A to Z of Vitamin & Mineral Deficiencies & Toxicities in Horses
By Jess Hallas-Kilcoyne
Of all the nutrients that a horse requires to survive, vitamins and minerals represent the smallest percentage of the equine diet. Yet these tiny substances are critically important when it comes to keeping your horse’s body functioning and in good health.
Knowing the importance of vitamins and minerals to horse health should not lead to the assumption, “if some is good, more must be better.” There is an optimal range (sometimes wider, sometimes more narrow) for each vitamin and mineral required by the horse’s body, and the toxicity that can result from stepping past the upper limit of that range is often no less dangerous to a horse’s health than the deficiency caused by failing to meet the lower threshold. Supplementation, therefore, of any vitamin or mineral is unadvisable without the knowledge and approval of a veterinarian or equine nutritionist.
What follows is a brief overview of the vitamins and minerals – from vitamin A to the trace mineral zinc – that are most essential to horses, and the symptoms that commonly accompany their deficiency and toxicity.
Vitamins are organic compounds that are vital to a wide variety of biochemical processes in your horse’s body. They can be divided into two categories: fat soluble (vitamins A, D, E, and K) and water soluble (B vitamins and vitamin C). Fat soluble vitamins are more readily stored in the body (and thus are more likely to build up toxic levels when present in excessive amounts), while any excess of water soluble vitamins is excreted fairly promptly if it remains unused by the body.
Another way to classify vitamins is according to their source. Vitamins C, D, and niacin (one of the B vitamins) are all manufactured by the horse’s body from other organic compounds he has ingested. The other B vitamins and vitamin K are produced by the beneficial microbial organisms living in the horse’s caecum and large intestine. Vitamins A and E are the sole vitamins not produced within the horse’s body, and therefore must be supplied in the horse’s diet.
Photo (above): Generally speaking, a diet that is high in pasture and good quality hay and lower in concentrates will provide the average horse with essential vitamins and minerals in the quantities required, without the need for supplementation. Photo: ©CanStockPhoto/Rogera
Besides being crucial for good vision, vitamin A also plays a role in bone and muscle growth, reproduction, and healthy skin. The main precursor for vitamin A, beta-carotene, is present in fresh pasture and good quality hay in levels that are more than sufficient to satisfy the horse’s requirement. Vitamin A will be stored in the liver for up to six months, to be released as needed by the horse’s body.
Photo (above): By late winter, if fresh pasture and good quality hay have remained unavailable over a period of several months, a horse might be at risk of developing a deficiency in certain nutrients, such as vitamin A. Photo: Robin Duncan Photography
Deficiency: Vitamin A deficiency may develop when a horse’s diet is devoid of good quality green forage (not uncommon in the winter when pasture is unavailable and hay quality is often poor) for longer than six months. Symptoms of deficiency include night blindness, tearing of the eyes, bone and muscle growth defects, a dull coat, reproductive problems, and increased susceptibility to disease and infection.
Toxicity: Typically, vitamin A toxicosis only occurs as a result of over-supplementation in the diet. Mild toxicity can cause stunted growth and poor skin condition, while a more severe toxicity may result in depression, weight loss, hair loss (alopecia), neurological dysfunction, serious bone deformities, and death.
Vitamin B Complex
The vitamin B complex includes B1 (thiamine), B2 (riboflavin), niacin, pantothenic acid, B6 (pyroxidine), biotin, B12 (cobalamine), and folacin (folic acid or folate). The B vitamins are essential in the horse’s body for a wide variety of functions – including metabolism of carbohydrates, proteins, and lipids; energy metabolism; and the synthesis of epinephrine and norepinephrine, red blood cells, and DNA – that serve just about all the horse’s body systems.
Because they are abundantly present in most types of green forage, B vitamins in adequate quantities should be provided by a normal diet. Additionally, being water-soluble, they are unlikely to ever reach toxic levels in the horse’s body.
Deficiency: No signs of deficiency have been reported in horses for any of the B vitamins with the exception of B1 (thiamine) deficiency, which can occur when horses eat bracken ferns. Symptoms of thiamine deficiency include loss of appetite, poor coordination, edema, polyneuritis (inflammation of the nerves), convulsions, irregular heartbeat, gastrointestinal hemorrhage, and diarrhea. Thin or shelly hooves may be a sign of possible biotin deficiency, but research has not yet confirmed the optimal level of biotin in horses.
Toxicity: No signs of excess amounts of any of the B vitamins have been reported in horses.
Vitamin C (ascorbic acid) is an antioxidant that protects body cells from harmful free radicals. It also contributes to the formation of collagen in the body, and strengthens bones and teeth. Vitamin C is synthesized from glucose in the horse’s liver and is not a dietary requirement for this species as it is for humans.
Deficiency: The horse’s body may develop an increased need for vitamin C during periods of stress or illness, and in old age. Absorption of supplemental vitamin C by the horse’s intestine is poor; therefore, vitamin C must be fed in fairly high amounts in order to have an effect.
Toxicity: Being water soluble, any excess vitamin C in the horse’s body is quickly eliminated.
Sometimes called the “sunshine vitamin,” vitamin D is a hormone produced by 7-dehydrocholesterol in the skin when it receives adequate exposure to the sun’s ultraviolet rays. A horse that receives sufficient amounts of sunlight (at least six to eight hours per day) should not require dietary supplementation. Additionally, vitamin D is present in good quality hay, although the amount decreases the longer the hay is stored. Vitamin D assists the intestinal absorption of calcium and phosphorus, which are crucial to maintaining healthy bones and joints.
Deficiency: The inadequate amounts of calcium and phosphorus that result from vitamin D deficiency can cause defective calcification of the bones. This condition, characterized by the softening, deformation, and stunted growth of the horse’s bones, is known as rickets in the young of most species, osteomalacia in adults.
Toxicity: Toxic levels of vitamin D in horses cause depression, reduced appetite and weight loss, stiffness, and accumulation of calcium deposits in the heart, lungs, kidneys, and other organs. Chronic toxicity may be fatal.
While vital to the reproductive, muscular, nervous, circulatory, and immune systems, vitamin E’s most important function is perhaps as an antioxidant protecting cell membranes from peroxidative damage. Fresh pasture is the best source of vitamin E, although vitamin E is also present in good quality hay, particularly alfalfa. However, the amount of vitamin E in hay decreases with storage over time.
Photo (above): Alfalfa hay, while not an appropriate forage choice for all horses, is an excellent source of calcium, magnesium, and vitamin E. However, care must be taken to make sure alfalfa’s richness in these nutrients is balanced by the rest of a horse’s diet to ensure all his nutritional requirements are met. Photo: eXtensionHorses/Flickr
Deficiency: Signs of vitamin E deficiency, such as nutritional muscular dystrophy (muscle wastage and malformation), tend to be similar to those associated with selenium deficiency. Prolonged deficiency in horses aged two and older can cause equine motor neuron disease (EMND), which is characterized by trembling, constant weight shifting in the hind limbs, loss of muscle tone, and periods of prolonged recumbency.
Toxicity: Relatively high doses of vitamin E appear to be well tolerated by horses since signs of vitamin E toxicity have not yet been produced, but very high intakes may interfere with the absorption of other fat soluble vitamins.
While it has many functions in the body, vitamin K’s primary role is to activate blood coagulation. Vitamin K is also present in hay and is manufactured by the bacteria in the horse’s large intestine in adequate amounts so that it is extremely rare for supplementation to be needed.
Deficiency: Vitamin K deficiency can occur as a result of sweet clover poisoning, or any prolonged intestinal disease that compromises the bacteria in the gut. Clinical signs include decreased blood coagulation which usually presents initially with bleeding from the nose and may progress to internal bleeding, the symptoms of which are pale mucous membranes, weakness, and a rapid, irregular heartbeat.
Toxicity: Vitamin K toxicity is exceedingly rare and usually results in death caused by acute renal failure.
Unlike vitamins, minerals are inorganic substances, yet their role in equine body functions is no less important. There are 14 minerals that are considered essential for horses, and these can be classified as one of two categories: macrominerals and microminerals, or trace minerals. Macrominerals are required in relatively large quantities in the diet and include calcium, phosphorus, magnesium, potassium, sodium, chlorine (as chloride), and sulfur. Trace minerals, which are required in much smaller quantities, include iron, copper, iodine, zinc, manganese, cobalt, and selenium.
All minerals can be dangerous when present in high enough levels, yet toxicity in horses is rare. For that matter, so is deficiency. Far more problematic, because it is far more common, is imbalance of levels of minerals whose specific ratio to each other is required to optimize the horse’s health and wellbeing. Calcium and phosphorus are one such pair, sodium and chloride another.
Calcium (Ca) & Phosphorus (P)
Calcium and phosphorus are vital macrominerals in the horse’s body, working together to build and strengthen bone. Calcium is also essential to neuromuscular and cardiac function, cell membranes, blood clotting, and temperature regulation. Phosphorus, in addition to bone growth and maintenance, is responsible for aiding in energy metabolism and various cellular functions.
Calcium and phosphorus are presented together because it is absolutely critical that they be present in the horse’s body in the correct proportion (with at least as much calcium as phosphorus). A 1:1 ratio of calcium to phosphorus is considered ideal, although an excess of calcium can be tolerated as long as the level of phosphorus has reached a certain threshold. However, the amount of phosphorus should never exceed the quantity of calcium.
Forages, especially legume hay, are rich in calcium, while grains have a naturally high phosphorus content. Therefore, a low-forage, high-concentrates diet could result in calcium deficiency/excess phosphorus. Calcium deficiency can also be caused by ingestion of certain plants (e.g. sorrel, dock, rhubarb, etc.) that inhibit calcium absorption.
Imbalance: Excess phosphorus can impair calcium absorption, resulting in calcium deficiency. Calcium deficiency can result in abnormal bone development in young horses, osteomalacia or osteoporosis in older horses, hyperparathyroidism (“big head disease”), loose teeth, and weight loss. These same symptoms may also be caused by phosphorus deficiency.
Magnesium has an important function in muscle contraction, nerve impulses, and energy metabolism, as well as being one of the major minerals present in the horse’s skeletal structure. A horse’s daily requirement for magnesium is usually satisfied by the normal presence of this macromineral in most hays and grains.
Deficiency: Rare in horses, an insufficient quantity of magnesium in the body is believed to be a possible causal factor in excitability and muscle tremors or cramping. It may also play a role in insulin resistance and equine metabolic syndrome but, while a link between magnesium deficiency and diabetes in humans has been demonstrated, there is no scientific evidence to confirm this theory.
Toxicity: Over-supplementing with magnesium can have a severely negative impact on the horse’s digestive and metabolic systems.
Potassium is an electrolyte that is vital for maintaining cellular osmotic pressure and electrical activity, as well as regulating the body’s acid/base balance, and thus plays a crucial role in muscle and nerve function. Relatively high concentrations of potassium are found in most types of forage.
Photo (above): Hard-working horses are more likely to develop potassium deficiency, particularly when training or competing in hot, humid climates. A commercial electrolyte supplement can help prevent potassium loss and/or replenish potassium levels in the horse’s body. Photo: Rayand/Flickr
Deficiency: Because sweating results in potassium loss, horses in hard work, especially those in hot, humid climates, may experience symptoms of potassium deficiency, which include muscle cramps and/or weakness, fatigue, impaired ability to sweat, and decreased response to thirst. A commercial electrolyte supplement will replenish potassium levels in the body.
Toxicity: For most horses, excess potassium in the body is harmlessly excreted in the urine. However, potassium intake should be carefully restricted in horses with hyperkalemic periodic paralysis (HYPP), a genetic disease characterized by a buildup of potassium in the body that can severely impair muscle function.
Sodium (Na) & Chloride (Cl)
Both sodium and chloride have an important function in regulating body fluids and conducting the electrical impulses in nerves and muscles, and they are the most important of the minerals lost through sweat and urine during exercise (otherwise known as electrolytes). When combined, these two minerals form sodium chloride, or salt.
To compensate for the fact that most natural forages contain very little sodium, sodium chloride is often added to concentrates. As a general rule of thumb, salt should always be made available to horses, either free choice in the form of a salt block or by adding it to the feed. If offered free choice, the horse’s salt consumption should be monitored.
Photo (above): Salt can be offered free choice in the form of a salt lick or block, but the horse’s use of the lick should be monitored to prevent overconsumption. Photo: Katie Brady/Flickr
Deficiency: While uncommon in normal circumstances, sodium chloride deficiencies can occur in physically stressful situations involving intense exercise and/or hot, humid climates. Deficiency usually presents with uncoordinated muscle movement and chewing, decreased sweating, decreased performance, dehydration, constipation, loss of appetite, and general weakness.
Toxicity: Because excess sodium and chloride are both readily excreted in urine, high salt intake is usually only a problem if the horse doesn’t have access to clean, fresh water. Salt toxicity can result in colic, diarrhea, frequent urination, weakness, and death.
Sulfur is an important component of biotin (vitamin B ) and thiamine (B ), the amino acids methionine and cysteine, insulin, chondroitin sulfate, and heparin (an anticoagulant), making it essential to the health of many of your horse’s body systems. Organic sulfur (as opposed to inorganic sulfur, which is not absorbed by the horse’s body) is present in most forages and grains in adequate amounts to satisfy a horse’s requirements.
Deficiency: Sulfur deficiency has never been reported in horses and appears an unlikely problem.
Toxicity: Sulfur toxicity is very rare except in extreme cases of overdose. High sulfur intake is not likely to be cause for concern since sulfur is readily excreted in the horse’s urine and feces.
Cobalt is used by the bacteria in the horse’s hindgut in the synthesis of cobalamine (vitamin B12) – this is its only known function in the body.
Deficiency: No deficiencies in cobalt have been reported in horses.
Toxicity: No excesses in cobalt have been reported in horses.
Copper is the trace mineral heavily associated with the formation and maintenance of elastic connective tissue, metabolism of iron, energy production within the cellular mitochondria, and production of melanin. The copper requirement for most horses is thought to be met by the forages in a normal diet, but more research is needed before the exact requirements are definitively known.
Deficiency: Copper deficiency in horses is rare, but has been implicated in developmental orthopedic disease in young horses as well as arterial ruptures and chronic anemia. Loss of pigment in the hair coat may also be a sign of a deficiency.
Toxicity: For toxic levels to occur, copper intake must be relatively high, making copper toxicity in horses very rare. Possible symptoms may include liver damage.
Iodine’s only known function is as a vital contributor to the synthesis of the thyroid hormones thyroxin (T4) and triiodothyronine (T3), which affect every system in the horse’s body, including muscle metabolism and the nervous, respiratory, and cardiovascular systems. Most horses receive adequate iodine from their normal diet, the exception being horses fed forages grown in iodine-deficient soils.
Deficiency: Inadequate amounts of iodine in a horse’s body may result in a goiter (an enlarged thyroid gland) on the underside of the throat along the jaw. Other signs of an iodine imbalance include poor coat and hair loss, lethargy, low body temperature, and increased susceptibility to infectious diseases. Iodine-deficient mares also tend to give birth to stillborn or weak foals.
Toxicity: Iodine toxicity tends to produce clinical signs similar to those resulting from iodine deficiency. However, toxic levels of iodine are more common than deficits, and more likely to be fatal in extreme cases.
Iron is most often associated with exercise because adequate levels of iron in the horse’s body are vital to maintaining the functionality of hemoglobin, the molecule in red blood cells responsible for their ability to transport oxygen throughout the body. Most forages contain plentiful amounts of iron to meet the needs of a horse.
Deficiency: A deficiency of iron is rare, but when it does occur can lead to anemia (low red blood cell count) of which the first symptom usually noticed is decreased performance and fatigue.
Toxicity: Far more common than iron deficiency, iron toxicity can make a horse more susceptible to bacterial infections and impair the absorption of other minerals, including copper and zinc. Foals with iron toxicosis may experience depression, dehydration, diarrhea, liver failure, and death.
Manganese is essential to the metabolism of fats and carbohydrates and the synthesis of chondroitin sulfate, an important structural component of cartilage – functions which are mostly involved in energy use and skeletal formation. Most horses receive adequate intake of manganese through a normal diet that includes fresh pasture, good quality forages, and grains.
Deficiency: Manganese deficiency has not been observed in horses, but would likely result in defective bone growth such as limb deformities in foals and possibly a reduced amount of energy available for use by the horse’s muscles, which could result in decreased performance.
Toxicity: Of all the trace elements, manganese is one of the least toxic when present in very high concentrations; however, excess manganese might hinder the absorption of phosphorus.
Selenium’s most important role in the horse’s body is as an antioxidant, working alongside vitamin E to protect body tissues from free radicals at the cellular level. It also has a function in controlling thyroid hormone levels, preserving muscle integrity, and maintaining the horse’s immune system. The roles of selenium and vitamin E are so closely related that a deficiency in one can be compensated for by a sufficient quantity of the other. The major source of selenium in the equine diet is in forages which, if grown in selenium-deficient soils, may not contain adequate amounts of this trace mineral.
Deficiency: Mild selenium deficiencies can present with subtle signs such as poor coat quality, work intolerance, and reduced immune function. Gross deficiency can result in cardiovascular problems, muscle disorders such as “typing up,” cataracts, and impaired reproductive function.
Toxicity: The margin between ideal and toxic levels of selenium is very slight, and selenium toxicity can be far more dangerous than a deficiency. Symptoms of toxicity include heart and liver damage; degeneration of bones, joints, and muscles; cracking or complete sloughing off of hooves; and death.
Photo (above): Always consult your veterinarian before incorporating a vitamin or mineral supplement into your horse’s diet. In many cases, the toxicity that arises from overfeeding certain vitamins and minerals is equally (in some cases, even more) harmful to your horse’s health as deficiency. Photo: BLW/Wikimedia Commons
Zinc plays a crucial role in the metabolism of carbohydrates and proteins, and in bone development, and is important for healthy skin and hooves. Zinc is present in varying amounts in most forages and grains, with pasture generally containing higher quantities than dry forages.
Deficiency: Zinc deficiency can cause low insulin levels and reduced glucose tolerance leading to increased insulin resistance. Inadequate levels of zinc might also result in poor coat and hoof quality, reduced immune function, and increased likelihood of developmental orthopedic disease in foals.
Toxicity: Symptoms of zinc toxicosis, usually a result of horses grazing on pasture contaminated by a nearby metal refinery, include lameness or stiffness, bony limb deformities, and growth plate enlargement in foals. Excess zinc can also impair the metabolism of copper.
This article originally appeared in the September 2013 issue of Canadian Horse Journal.
Main Article Photo: Vitamin D is often called the “sunshine vitamin” because its production is triggered when the horse’s skin is exposed to sunlight. A minimum of six to eight hours of turnout each day is generally sufficient to ensure adequate levels of vitamin D in the horse’s body.