Effects of an inadequate dietary intake of magnesium on osteogenesis in dairy cows during the dry period.

The osteogenic activity in the rib bones of 33 dairy cows (14 of parity 1 or 2, and 19 of parity 3 or more) during their last two months of pregnancy, and the effects upon it of a deficient supply of dietary magnesium, were studied by means of intravenous infusions of the fluorochromes oxytetracycline hydrochloride, calcein and chlortetracycline given nine weeks, five weeks and one week, respectively, before the expected time of parturition. Seven weeks before this date the cows' ration was changed to one containing either 0.22 per cent magnesium (LMg) or 0.82 per cent magnesium (HMg) in the dry matter, and the potassium content of both rations was increased to approximately 4.1 per cent in the dry matter to reduce the absorption of magnesium. Four of the 10 LMg-high parity cows showed signs of periparturient hypocalcaemia. A significant decrease in the number of labelled osteons towards parturition was demonstrated in both the low parity (P less than 0.05) and high parity cows (P less than 0.001). The mean bone apposition rate in high parity cows was significantly lower (P less than 0.001) than in low parity cows. The different levels of magnesium in the diets of the cows had no significant effects on the numbers of osteons which were labelled nine weeks, five weeks or one week before parturition, or on the numbers of osteons which took more than one label or on the bone apposition rate. These results show that in these cows the rate of bone remodelling decreased as parturition approached.