Relationship of milk secretion to hypocalcemia in the dairy cow.

After parturiton, alternate periods of milking for 4 days and nonmilking for 3 days produced marked changes in plasma concentrations of calcium, phosphorus, magnesium, chloride, and parathyroid hormone in Jersey cows. A 3- to 4-day period of hypocalcemia, increased parathyroid hormone concentrations, hypophosphatemia, and hypermagnesemia was associated with initiation of each milking period. These measurements did not return to normal until 6 to 7 days after initiation of milking. An "overshoot" of the calcium and phosphate concentrations after this adaptive period occurred in many of the cows. There was little difference in the time sequence of the hypocalcemic period that was associated closely with parturition and initiation of milk secretion in the cow and of hypocalcemic periods that could be induced by the alternate periods of milking. A "lag time" in the calcium homeostatic mechanisms of the cows was present, and for the calcium demand placed on the cows in these experiments the "lag time" was 5 to 7 days. This "lag time" appears to be important in development of parturient by hypocalcemia of dairy cows.