The macrominerals calcium and phosphorus are the most abundant minerals in the horse’s body. They are important for bone strength, cellular function, muscle health and more.

Ensuring adequate calcium and phosphorus intake is particularly critical for pregnant and lactating mares, as well as growing horses. Milk production and bone development both increase the demands for these minerals.

Calcium and phosphorus levels vary in the feeds and forages you give your horse. Careful selection of feeds is warranted to avoid deficiencies and to maintain a balanced ratio of these nutrients in the diet.

Long-term imbalances in these minerals can lead to bone abnormalities and poor performance. Sampling your hay and feeding a well-balanced vitamin and mineral supplement can prevent complications associated with inadequate mineral intake.

For assistance with balancing your horse’s calcium and phosphorus intake, submit your horse’s diet online to receive a complementary evaluation from our equine nutritionists.

Calcium for Horses

Calcium is a major structural mineral. 99% of the calcium in the body is located in the bones and teeth. The remaining 1% of calcium is found in soft tissue and bodily fluids. [1]

Calcium is involved in many bodily processes such as muscle function, blood clotting and enzyme activity. [2]

The level of calcium in circulation is tightly regulated within an optimal range. Hormones control how the body uses calcium and its movement from the bone to other tissues.

These hormones – parathyroid hormone and calcitonin – ensure blood levels of this mineral stay consistent through a process known as calcium homeostasis.

Calcium homeostasis is negatively impacted if inadequate calcium is provided in the diet or if phosphorus intake is too high.

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Calcium Absorption

Calcium is absorbed in the small intestine in the form of Ca2+ ions. These ions bind to a calcium-binding protein (calbindin), which transports this mineral to the bloodstream.

The absorption of calcium from the gut is directly impacted by the level of phosphorus in the diet. Phosphorus in forages and grains is predominantly in the form of phytate. Phytate can bind calcium and make it unavailable for absorption. [8]

The ingestion of oxalates in hay and other feeds can also impair calcium absorption, contributing to secondary deficiency. Oxalates are naturally occurring compounds in plants that bind to minerals and reduce their bioavailability. Oxalates are especially a concern in tropical forages. [9]

Vitamin D also plays an essential role in calcium absorption. Vitamin D is important for the ability of cells to uptake calcium, phosphorus, magnesium, zinc, copper and manganese.

It is important to supplement vitamin D in growing horse diets to allow for optimal absorption of these minerals.

Calcium Sources

The majority of calcium in your horse’s diet will be provided from hay and pasture. Fresh pasture grass and legume hays contain more calcium than grass hays.

The following chart lists approximate calcium concentrations as a percentage of dry matter for common hays:

  • White clover: 1.90 %
  • Red clover: 1.38 %
  • Alfalfa: 1.28 %
  • Lespedeza: 0.88 %
  • Prairie grass: 0.57 %
  • Bermudagrass: 0.47 %
  • Kentucky bluegrass: 0.47 %
  • Bromegrass: 0.44 %
  • Timothy: 0.41 %
  • Tall Fescue: 0.39 %
  • Orchardgrass: 0.38 %

There can be significant variation in calcium levels depending on when the hay is harvested, how mature the plant is and the type of soil. Testing your hay is important so you know the actual level of key nutrients in your horse’s diet.

Supplemental Sources

If your horse’s diet is low in calcium, supplements or feeds can be used to increase intake.

Below are some of the main ingredient sources of calcium that are typically added to commercial feeds or mineral and vitamin premixes:

  • Calcium carbonate (limestone) – Contains 38% calcium
  • Calcium chloride dihydrate – Contains 28% calcium
  • Calcium hydroxide – Contains 55% calcium
  • Dicalcium phosphate – Contains 28% calcium, 19% phosphorus

Feed Sources

Sometimes additional feed sources are needed to increase calcium concentrations in the diet.

Grains are typically poor sources of calcium, but the following feeds can be added to your horse’s diet if they need more calcium:

  1. Beet pulp – Contains 1% calcium
  2. Alfalfa pellets or cubes – Contains 1.5% calcium
  3. Milk replacer – Contains 1.0 – 1.5% calcium

Phosphorus for Horses

Phosphorus has a wide range of essential functions within the horse’s body, both structurally and at a molecular level.

This mineral helps provide structure and strength in bones. Phosphorus is found in bone as hydroxyapatite (Ca5(PO4)3OH). [2] Hydroxyapatite is a precursor for bone and tooth construction.

Broodmares and foals are most at risk of phosphorus deficiency. Inadequate intake of this mineral can lead to problems with bone formation or slow growth in young horses.

Phosphate is also a component of the body’s main source of energy, adenosine triphosphate (ATP). It is also found within cell membranes and is a component of DNA and RNA.

Dietary phosphorus is mainly absorbed in the large intestine as phosphate (PO4). [2]

In most plant tissues, phosphorus is stored as phytate – an anti-nutritional factor that can negatively impact digestibility. This molecule requires an enzyme called phytase to break down phosphorus to make it digestible.

Horses appear to have adequate phytase produced by microbes in the hindgut to liberate the phosphorus in plants. [10] However, supplemental phytase can be beneficial for improving calcium digestibility. [8]

Phosphorus Sources

Forage will provide a large portion of a horse’s daily phosphorus needs. However, additional phosphorus is usually needed to meet nutrient requirements in most equine diets.

The following are concentrations of phosphorus as a percentage of dry matter in common forages:

  • Oats – Immature: 0.39 %
  • Timothy – Prebloom: 0.36 %
  • White clover: 0.29 %
  • Kentucky bluegrass: 0.27 %
  • Lespedeza: 0.25 %
  • Alfalfa: 0.24 %
  • Orchardgrass: 0.23 %
  • Red clover: 0.22 %
  • Tall Fescue: 0.21 %
  • Prairie grass: 0.17 %
  • Bermudagrass: 0.16 %
  • Timothy: 0.17 %
  • Bromegrass: 0.15 %

Supplemental Sources

Sources such as wheat bran, rice bran and monosodium phosphate can be used to supplement a diet with phosphorus.

  • Monosodium phosphate – Contains 23% phosphorus
  • Monocalcium phosphate – Contains 21% phosphorus, 16% calcium

Feed Sources

  • Wheat bran – Contains 1.38% phosphorus
  • Wheat middlings – Contains 1.24% phosphorus
  • Rice bran – Contains 2.22% phosphorus

Calcium & Phosphorus Requirements for Horses

The calcium and phosphorus requirements for horses are well-documented in the NRC and largely depend on age and physiological status.

Ratios are important to keep an eye on, but total intake needs to be considered as well to ensure there are no deficiencies in the diet.

Mature Horses

A 500 kg (1100 lbs) mature horse at maintenance (not in work) will require roughly 20 g of calcium and 14 g of phosphorus in their diet. [4]

This requirement increases for horses in light, moderate or heavy work. A 500 kg (1100 lbs) mature horse in heavy work will require 40 g of calcium and 29 g of phosphorus.

The following table shows calcium (Ca) and phosphorus (P) requirements for mature horses at various levels of activity.

Maintenance & Light Work

Body Weight Maintenance Light Work
Ca P Ca P
400 kg (880 lb) 16 g 11 g 24 g 14 g
500 kg (1100 lb) 20 g 14 g 30 g 18 g
600 kg (1320 lb) 24 g 17 g 36 g 22 g


Moderate & Heavy Work

Body Weight Moderate Work Heavy Work
Ca P Ca P
400 kg (880 lb) 28 g 17 g 32 g 23 g
500 kg (1100 lb) 35 g 21 g 40 g 29 g
600 kg (1320 lb) 42 g 25 g 48 g 35 g


Gestation & Lactation

Gestation and lactation represent the largest change in a horse’s calcium and phosphorus requirements.

The rapid fetal growth in the last month of pregnancy and peak milk production in the first month of lactation require the highest calcium and phosphorus intake for the broodmare.

During the last phase of pregnancy, an additional 0.032 grams of calcium are required per kilogram of body weight for the mare. A 500 kg (1100 lb) mare will require 36 grams of calcium and 26 grams of phosphorus per day. [4]

The following table shows calcium and phosphorus requirements fr the broodmare at different months of gestation.