Most horse owners know their horses need dietary calcium (Ca) and phosphorus (P) in the right amounts to maintain a healthy skeleton. Phosphorus in bones not only provides structural support for the skeleton, but it also acts as a reserve of P for other bodily functions. Phosphorus is important in cell membranes and in reactions requiring cellular energy. Phosphorus also helps form the backbone of DNA and contributes to the pH and electrolyte balance in body fluids. In a nutshell, P is an essential nutrient that animals cannot live without.