Abstract
After parturition, alternate periods of milking for 4 days and nonmilking for 3 days produced marked changes in plasma concentrations of Ca, P, Mg, Cl, 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 6-7 days after initiation of milking. An overshoot of the Ca 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 Ca homeostatic mechanisms of the cows was present, and for the Ca demand placed on the cows in these experiments the lag time was 5-7 days. This lag time appears to be important in development of parturient hypocalcemia of dairy cows.