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, fractu