During periods of dietary calcium deficiency, calcium is mobilised from the bone to keep blood calcium levels ‘normal’. When a dietary calcium deficiency occurs for prolonged periods of time the calcium mobilised from the bone is replaced by fibrous tissue, resulting in weak bones and a ‘Big Head’ appearance. Most commonly seen in horses grazing tropical grasses such as kikuyu, buffel, setaria, green panic, para grass, pangola grass, guinea grass, signal grass and purple pigeon grass. These grasses contain substances called oxalates that bind up calcium, making it unavailable for absorption through the bowel. Horses eating moderate to high amounts of high phosphorous feeds like cereal grains can also have a calcium blocking effect.