selenium status is available, with the blood-sample storage time and temperature being critical for accurate results
Source — pasture, hay, and grain can supply selenium, but supplements are the most reliable source because soil levels of selenium in the U.S. vary so widely; 37 states have areas known to be selenium deficient; 8 states have areas that are selenium-deficient and also areas where selenium levels are so high they can be toxic; selenium content can vary from field to field or even within the same field; plants grown on soil with a more basic pH are more likely to take up selenium, especially in areas with little rainfall; and soil with a more acidic pH is more likely to produce plants that are selenium deficient; for these reasons, a feed with equal selenium content may be toxic in one region and deficient in another; be careful with selenium supplements because the margin of safety is relatively narrow; to estimate the total selenium your horse is ingesting, have your forage source (hay and pasture) analyzed for selenium content and pay attention to the selenium content in all feed and supplements you are giving
Deficiency — causes an affliction known as “white muscle disease,” more common in young horses; signs include: weakness, impaired movement, difficulty swallowing, trouble nursing, problems with respiratory and heart function, degeneration of smooth, skeletal, and heart muscle; in selenium-deficient areas, some veterinarians recommend giving newborn foals a selenium/vitamin E injection
Toxicity — can result from ingestion of plants containing high levels of selenium or from overuse of supplements; margin between needed and toxic levels is very narrow; signs of toxicity include sweating, blind staggers, colic, diarrhea, increased respiratory and heart rates, loss of appetite, unthriftiness, head pressing, hair loss (especially mane and tail), and hoof loss with separation at the coronary band; a garlic odor on the breath is a good clue indicating possible selenium toxicity.
Function — part of the structure of enzymes involved in protein and carbohydrate metabolism; a component of skin and hair; involved in bone growth and development
Source — commercial feeds and supplements; zinc is absorbed based on the need of the body; uptake can be affected by the levels of other minerals in the body
Deficiency — not usually a problem in the horse; excess calcium in the diet decreases the absorption of zinc in horses; DOD (developmental orthopedic disease) or bone-growth problems can occur in young horses
Toxicity — toxicity has been seen in horses grazing pastures near industrial plants with high levels of zinc in air emissions; excess zinc in water can be a problem — this may occur when copper and galvanized pipes are joined during the plumbing process; seen more in young animals; signs include bone-growth deformities, stiff gaits, lameness, reluctance to move, low head carriage, arched back, anemia, poor condition, and decreased growth rate, blood and organ tissues contain high levels of zinc.