Effects of calcium and magnesium intakes and feeding level during the dry period on bone resorption in dairy cows at parturition.

A study was made of the influence of low and high calcium intakes at two feeding levels (experiments 1 and 2), and of different magnesium intakes (experiment 3) during the dry period, on the cellular resorptive activity in bone of dairy cows at parturition. In experiment 1 (26 cows) the feed intake was 1.12 times maintenance and the calcium intake was either 13.0 g d-1 or 83.5 g d-1. In experiment 2 (33 cows) the feed intake was 1.9 times maintenance and the calcium intake was either 26.4 g d-1 or 87.2 g d-1. In experiment 3 the cows feed intake was 1.4 times maintenance, the calcium intake was 71 to 72 g d-1 and the magnesium intake was either 16.6 g d-1 or 71.0 g d-1. Bone biopsies were taken from the tuber coxae between three and eight hours after parturition. In experiment 1 the cellular resorptive activity in bone from older cows was higher in the cows receiving the low calcium diet. In experiment 2 the differences between the bone resorptive activities of the two groups were small. In experiment 3 the bone resorptive activity was lower in the cows with a deficient supply of magnesium. There were significant relationships in young cows, but not in older cows, between the cellular resorptive activity in bone and the rate of calcium mobilisation measured biochemically. These results suggest that in older dairy cows bone resorption makes only a minor contribution to the total rate of calcium mobilisation at parturition and is therefore of minor importance for the prevention of periparturient hypocalcaemia.