SUMMARYPlasma osteocalcin, or plasma bone GLA protein (BGP), total plasma alkaline phosphatase activity and urinary hydroxyproline excretion of twenty-four pregnant dairy cows (thirteen in their first or second pregnancy, i.e. low parity, and eleven in their third or more pregnancies, i.e. high parity) were measured from 7 weeks before parturition until 1 week after parturition. Seven weeks before parturition the cows' ration was changed to one containing either 0-22 % magnesium (low magnesium, LMg) or 0-82 % magnesium (high magnesium, HMg) in the dry matter, and the potassium content of both rations was increased to approximately 4-1 % in the dry matter to reduce the absorption of magnesium. Plasma BGP levels decreased significantly (P < 0-01) as parturition approached while total plasma alkaline phosphatase activity and urinary hydroxyproline excretion did not. Magnesium supply and parity had no significant effect on this decrease. The overall plasma concentration of BGP, total plasma alkaline phosphatase activity and the urinary hydroxyproline/creatinine ratio in the prepartum period were affected by parity (P < 0 05) with higher values in the lower parity cows. A significant positive correlation (r = 0 58, P < 0-01) was found in all cows between plasma BGP level at parturition and the percentage of the bone surface covered with osteoblasts; however, plasma BGP was not correlated either with other histomorphometric variables or with total alkaline phosphatase activity during this time.
SUMMARY. The bone activity and bone mineral content in rib bones resected from 33 dairy cows between 3 and 8 h after parturition were measured, and the effects upon them of a deficient supply of dietary magnesium (Mg) during the last seven weeks of pregnancy were studied. The cows were fed a diet containing either 0.22% magnesium (low Mg) or 0.82% magnesium (high Mg) in the dry matter (DM), and the potassium content of both rations was increased to approximately 4.1% in the DM to reduce the absorption of magnesium. In the cows fed the low-Mg diet a fall in plasma Mg concentration was observed In the low-Mg, low-parity cows the plasma Mg concentrations at parturition were higher than in the low-Mg, high-parity cows, i.e. 0.83 mmol/l and 0.54 mmo1/1, respectively. After parturition four cows in the lowMg, high-parity group showed clinical signs of hypocalcaemia but none of the other groups did so. The bone formation in low-parity cows was significantly (P < 0.05) affected by Mg supply, with higher percentages of both trabecular surface covered by osteoid and osteoid volume in the low-Mg group. In the high-parity cows no significant differences in bone formation were found between the low-and high-Mg groups. An inadequate Mg supply resulted in a significantly (P < 0.05) higher Ca content in the bone ash of low-parity cows and a significantly (P < 0.05) higher bone ash percentage in the bone of high-parity cows.
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