Calcium and Phosphorus Ratios in Equine Diets

Ensuring your horse maintains a balanced diet is one of the most important aspects of horse care. In particular, ensuring your horse maintains a balanced calcium and phosphorus ratio in his diet is critical, as horses with calcium or phosphorus deficiencies or toxicities are prone to various disorders, according to Ramiro E. Toribio, DVM, MS, PhD, Dipl. ACVIM, associate professor in the Department of Veterinary Clinical Sciences at The Ohio State University, who recently composed a literature review on the topic.

In horses calcium helps maintain normal brain and nerve function and aids in heart, skeletal muscle, and intestinal contraction. Phosphorus helps regulate muscle and heart contraction, cell integrity, and glucose use.

The calcium to phosphate ratio in the equine diet is important because the two work closely together: "A balanced equine diet must have 0.15-1.5% of calcium and 0.15-0.6% of phosphorus in feed dry matter," Toribio explained. "A calcium to phosphorus ratio of less than 1:1 can have negative consequences on the skeleton." Simply put, a horse needs at least as much calcium in his diet as phosphorus, never the reverse.


Most animals with a calcium or phosphorous deficiency show subtle clinical signs, generally because most of the resulting damage is internal. When blood levels are low, the horse's body will draw calcium and phosphorus from the bones to carry out bodily functions, which can lead to some serious consequences, as described below.

Chronic calcium deficiency is rare and is associated with abnormal skeletal development, lameness, weak bones, fractures, poor growth, and poor performance. Acute calcium deficiency also is rare and is associated with neurologic signs (e.g., seizures), muscle trembling, decreased intestinal motility, and, in pregnant mares, dystocia (difficult birth) or retained placenta.

Phosphorus-deficient horses often show clinical signs such as muscle weakness and trembling, said Toribio. Additionally, lack of phosphorus can hinder horses' ability to regulate their energy needs properly, which can lead to high blood levels of glucose and fats.


Phosphorus toxicity is more common in horses than calcium toxicity, Toribio said. Horses with phosphorus toxicity often display similar clinical signs as horses with a calcium deficiency; the excess phosphorus binds to the calcium in the intestine, decreasing the body's absorption of the latter. The horse's body also tries to compensate for the excess phosphorus by using calcium from the bones, essentially weakening the skeleton.

Calcium toxicity is rare in horses and provided the horse receives enough phosphorus in his diet, there's little risk of detrimental effects.

Toribio noted that while most horses receive a good balance of calcium and phosphorus from a daily diet of high-quality forage coupled with a commercial grain mix (some forages are low in phosphorus, so hay should be tested if the horse is eating a forage-only diet), some, including foals, high performance horses, and pregnant or lactating mares, could require a higher daily amount of the two minerals. Owners can fulfill an increased calcium and phosphorus requirement easily by providing the horse with a balanced vitamin and mineral supplement.

Simple blood tests can detect calcium or phosphorus toxicity or deficiency, so it's advisable to work with your veterinarian if concerns mount about a particular horse's daily intake of the two required minerals. Also consider having the nutrient contents in your horse's hay and grass tested to ensure he's receiving the appropriate amount of vitamins and minerals. Finally, your veterinarian or equine nutritionist can help you plan a balanced diet for your animal, complete with vitamin or mineral supplement suggestions if necessary.

The article, "Disorders of calcium and phosphate metabolism in horses," appeared in April in Veterinary Clinics of North America: Equine Practice. The abstract is available on PubMed.

About the Author

Marie Rosenthal, MS

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