Big head is a calcium deficiency of horses grazing introduced tropical pasture grasses.
It is caused by crystals of calcium oxalate in the grass blades that prevent the horse from absorbing calcium from the grass during digestion. Some or all horses on a pasture may develop big head. The disease can develop within 2 months of horses being put on hazardous pastures but commonly takes 6 to 8 months. Mares and foals are more susceptible than stallions and geldings, but all can get the disease. Cases of big head have occurred on pastures of the introduced tropical species buffel grass, green panic, setaria, kikuyu, guinea grass, para grass, pangola grass and signal grass.