“Calcium and phosphorus, two very important minerals, make up approximately 70% of the total mineral content in the equine body. Most people, including young children, have seen the milk commercials on television that inform their viewer of the important role that calcium plays in creating strong, healthy bones. Calcium plays this role in many species and is especially important in large breed animals such as the horse, who is often asked to perform hard work and athletically challenging tasks. In addition to creating strong bones in the equine body, calcium is also important for several metabolic functions such as nerve conductions and muscle contractions. Without a sufficient amount of calcium, the horse’s bones would grow weak and he would be at greater risk for injury. Due to the importance of these vital functions of calcium, the equine’s body closely regulates the levels of calcium in the bloodstream. Vitamin D and hormones such as the parathyroid hormone (PTH) play an important part in maintaining adequate calcium levels in the blood stream. When there is not enough calcium in the horse’s diet the PTH begins to break down calcium from the bones, which store 99% of the body’s calcium, and moves it into the bloodstream. When there is a long-term deficiency of calcium in the equine’s diet, significant re-absorption of bone can occur, which can lead to decreased bone strength and density, or abnormal bone formation. Many racetrack injuries, laminitis, and tendonitis have been attributed to inadequate calcium in the diet of young racehorses. Even if there is adequate calcium, a large concentration of other minerals such as phosphorus will decrease the absorption of calcium. Thus, there must be a balanced ratio of these minerals. If improper Ca:P levels in the diet continue for an extended period of time, serious problems such as nutritional secondary hyperparathyroidism will develop.