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.

Deficiency Symptoms:

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.

Toxicity Symptoms:

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.

Calcium and phosphorus are macrominerals that should be provided in a ratio of approximately 1.5:1 Ca to P. They support healthy bones and metabolic processes throughout the body. Deficiencies can result in weak bones or abnormal growth.


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. [6]

Supplemental forms of magnesium are also available. Magnesium oxide is a cost-effective option because it is more concentrated than other sources and has good oral bioavailability. [4]

Deficiency Symptoms:

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
  • Tying-up
  • Poor coordination
  • Nervousness and anxiety

Horses with high magnesium requirements are most susceptible to deficiency. [5] Lactating mares, heavily exercised horses, or stressed horses have higher requirements for this mineral.

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.

Toxicity Symptoms:

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 v