Why is the calcium:phosphorus ratio so important in horses? Around 99% of calcium found in the body is found in the bones and teeth of your horse, yet only 35% of equine bone is comprised of calcium. Phosphorus is another major component of bone, making up around 14-17% of the equine skeleton. Additionally, it is required for the energy transfer process with adenosine diphosphate (ADP) and adenosine triphosphate (ATP).