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. 
Calcium is involved in many bodily processes such as muscle function, blood clotting and enzyme activity. 
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.
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. 
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. 
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.
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.
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
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:
- Beet pulp – Contains 1% calcium
- Alfalfa pellets or cubes – Contains 1.5% calcium
- 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).  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). 
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.  However, supplemental phytase can be beneficial for improving calcium digestibility. 
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 %
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
- 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.
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. 
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|
|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|
|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. 
The following table shows calcium and phosphorus requirements fr the broodmare at different months of gestation.
|Body Weight||1st-6th month||7th-8th month||9th-11th month|
|400 kg (880 lb)||16 g||11 g||22 g||16 g||29 g||21 g|
|500 kg (1100 lb)||20 g||14 g||28 g||20 g||36 g||26 g|
|600 kg (1320 lb)||24 g||17 g||34 g||24 g||43 g||32 g|
The first month of lactation is considered the peak lactation period. Calcium requirements for the broodmare increase to 56 grams and phosphorus to 36 grams per day. 
At this point, roughly 50% of the minerals in a mare’s milk are calcium and phosphorus. 
|Body Weight||1st-2nd month||3rd-4th month||5th-6th month|
|400 kg (880 lb)||47 g||32 g||45 g||29 g||32 g||20 g|
|500 kg (1100 lb)||59 g||30 g||56 g||36 g||40 g||25 g|
|600 kg (1320 lb)||71 g||46 g||67 g||43 g||47 g||30 g|
Weanling foals and yearlings require more calcium and phosphorus in their diets to support the rapid growth of their bones, muscles and other tissues.
The daily requirement of each will depend on the horse’s estimated mature body weight and daily growth rate.
For a yearling that is estimated to grow to 500 kg (1100 lb), the calcium requirement is 37 grams and the phosphorus requirement is 20 grams per day. 
|Mature Weight||4 – 7 months||8 – 14 months||15 – 24 months|
|400 kg (880 lb)||31 g||17 g||30 g||17 g||30 g||16 g|
|500 kg (1100 lb)||39 g||22 g||38 g||21 g||37 g||21 g|
|600 kg (1320 lb)||47 g||26 g||46 g||26 g||45 g||25 g|
Calcium to Phosphorus Ratio
Dietary minerals have complex interactions that can impact the absorption and metabolism of other minerals in the body. This is why it is important to supplement minerals in balanced ratios.
Maintaining an appropriate ratio of calcium to phosphorus in the diet is important, especially for pregnant, lactating and growing horses.
The ratio of calcium to phosphorus (Ca:P) should be around 1.5 – 2:1 for most mature horses. This means that there should be 1.5 – 2 grams of calcium for every 1 gram of phosphorus in the diet.
Ratios of up to 9:1 can be tolerated, as long as the horse is meeting their minimum daily phosphorus requirement. 
Growing horses can tolerate up to a 6:1 ratio of calcium to phosphorus, so long as they are getting enough phosphorus in the diet. 
The Ca:P ratio of the diet should never go below 1:1 or else dietary phosphorus will interfere with calcium absorption.  This can have negative effects on bone development for growing and mature horses.
Individual components in the diet may have ratios outside these ranges. What is important is the ratio of the overall diet.
Furthermore, a diet may have the correct ratio of calcium to phosphorus but may not provide these minerals in adequate amounts. Ensure that there is sufficient total calcium and phosphorus in your horse’s diet to avoid deficiency.
Our nutritionists can help you determine the calcium to phosphorus ratio in your horse’s diet. Submit your horse’s information online for a free diet evaluation and balancing.
Ratios in Feeds & Forages
The following chart lists the calcium to phosphorus ratio for common forages used in the equine diet.
Below is the calcium to phosphorus ratio for common concentrates and feeds that are used in horse rations.
|Dried Distillers Grains||1:14|
Calcium & Phosphorus Disorders
Imbalances in a horse’s calcium and phosphorus intake can negatively impact welfare and result in a variety of health conditions. Working with a nutritionist to balance your horse’s diet can help you avoid common problems.
Thankfully, many of these conditions are rare today because of improved vitamin and mineral formulas and fortified feeds.
To maintain levels of these minerals in the blood and tissues, the horse’s body will mobilize stored calcium and phosphorus from bones. Over time, this leeching of minerals could lead to conditions that affect the bones, muscles and cartilage.
Long-term deficiency in phosphorus can result in muscle weakness, consumption of manure or dirt (pica), lameness and weight loss. 
In growing horses, inadequate intake of calcium, phosphorus or vitamin D can result in a condition known as Rickets. In horses with Rickets, improper bone mineralization leads to bowed bones and enlarged joints.
In mature horses, deficiency in these minerals can manifest as osteomalacia. This condition makes bones weak, brittle and more prone to fractures.
Horses that are deficient in calcium but consume too much phosphorus can develop a condition called nutritional secondary hyperparathyroidism.
This condition, also known as Big Head or Miller’s Disease, can cause the facial bones to swell and soften.  It is caused by increased parathyroid hormone levels, which rise to maintain balanced calcium levels in the blood.
Long-term imbalances in minerals can lead to bone abnormalities, poor performance and other health problems.
A properly balanced equine diet will supply calcium and phosphorus at the recommended rates according to your horse’s physiological status.
By selecting appropriate forages and balancing them with suitable feeds, you can ensure that your horse has the ideal ratio of calcium and phosphorus in their diet
Feeding a well-balanced vitamin and mineral supplement and sampling your horse’s hay can help to support optimal health.
Submit your horse’s information online for a free diet evaluation from our equine nutritionists. A diet evaluation will tell you if your horse is meeting their needs for calcium and phosphorus and whether these minerals are being fed in a balanced ratio.
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- Toribio, R. E. Disorders of Calcium and Phosphate Metabolism in Horses. Vet. Clin. North Am. Equine Pract. 2011.
- Coenen, M. Macro and trace elements in equine nutrition. In: R.J. Geor, P.A. Harris, M. Coenen. Equine Applied and Clinical Nutrition. 2013.
- H.F. Schryver et al. Calcium and phosphorus in the nutrition of the horse. Cornell Vet. 1974.
- National Research Council. Chapter 5: Minerals. Nutrient Requirements of Horses. 2007.
- Novak S. & Shoveller A.K. Chapter 6: Minerals for Horses. Nutrition and Feeding Management for Horse Owners. 2008.
- Ralston, S.L. Nutritional Diseases of Horses and Other Equids. Merck Veterinary Manual. 2021.
- Selle, P.H. et al. Consequences of calcium interactions with phytate and phytase for poultry and pigs. Livest Sci. 2009.
- van Doorn, D.A. et al. The apparent digestibility of phytate phosphorus and the influence of supplemental phytase in horses. J Anim Sci. 2004.
- Blaney, B.J. et al. The effects of oxalate in some tropical grasses on the availability to horses of calcium, phosphorus and magnesium. J Agri Sci. 1981.
- Brinkley-Bissinger, K. et al. Phytate supplementation has minimal impact on mineral digestibility in horses. J Anim Sci. 2020.
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