Milk fever and subclinical hypocalcemia are the most important macro-mineral metabolic disorders that affect transition dairy cows. Many studies have shown that cows with subclinical hypocalcemia are also prone to many diseases and disorders. The drain of Calcium (Ca) during early lactation represents a significant increase in Ca demand over that for late fetal growth and physiological maintenance. The requirements of the mammary gland for Ca often exceeds the ability of the cow to replenish the plasma Ca pools. Blood Ca concentrations remarkably decline in dairy cows around calving, with the lowest concentrations occurring about 12 to 24 hours after calving. To maintain Ca homeostasis after calving, at the start of lactation, Ca compensating mechanisms are activated. These mechanisms involve a coordinated effort among the hormones 1,25-dihydroxyvitamin D3, parathyroid hormone (PTH), and calcitonin. Hypocalcemia is associated with an increased risk of several important health conditions such as mastitis, retained placenta, metritis, abomasum displacement and immune insufficiency, particularly in transition period. The incidence of subclinical hypocalcemia approaches 40-50% in multiparous cows after calving in dairy herds. In spite of developments in preventive approaches, tremendous economical impact of hypocalcemia on health, production and fertility of dairy cows is a major concern for dairy herd owners. The paramount advances in dairy health have been the paradigm shift from treatment of clinical illness to disease prevention and redefining disease more broadly,to include subclinical conditions. Herd-based tests are now available for use in routine herd monitoring and for investigating dairy herds with metabolic subclinical problems. This review provides the criteria for hypocalcemia monitoring and interpretation of the results in dairy herds.

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