Forages are usually high in calcium, and grain is high in phosphorus. The ratio of calcium to phosphorus in the horses diet is very important; he needs at least as much calcium as phosphorus. Most researchers feel that the balance should be somewhere between 1:1 and 2:1 (1.2 parts calcium to one part phosphorus is ideal) and should not exceed four parts calcium to one part phosphorus. The usual ratio in pasture grass is about 1.2:1, which is perfect for horses. Young horses are often fed grain to speed their growth, but if grain (with its higher phosphorus content) is added to the diet then more calcium should be added (alfalfa hay is a good source) to balance it.

Too much phosphorus in a ration binds with calcium and prevents its absorption (but excess calcium has little effect on the absorption of phosphorus). Feeding too much grain can be detrimental because the grain not only provides more phosphorus than needed but also ties up the calcium in a compound that is not absorbed. Feeding more calcium does not solve the problem because it is not properly absorbed. Thus overfeeding a young horse with grain can result in soft bones, crooked legs, and unsoundness. Signs of calcium deficiency include abnormal bone development in foals and yearlings, decreased bone density in adult horses (and sometimes stiffness and lameness), weight loss, fragile bones, and loose teeth. Big head in adult horses (enlargement of the jaws, from bone being replaced by fibrous connective tissue) is caused by overfeeding phosphorus and is often due to reliance on bran in the diet, which is very high in phosphorus and low in calcium.

Deficiencies of either calcium or phosphorus can result in the body drawing upon bone supply to maintain other functions dependent upon that mineral. In 156 CH05.qx 5/21/04 8:27 AM Page 157 G e n e r a l N u t r i t i o n adult horses, this results in weakening of bones (or osteoporosis, in which bones become fragile and brittle). A calcium deficiency can cause rickets in young horses. Under most circumstances, however, a horse eating forage never develops a calcium shortage because hay is high in calcium.

Inappropriate calcium metabolism (too high or too low) can affect athletic performance because calcium plays a role in muscle contraction. The ability to mobilize calcium is regulated by the hormone calcitonin, the production of which is affected by dietary calcium. If dietary calcium is high, the body is lulled into a false sense of plenty and calcitonin production is low; the horse may suddenly face a shortage during athletic activity. This is a big factor in some cases of muscle tying up or metabolic collapse. It also can be a factor in post-exertion colics when decreased gut motility occurs due to insufficient calcium. Excess calcium in a diet may interfere with absorption of trace minerals (especially iron, zinc, and copper) and in the young horse can lead to skeletal problems such as OCD lesions. Too much calcium in the bloodstream can occur from kidney disease. If the kidneys are not working properly to excrete calcium in the urine, calcium quickly builds up.