Vitamin D status and hypocalcemic response to protamine in exercised and non-exercised dairy cows.

Bone resorption was studied in post parturient cows using intravenous injection of protamine. Protamine inhibits bone resorption and the protamine-induced hypocalcemia can be taken as a measure of this process. The studies were performed in a herd where half of the cows had been tied indoors for 2 1/2 years, the other half had been given daily exercise during the same period. All animals were fed in the same way. The hypocalcemic response to protamine was the same in both groups, which indicates that exercise did not change bone resorption. The serum levels of calcium, inorganic phosphorus and magnesium around parturition did not differ. The daily exposure to daylight increased the serum level of 25-(OH)-cholecalciferol in late summer in the exercised group.