Adequate mineral intake in your horse’s diet is essential for overall well-being and function. Minerals play important roles in supporting tissue structure, enzyme reactions, energy metabolism, and maintaining electrolyte balance.
Mineral deficiencies can contribute to poor hoof and coat quality, joint issues, metabolic concerns, and growth issues. Excess intake of minerals can also cause health issues.
Minerals make up a small portion of the total equine diet, but can have a significant impact on your horse’s health and performance. These elements must be provided by the diet because they cannot be synthesized within the horse’s body.
Minerals that are required in larger amounts are known as macrominerals and include sodium, chloride, magnesium, calcium, phosphorus, and potassium. Minerals required in smaller amounts are known as trace or microminerals and include selenium, copper, zinc, manganese, iron, iodine and cobalt.
Mineral requirements for your horse change depending on its age, reproductive status and activity level. The ratios of different minerals in the diet must also be carefully balanced to meet your horse’s needs and support optimal health and performance.
You can analyze your horse’s diet online to check the levels of different minerals provided by your feeding plan. Our equine nutritionists can also help you balance your horse’s diet for free.
Mineral Requirements for Horses
Minerals are inorganic elements that are required in the horse’s diet in relatively small amounts compared to macronutrients.
The National Research Council’s (NRC) Nutrient Requirements for Horses (2007) outlines recommended intake levels of most minerals based on available research in horses or extrapolation from other species.
NRC requirements for minerals and vitamins represent the minimum required to prevent symptoms of deficiency. NRC values do not necessarily reflect requirements for optimal health which is why most nutritionists will recommend supplementing above NRC levels.
Macrominerals are typically required in quantities measured by grams whereas trace or microminerals are typically required in quantities expressed by milligrams (one thousandth of a gram).
Minerals are found in small amounts in forages, grains, horse feeds and in other additives used in the horse’s diet. The levels of minerals in forages and crops vary depending on the soil mineral levels, plant species and harvest conditions.
These nutrients are supplied as organic compounds (bound to carbon atoms) and inorganic compounds (no carbon atoms present); organic mineral complexes tend to be better absorbed and utilized in the horse’s body.
It is important to note that minerals are rarely present in the horse’s diet at the correct amounts or ratios unless a horse is being fed a mineral and vitamin supplement at the recommended feeding rate. In a study of 200 equine diets, over 90% were found to be deficient in one or more important trace minerals or vitamins. (Bruggink et al., 2018 unpublished)
In a recent review, Mad Barn analyzed the diets of over 6,500 horses for nutritional deficiencies. The vast majority of diets were over-supplying energy and protein while undersupplying key electrolytes and antioxidant minerals.
It is always recommended to submit forage samples for analysis to determine their mineral content and guide appropriate supplementation strategies to meet your horse’s needs.
Well-balanced vitamin and mineral supplements can help fill in any gaps in dietary intake from forage, grains and other feedstuffs.
Macrominerals are required in the horse’s diet in larger amounts. The quantities required are usually represented as a percentage of total dry matter in the diet or as grams per kilogram of feed.
The macrominerals for horses are:
- Calcium (Ca)
- Phosphorus (K)
- Magnesium (Mg)
- Potassium (K)
- Sulfur (S)
- Sodium (Na)
- Chloride (Cl)
These macrominerals are involved in the structure and function of tissues including bone formation, muscle contractions, fluid balance in cells, and nerve transmission.
Calcium & Phosphorus
Calcium and phosphorus have well-established roles in maintaining strong bones and teeth. Almost 99% of calcium and 80% of phosphorus in the body can be found in the bones and teeth.
These minerals are also important for supporting enzymatic reactions and they are components of cell membranes.
The levels of calcium and phosphorus in the diet need to fall within a specific ratio to ensure proper absorption and function.
Roles of calcium:
- Structure and function of bone
- Muscle contraction
- Function of cell membranes
- Enzymatic co-factor
- Blood coagulation
Roles of phosphorus:
- Structure and function of bone
- Component of cell membranes in the form of phospholipids and phosphoproteins
- Component of adenosine tri-phosphate (ATP) – the major energy currency of cells
Requirements for calcium and phosphorus have been published by the NRC for horses that are growing, horses that are pregnant, and for horses at maintenance.
Maintenance: A 500 kg (1100 lb) mature horse at maintenance requires 20 grams of calcium and 14.3 grams of phosphorus. This is equivalent to a Ca:P ratio of 1.4:1.
Gestation: During the last 3 months of gestation, an additional 0.032 grams of calcium are needed per kg of bodyweight. For a 500 kg mare, this is requirement is equivalent to 36 grams of calcium per day. Phosphorus requirements increase to 26 grams per day. This is equivalent to a Ca:P ratio of 1.4:1.
Lactation: During peak lactation, calcium and phosphorus requirements for a 500 kg mare are 56 grams and 36 grams per day, respectively. Mares often lose bone density in the first year post-foaling because calcium and phosphorus are mobilized from bone tissue. This is usually normalized by 24 months.
Growth: The calcium requirement for a growing horse is based on the current body weight and predicted daily growth. For a yearling that will reach a mature weight of 500 kg, the calcium and phosphorus requirements are estimated at 37 grams and 20 grams per day. This represents a higher Ca:P ratio of approximately 1.8:1.
In general, calcium and phosphorus should be given in at least equal amounts (1:1 ratio), but preferably higher levels of calcium to phosphorus.
A ratio between 1.2:1 and 1.5:1 (20% to 50% higher calcium than phosphorus) is considered ideal for most horses.
For growing horses, Ca:P ratios up to 6:1 are acceptable as long as the phosphorus requirement is met.
Legume hays, such as alfalfa and clover, are good sources of calcium. Alfalfa typically contains 0.8 – 2% calcium and 0.1 – 0.3% phosphorus, on a dry matter basis.
Grains such as oats, barley, and corn, contain lower amounts of calcium (0.05 – 0.1% dry matter) and higher phosphorus levels (0.3 – 0.4% dry matter). Horses on high grain diets often require mineral supplements that contain added calcium to ensure the appropriate ratio is maintained.
Phosphorus is typically bound within phytate in cereal grains and hay. Horses are unable to absorb phosphorus from phytate unless it is broken down by the enzyme phytase.
Although there is some phytase in the hindgut, most horses require supplemental inorganic phosphorus to meet their needs. Phytase is sometimes added to dietary supplements for horses.
Supplemental calcium is commonly given in the form of calcium carbonate (limestone), sulfate or oxide. All supplemental calcium sources demonstrate approximately 50% absorption rate in the digestive tract of horses.
Supplemental phosphorus is commonly given as inorganic sources, including monosodium phosphate or monocalcium phosphate.
Monosodium phosphate tends to have lower iron contamination and is the preferred source for horses with metabolic concerns such as insulin resistance, Cushing’s disease (PPID) or otherwise high iron intake.
Horses fed sub-optimal levels of these minerals or a calcium-to-phosphorus ratio less than 1:1, are at risk of deficiency symptoms.
Horses, like all animals, maintain strict control of calcium levels in the blood. When levels in the blood drop, hormones trigger calcium mobilization from bone, increased kidney retention, and enhanced gut absorption.
Long-term intake of low calcium or phosphorus can result in weak bone structure known as osteomalacia.
Low calcium levels in the body can also occur when phosphorus levels in the diet are too high; this interferes with calcium absorption from the gut.
This can result in a condition called nutritional secondary hyperparathyroidism (NSH) which involves persistently elevated parathyroid hormone levels in an effort to maintain blood calcium levels.
In growing horses, insufficient calcium or phosphorus can cause rickets. Rickets is a condition in which the bone is not properly mineralized. It can result in enlarged joints and crooked long bones.
In mature horses, low calcium intake can create weak bones and shifting lameness.
Vitamin D deficiency may also result in these conditions because insufficient vitamin D can impair calcium absorption and cause a secondary deficiency.
In horses, Miller’s disease can result from excess dietary phosphorus which inhibits calcium absorption. This disease is characterized by enlarged bones in the face and is often referred to as “big head disease.”
The maximum level of calcium in the diet should be 2% of total dry matter. The maximum tolerable concentration of phosphorus in the diet is 1%. These values may be adjusted to maintain an appropriate calcium to phosphorus ratio.
Magnesium is an important macromineral that acts as a cofactor for over 300 metabolic processes in the body. It is important for muscle and nerve function, bone health, and energy production.
Severe deficiency is rare as common feeds typically contain 0.1 – 0.3% magnesium. Forage usually provides enough magnesium to meet the horse’s minimum requirements.
However, additional magnesium supplementation may have a calming effect and support muscle recovery from exercise.
- Along with calcium, involved in the contraction and relaxation of muscles
- Important for nerve impulse transmission
- A component of bone; supports skeletal structure and bone formation
- Involved in ATP production as the primary energy source for cells
According to the NRC, the magnesium requirements for horses are:
Maintenance: 15 mg/kg BW (7.5 grams magnesium per day for a 500 kg horse)
Growth: Estimated at 0.07% of total ration
Pregnancy and Lactation: 15 – 30 mg/kg BW (7.5 – 15 grams magnesium per day for a 500 kg horse)
Work: 20 – 30 mg/kg BW (10 – 15 grams magnesium per day for a 500 kg horse)
Magnesium is available in various forages, usually at levels of 0.1 – 0.3%. Alfalfa hay is a good source of this mineral and has higher digestibility of magnesium compared to other forages. 
Magnesium deficiency, known as hypomagnesemia, has been documented in horses but is generally rare and is more commonly seen in cattle.
Deficiency is usually observed in the spring when rapidly growing grasses have low magnesium levels. These grasses often have high levels of potassium and low calcium.
This imbalanced nutrient profile can result in “grass staggers” and the following symptoms:
- Muscle tremors
- Poor coordination
- Nervousness and anxiety
High potassium and low calcium concentrations in the diet can reduce the absorption of magnesium. The ratio of potassium to calcium and magnesium for all life stages should be less than 2.2.
There are no reported adverse effects of high magnesium intake. Excess amounts are excreted in the urine. Over-supplementation should be avoided in horses with kidney issues.
Magnesium sulphate (Epsom salts) can cause diarrhea in horses. Avoid feeding Epsom salts unless under the guidance of a veterinarian.
Up to 98% of the body’s potassium is found within cells where it acts as an electrolyte.
Potassium and other electrolytes help to maintain fluid volume inside cells and cation-anion balance.
Heavily exercised horses lose significant amounts of potassium through sweat. Proper diet and electrolyte replacement is beneficial for exercise recovery and to replenish electrolytes lost in sweat.
- Maintains proper fluid levels within cells
- Aids in muscle contraction and nerve impulse transmission
- Component in many cellular enzymatic reactions
Mature horses require approximately 1% potassium in the diet, expressed as a percentage of dry matter. If forage is provided at greater than 50% of the diet, this requirement is easily met.
According to the NRC, a mature 500 KG (1100 LB) horse at maintenance requires 25 grams of potassium per day. Lactating and exercising horses require more.
Potassium is readily found in horse feeds and forages. Concentrations of potassium in forage are approximately 1 – 4%; grains contain 0.2 – 0.7%.
Horses in heavy work benefit from supplementation along with other electrolytes including sodium, calcium, and magnesium.  Horses with ulcers, colic, and diarrhea may also benefit from electrolyte supplementation.
Feeding electrolytes is recommended on hot days as well as before and after heavy exercise.
Potassium deficiency is rare in horses because forage typically contains adequate amounts of this mineral. Horses consuming high-grain diets are more likely to become deficient.
Athletic horses can become deficient if the potassium loss through sweat is not replenished.  Symptoms of deficiency or suboptimal intake include:
- Muscle weakness
- Resistance to exercise
- Reduced food intake
Excess potassium is excreted in the urine and toxicity is rarely a problem. Clinical cases of toxicity have been documented in Quarter horses affected by hyperkalemic periodic paralysis (HYPP).
If your horse has HYPP, soaking their hay can help reduce potassium intake to manage this condition.
Sulfur is a component of many molecules in the horse’s body, including the amino acids methionine and cysteine. Insulin, coenzyme A, glucosamine, and other important compounds also contain sulfur.
This mineral is important for hoof health, joint function, and coat quality as well as metabolic health.
This mineral is also found in the molecular structure of the B-vitamins thiamin and biotin. Thiamin is important for metabolizing energy from sugars. Biotin is critical for keratin production – the main protein in hooves and hair.
- A component of sulfur-containing amino acids
- Used to make proteins such as keratin, the major component in hooves and hair
- Incorporated into glutathione which acts as a powerful antioxidant
- Maintains healthy connective tissue and cartilage as a constituent of collagen
The requirements for sulfur are not well-defined in horses. A dietary intake of 0.15% on a dry-matter basis appears to be adequate to avoid deficiency.
This concentration is usually achieved by ensuring your horse is meeting their protein requirements.
Organic sulfur is readily available in plants as amino acids within proteins. Most feeds contain enough sulfur to meet the daily requirement of horses.
There are no reports of sulfur deficiency in horses.
In other species, sulfur deficiency can cause reduced appetite, decreased growth, and hair loss.
Excess sulfur is excreted in the feces and urine of horses. Toxicity is rare.
Feeding extremely high levels of inorganic sulfur may cause lethargy and jaundice. This is unlikely to occur with typical supplementation regimens. 
Sodium is the major electrolyte in the horse’s body that works to maintain fluid levels. Chloride is important for the transmission of nerve impulses.
These minerals are often considered together as they are usually found together in the equine diet as sodium chloride (NaCl) or salt.
It is recommended to provide loose, free-choice salt to your horse at all times. Most horses are deficient in sodium and do not meet their salt requirement from forage alone.
- Electrolyte that supports fluid balance
- Proper nerve transmission
An adult horse at maintenance requires 0.1% sodium and 0.3% chloride concentration in the total diet. This is equivalent to 0.5 – 0.7% sodium chloride.
This requirement can be met by adding 2 tablespoons (30 grams) of loose salt directly to your horse’s feed. It is also recommended to offering free-choice loose salt.
Endurance and performance horses benefit from higher levels of sodium chloride in their diet. Horses in hot weather also require more because of the electrolytes lost in sweat.
Horse feeds and forages are generally not good sources of sodium chloride. They often contain less than 0.1%, which is below the requirement for horses.
Horses appear to have some ability to gauge their own salt requirements. Providing free-choice loose salt will allow your horse to consume an adequate amount to meet its requirements. Loose salt is preferred over salt blocks which do not generally encourage adequate intake.
Exercising horses lose large amounts of sodium through sweat. Lactating mares are susceptible to deficiency from the loss of sodium in their milk.
Signs of deficiency can include:
- Dehydration and reduced thirst
- Inactivity or lethargy
- Reduced sweat output
- Pica (eating soil, rocks, or other strange items)
- Weight loss
- Loss of coordination
On rare occasions, if a horse consumes too much salt they may experience diarrhea, increased urine output, colic, fatigue, and poor coordination.
Horses have a high tolerance to salt and toxicity is extremely rare. If a horse consumes a diet with greater than 2% salt and has limited or no access to water, then toxicity may occur. Prolonged toxicity could be fatal.
Microminerals are required in much smaller quantities than macrominerals. The requirements for these essential nutrients are stated in terms of parts per million (mg per kg of diet).
The microminerals that are most important in the equine diet include:
In general, less is known about the micromineral requirements of horses. Recommendations are based on the amounts required to avoid symptoms of deficiency.
Iodine is required to synthesize the thyroid hormones T3 and T4. These hormones help regulate the body’s basal metabolic rate.
Improper iodine levels in the diet can result in hypothyroidism or low thyroid hormone levels. This condition can cause an enlarged thyroid gland known as a goiter.
Pregnant and lactating mares should be supplemented with iodine to prevent developmental issues in foals.
- Essential for thyroid hormone synthesis
- Required for proper growth and development
The NRC estimates that dietary intake of 0.1 mg iodine per kg of dry matter is sufficient for an adult horse.
Pregnant mares may require more. Based on research in other species, horses in late gestation are advised to consume 0.4 mg per kg of dry matter.
Horse feeds usually provide 0 to 2 ppm iodine; this is equivalent to a total dietary intake of approximately 0 – 20 mg. A horse being fed the recommended amount of a complete feed generally obtains enough iodine to meet their requirements.
The iodine content of pasture and forage varies by geographical region; soil type, climate, and fertilizer use may impact concentrations of this mineral. 
Horses fed a forage-only diet may benefit from supplementation in the form of iodized salt or kelp meal.
The major symptom of iodine deficiency is an enlargement of the thyroid gland, also known as a goiter.
Other symptoms of deficiency may include:
- Loss of appetite
- Poor coat quality and hair loss
- Weakness or fatigue
Deficiency in a pregnant mare may result in developmental issues for the foal. Foals born to deficient broodmares may be affected by hairlessness, weakness, enlarged thyroid, or stillbirth. 
High nitrate in the diet of pregnant mares has also been implicated in the development of congenital hypothyroidism in foals. 
Excessive iodine intake can also result in an enlarged thyroid gland. This can occur if feeding too much kelp meal.
Other signs of too much iodine include discharge from the nose and eyes, reduced immune function, and fever.
Copper supplementation can help to support coat colour and quality, joint and hoof health, energy production, and immune function in horses. 
- Functions as an antioxidant to combat oxidative stress
- Involved in bone collagen stabilization
- Maintains connective tissues and is involved in synthesizing elastin
- Mobilizes iron from bodily stores
- Involved in melanin synthesis, the main colour pigment in skin
- Involved in keratin synthesis, the main protein in hoof and hair
According to the NRC, the estimated copper requirement for an adult horse at maintenance is 10 mg per kg dry matter. This is approximately 100 mg per day.
Pregnant mares may benefit from higher levels up to 300 mg per day. Studies have shown benefits for foals born from mares supplemented with 300 mg of copper per day in late gestation.
Foals need higher levels of dietary copper to support growth. Feeding 100 mg per day can support bone development and joints during growth. 
Performance horses or moderately exercised horses may also benefit from higher intake. Some copper is lost in sweat; a minimum feeding rate of 125 mg per day is recommended for exercising horses.
High levels of iron and zinc in the diet can reduce copper absorption and utilization. A ratio of 4:3:1 of iron to zinc to copper is typically recommended.
Horses naturally obtain copper from forage and grain. However, this mineral is commonly deficient in the equine diet.
Most commercial horse feeds and vitamin/mineral premixes contain supplemental copper in an inorganic form such as copper sulphate or copper chloride.
The absorption of inorganic copper is impaired by high levels of iron or zinc in the diet.
Chelated copper is an organic molecule bound to amino acid proteinates. This form of the mineral can bypass copper transport mechanisms in the gut and is less affected by zinc and iron levels.
Mad Barn uses 100% organic trace minerals in our formulas because organic minerals are better absorbed and utilized within the horse’s body.
A copper deficiency can result in a variety of symptoms including:
- Faded coat colour
- Split or cracked hooves
- Hives or sweet itch
- Loss of appetite
- Yellowing of the eyes, skin, or gums
- Low energy
If left untreated, deficiency can lead to serious consequences such as osteochondrosis or osteochondritis dissecans (OCD), aortic rupture, or uterine artery rupture.
Copper toxicity is unlikely in horses. Chronic exposure to levels greater than 1000 mg/kg per day could lead to organ damage or failure. 
However, reports of toxicity have only occurred in research studies using specially formulated copper supplements that are not available on the market.
The upper tolerable limit for horses is 250 mg per kg of dry feed per day. This is equivalent to approximately 2500 mg of copper per day for a 500 kg horse.
Iron is a trace mineral that is required to form hemoglobin, the protein in red blood cells that carries oxygen throughout the body. It is also important for supporting enzymatic reactions in the body.
Many equine diets contain excess iron because it is found in all plant material, soil, and water.
High iron levels can interfere with copper and zinc absorption in the gut leading to secondary deficiencies in these minerals. The recommended iron to zinc to copper ratio is 4:3:1.
- Transport oxygen throughout the body
- Supports enzymatic reactions
Mature horses at maintenance require 40 mg iron per kg of diet, equivalent to approximately 400 mg per day.
For lactating mares or growing foals the requirement is 50 mg iron per kg diet due to the higher demands of growth and lactation.
Iron concentrations are high in forages, usually between 100 to 250 mg per kg. Levels are slightly lower in grains, typically up to 100 mg iron per kg.
Horses may consume incidental iron if they ingest soil while grazing. The highest levels of this mineral are found in clay-based soils. Hay and other feeds should be kept off the ground to avoid excessive soil intake.
Water is also a source of iron; groundwater typically contains higher amounts than surface water. Iron absorption from water is minimal.
It is recommended to avoid supplements or feeds that contain added iron, unless under the guidance of a veterinarian. Check the ingredient list for “iron” or “ferrous” to determine whether there is added iron.
Forages and feeds naturally contain high levels of iron and deficiency is not usually a concern.
Horses that are deficient in iron can develop anemia. This condition results from low blood iron levels, which limits oxygen transport to tissues.
Symptoms of anemia include fatigue, muscle weakness, lethargy, and dullness.
Excess dietary iron can be dangerous and lead to nutrient imbalances. Supplementation of iron is not advised, except for anemic horses under the supervision of a veterinarian.
The upper tolerable limit for iron intake is 500 mg iron per kg of dry matter or approximately 5,000 mg per day.
Signs of iron toxicity can include poor coat quality, allergy-like symptoms, and hoof problems. More serious consequences can include:
- Compromised immune function
- Oxidative stress and cell damage
- Insulin resistance and Equine Metabolic Syndrome
- Secondary copper or zinc deficiency
- Liver failure
Zinc is an essential trace mineral with many important roles for horses. It is involved in antioxidant protection, insulin sensitivity, and reproductive health.
Horses require zinc for protein synthesis, bone development, and growth. Zinc also plays a role in immune function, gene expression, wound healing and skin, coat and hoof health.
Zinc is found in low levels in common forages and typically needs to be supplemented in the equine diet. It should be fed in balance with iron and copper.
- Regulates metabolism of carbohydrates and energy
- Required for bone growth
- Used for protein synthesis, including keratin and collagen synthesis
- An important antioxidant
Horses at maintenance have an estimated daily requirement of 40 to 50 mg zinc per kg dry matter. A typical 500 KG horse will require 400 mg of zinc per day.
Levels of zinc in common feedstuffs are relatively low, ranging between 15 to 40 mg per kg dry feed.
Supplementing with a complete feed, ration balancer or mineral and vitamin product can help to ensure your horse meets their daily requirement for zinc.
Zinc is provided either in an inorganic form (i.e. zinc oxide) or an organic form (i.e. zinc proteinate).
Organic or chelated forms of zinc may be superior as the mineral is bound to an amino acid complex. This promotes efficient absorption with less risk of interference from iron and copper.
Zinc deficiency is common due to low levels of this mineral in forage as well as potential competition for absorption pathways by iron and copper.
These three minerals are often imbalanced in the equine diet, leading to secondary deficiencies even if the minerals are supplied in adequate total amounts.
Zinc deficiency is characterized by:
- Reduced appetite
- Poor coat quality, skin lesions, or poor hoof quality
- Increased thirst and urination linked to insulin resistance
- Bouts of laminitis
Symptoms of toxicity are serious. Growing horses are most at risk of excess intake if they graze on pasture with high zinc content in the soil due to industrial contamination.
Consuming too much zinc can cause:
- Abnormal bone formation
- Enlarged joints and bone plates
- Chronic swelling
- Stiff or rough coat
The upper tolerable limit for horses is 500 mg per kg of dry feed per day. This is equivalent to approximately 5000 mg of zinc per day for a 500 kg horse.
Selenium is an essential micromineral that works closely with vitamin E as an antioxidant.
Selenium is also an important component of the amino acids selenomethionine and selenocysteine. It is involved in growth and muscle function.
In North America, selenium content in the soil varies from region to region, resulting in varied levels of this mineral in pasture and hay. Midwestern US states generally have adequate levels whereas coastal regions and most of Canada have selenium-deficient forages.
- Supports antioxidant status
- Involved in immune function
- Component of proteins involved in muscle formation
- Involved in thyroid function
Selenium should be provided at a minimum level of 0.1 mg/kg of dry matter for mature horses at maintenance. This is equivalent to 1 mg per day for a 500 kg horse.
Working horses require 1.25 mg or more per day, depending on the exercise intensity and size of the horse.
The levels listed above are considered the bare minimum to avoid symptoms of deficiency. According to the NRC, intake of 2 to 3 mg daily supports optimal health in horses.
Horses can obtain selenium from forage and hay, so long as it is grown in selenium-rich soil. Grasses grown in soil with low selenium concentration will provide low levels of this mineral.
Selenium deficiency is quite common in North America as most parts of the continent have poor selenium content in the soil.
Between 30 to 50% of horses are estimated to be deficient in this micromineral. Supplements that provide selenium and vitamin E are beneficial for horses in these regions.
Organic forms of selenium such as selenized yeast are preferred over inorganic supplements such as sodium selenite. Selenized yeast is better used by the horse’s body and has a lower risk of toxicity.
The classic sign of selenium deficiency is white muscle disease which is characterized by:
- Fatigue and muscle weakness
- Poor coordination and difficulty moving
- Difficulty breathing and respiratory stress
- Heart problems
- Difficulty eating and swallowing
It is imperative that late gestation mares and lactating mares receive adequate dietary selenium and vitamin E. Mares can pass on their deficiency to foals that can exhibit the above symptoms. 
The upper tolerable limit of selenium is estimated at 20 mg per day for a 500 kg horse.
Selenium toxicity is rare except in cases of over-supplementation with products containing inorganic selenium. Deficiency is a vastly more common concern than toxicity.
Signs of toxicity include head pressing, excess sweating, diarrhea and colic. Horses consuming excess selenium may also experience rapid heart rate, increased respiratory rate, sluggishness, and blind staggers.
Toxicity is unlikely with organic selenium sources such as selenized yeast that consist primarily of seleno-amino acids. These compounds are metabolized differently and do not build up to toxic levels the way that selenide or selenite can.
Manganese is crucial for bone formation and antioxidant protection. It is also involved in maintaining healthy joints and supports the production of chondroitin sulfate.
- Involved in carbohydrate and lipid metabolism
- Required to make chondroitin sulfate, a component of cartilage
A 500 kg horse should consume 400 to 500 mg of manganese per day.
There is limited research available regarding requirements for this mineral for exercising horses and horses of different life stages (ie: growth, gestation, and lactation).
Forages typically contain 25 to 140 mg per kg dry matter. Manganese levels are lower in grains than in forages.
Supplemental sources of inorganic or organic manganese are available to increase dietary concentrations.
Manganese deficiency is not usually a concern for horses. Growing horses are more susceptible to deficiency due to the rapid growth of bones.
In other species, such as cattle and swine, manganese deficiency can result in:
- Malformation of bone
- Reduced or improper formation of cartilage
- Abnormal limb alignment
- Enlarged joints
There have been no reports of manganese toxicity in horses.
Cobalt is needed to form vitamin B12 (cobalamin). In horses, cobalt is converted to Vitamin B12 by the hindgut microflora.
Competition horses are sometimes given cobalt supplements to increase their oxidative capacity and support exercise performance. 
Cobalt is a necessary component of vitamin B12 which is important for carbohydrate and fat metabolism as well as athletic stamina.
Based on data in cattle, it is recommended for horses to consume approximately 0.1 mg of cobalt per kg of dry matter intake.
Pasture and grasses tend to be low in cobalt. Most horse feeds contain inorganic cobalt carbonate to support vitamin B12 production in the hindgut.
There are no reported cases of cobalt deficiency in horses.
If levels of this mineral are insufficient, horses may exhibit symptoms of vitamin B12 deficiency such as anemia.
The upper tolerable limit for cobalt intake is estimated at 10 mg per kg dry matter.
Long-term supplementation of cobalt at excessive levels can result in toxicity and negatively impact cardiovascular, reproductive, and thyroid health. 
More research is required in horses to understand the risk of toxicity. 
Chromium is involved in metabolic function and insulin regulation. It acts on numerous components of the insulin signalling pathway to increase glucose (blood sugar) uptake into tissues. 
- Involved in insulin signaling
- Enhances glucose uptake in insulin-responsive tissues
There are no published requirements for chromium intake in horses. Chromium levels in pasture, forage, hay, and feeds are presumed sufficient. However, metabolic horses may benefit from additional supplementation.
Levels of chromium in forage and common feeds are presumed adequate based on a lack of deficiency reported in the scientific literature.
There is research suggesting that metabolic horses can benefit from higher levels of this mineral in the diet. Supplemental forms of chromium include:
- Chromium yeast (Biochrome)
- Chromium propionate
- Chromium picolinate
Deficiency has not been reported in horses. In other species, chromium deficiency can cause:
- Impaired lipid and carbohydrate metabolism
- Insulin resistance; high levels of circulating insulin
- Glucose intolerance
Horses should not consume more than 5 mg per kg of dry matter per day. Absorption and bioavailability vary depending on the form of this mineral used. Organic forms are generally better metabolized in the body and less likely to result in toxicity.
Chromium toxicity has not been observed in horses. In cattle, toxicity can result in nausea, oxidative stress, diarrhea, muscle cramping, or fever.
Molybdenum is important for DNA (and RNA) synthesis as a cofactor of certain enzymes.
There is little known about the role of molybdenum in horses. Research suggests that too much molybdenum can interfere with copper absorption.
- Component in enzymes for purine metabolism
- Synthesis of DNA and RNA
There are no published requirements of molybdenum for horses. Horses are presumed to obtain sufficient amounts from pasture, forage, and feedstuffs.
Concentrations of molybdenum in horse feeds range between 0.3 to 8 mg per kg dry matter.
The NRC states that grass forages often have higher molybdenum levels than legumes.
Sandy soils may be low in this trace mineral. Some industrial areas are affected by molybdenum contamination in soils. Consuming forages grown on these soils can result in decreased copper absorption.
There are no reports of molybdenum deficiency in horses.
In other species, deficiency can result in poor growth and development.
Toxicity has not been well documented in horses. The NRC recommends that horses should not consume more than 5 mg per kg dry matter intake.
Higher intake can interfere with copper absorption and result in a secondary deficiency.
In livestock, the copper-to-molybdenum ratio is established at 4:1. More research is required to establish an appropriate ratio for horses.
Fluoride is used extensively in dental products for humans and horses. Fluoride is a major component of bone and teeth, helping to increase their hardness.
It is unclear whether fluoride should be categorized as an essential mineral. There have been no reports of fluoride deficiency across a range of different species.
Even in research studies, fluoride deficiency has not been observed. This mineral is widely available and inadequate intake is not a concern.
Fluoride is a structural component of bone and teeth. It is found in the form of the molecule fluorapatite, supporting bone and teeth hardness.
There are no published requirements for fluoride in horses.
The NRC has established an upper tolerable limit for fluoride intake at 40 mg per kg dry matter.
Fluoride is found in water, soil, forage, and horse feed. In groundwater, concentrations can range between 0.02 to 1.5 mg per litre.
As regulated by various governments, fluoride is often added to water for human consumption at a concentration of 4 mg per litre.
Soil concentrations of fluoride can vary. Forage provides 2 to 16 mg per kg of dry matter; grains provide between 1 to 3 mg per kg of dry matter.
There are no reported cases of fluoride deficiency in horses. This mineral is widely available in the soil, water, forage, and feed. Inadequate intake is not a concern.
In an experiment with rats, researchers limited fluoride intake in a laboratory setting. The lower intake resulted in poor growth rates.  This result has only been observed in laboratory settings.
Fluoride toxicity is referred to as fluorosis and can be dangerous for horses.
Fluorosis affects the bones and teeth. In growing animals, fluoride can accumulate in the teeth causing delayed replacement of teeth.
Teeth may show blotched enamel that can turn yellow, brown, or black. Fluorosis can also cause difficulty eating, and oral infections and lead to poor dental health.
Horses affected by fluorosis may have arched or humped backs. Skin and coat health may be impacted resulting in the hair coat feeling rough or dry.
Fluorosis can also negatively affect bones, resulting in heavy, enlarged bones that are rough and chalky white. Horses may exhibit signs of stiffness and lameness.
Minerals are critical for the well-being of your horse. Long-term deficiencies in vitamins and minerals can result in poor hoof health, weak joints, suboptimal coat quality and other issues.
Equine diets are commonly lacking in several key minerals including copper, zinc and selenium. Well-designed mineral and vitamin supplements will balance a wide range of forages and hays to ensure your horse’s diet meets their nutritional requirements.
Mad Barn’s Omneity mineral and vitamin supplement has been formulated based on thousands of North American equine diets to cover your horse’s core nutritional needs.
You can submit your horse’s diet online to evaluate inclusion rates of key minerals and vitamins. Our nutritionists can help you design a feeding plan that meets all of your horse’s mineral requirements and promotes optimal health and wellness.
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- National Research Council Chapter 5: Minerals. Nutrient Requirements of Horses. 2007.
- Lond L.D. Chapter 2: Minerals for Horses. Equine Clinical Nutrition: Feeding and Care. 1995.
- Novak S. & Shoveller A.K. Chapter 6: Minerals for Horses.Nutrition and Feeding Management for Horse Owners. 2008.
- Harrington D. & Walsh J. Equine magnesium supplements: Evaluation of magnesium oxide, magnesium sulphate and magnesium carbonate in foals fed purified diets . Equine Vet J. 1980.
- Stewart A. Magnesium disorders in horses. Vet Clin Equine 2011.
- Martens H, Schweigel M. Pathophysiology of grass tetany and other hypomagnesemias. Implications for clinical management. Vet Clin North Am Food Anim Pract. 2000
- Jansson A. et al. Effects of potassium intake on potassium, sodium and fluid balance in exercising horses . Equine Vet J Suppl. 1999.
- Maranon G. et al. The effect of methyl sulphonyl methane supplementation on biomarkers of oxidative stress in sport horses following jumping exercise
. Acta Vet Scand. 2008.
- Underwood E.J. Iodine and Goiter. The Mineral Nutrition of Livestock. 1996.
- Swerczek T.W. Effects of Nitrate and Pathogenic Nanoparticles on Reproductive Losses, Congenital Hypothyroidism and Musculoskeletal Abnormalities in Mares and Other Livestock: New Hypotheses. Anim Vet Sci. 2019.
- Jancikova P. et al. The effect of feed additive vcontaining vitamins and trace elements on the elements profile and growth of skin derivatives in horses . Annals Anim Sci. 2012.
- Stowe H.D. Effects of copper pre-treatment upon the toxicity of selenium in ponies. Am J Vet Res. 1925.
- Lofstedt J. White muscle disease in foals. Vet Clin North Am Equine Pract. 1997.
- Kinobe R.T. Towards the elimination of excessive cobalt supplementation in racing horses: a pharmacological review.. Res Vet Sci. 2016.
- Simonsen L.O. et al. Cobalt metabolism and toxicology – a brief update. Sci Total Environ. 2012.
- Spears J.W. Chromium propionate increases insulin sensitivity in horses following oral and intravenous carbohydrate administration. . J Anim Sci. 2020.
- Pagan J.D. et al. The effect of chromium supplementation on metabolic response to exercise in Thoroughbred horses. Proc 14th Equine Nutrition and Physiology Symposium . Proc 14th Equine Nutrition and Physiology Symposium. 1995.
- Mickelsen, O. et al. Water-cultured crops designed to study deficiencies in animals. J Agric Food Chem. 1953.
- Corke, M.J. An outbreak of sulphur poisoning in horses. Vet Rec. 1981.
- Pagan, J.D. Micromineral Requirements in Horses. KER, Inc.
- Hua, Y. et al. Molecular mechanisms of chromium in alleviating insulin resistance. J Nutr Biochem. 2012.