This sequence of events often prevailed in the past when millers used horse power to grind cereal grains into flour. The practice of the time was to give the by-products of the milling plus a percentage of the grain to the miller in payment for services rendered. The miller, in turn, used these resources to feed the horses that provided the power - a practice that caused his horses to receive a disproportionate amount of grain and bran. Today we know that these cereal products are high in phosphorus and contain little calcium - the very combination that sets up the opportunity for hypocalcemia to occur. The added consumption of bran supplied the trigger.
Bran contains phytic acid which chelates calcium further reducing the level of dietary calcium.
These factors in the ration fed to the miller's horses predisposed his horses to big heads even though they were fat and slick and presumed to be the pampered horses of the era. Although millers' horses are a thing of the past, today's race horses and show horses are receiving rations, low in roughage and high in grains to boost energy intake without causing large gut fill. This type of management can produce the same results - big head. A third cause of hypocalcemia involves inadequate amounts of the Vitamin D 3 which is necessary for the transport of calcium across the gut wall. Horses have the ability to manufacture Vitamin D 3 from sunlight and sun cured forages such as hay.