The periparturient cow undergoes a

The periparturient cow undergoes a transition from non-lactating to lactating at calving. The animal is tremendously challenged to maintain calcium homeostasis. Those that fail can develop milk fever, a clinical disorder that is life threatening to the cow and predisposes the cow to a variety of other disorders. Less dramatic sub-clinical hypocalcemia can also reduce productivity of cattle by reducing feed intake in early lactation. The cause and prevention of milk fever will be discussed, focusing on the role of diet cation–anion difference and use of low calcium diets. The periparturient period also typically causes minor perturbations in blood potassium and phosphorus concentrations. Occasionally these disturbances are severe enough to be the cause of recumbency and the “downer cow” syndrome. Pathogenesis of these syndromes will be discussed. Low blood magnesium concentrations are observed when animals are fed inadequate amounts of magnesium or some factor is present in the diet, which prevents adequate absorption of magnesium. Severe hypomagnesemia can cause tetany and the downer cow syndrome, but more commonly moderate hypomagnesemia impairs the ability of the cow to maintain calcium homeostasis and hypocalcemia occurs secondary to the hypomagnesemia.