Calcium (Ca) and Phosphorus (P) are two of the most plentiful minerals in your horse and are required for bone structure, muscle function and hundreds of other metabolic functions. However they compete for uptake in the digestive tract.
When there is more phosphorus than calcium in the diet, calcium is prevented from being absorbed in the diet so the body extracts calcium from the bones to maintain a precise and critical level in the blood. All grains are high in phosphorus and when fed over time cause calcium deficiencies.
Feed manufacturers add calcium and phosphorus in a ratio so Ca is greater than P but in doing so, they prevent the absorption of Magnesium (Mg).