PARTURIENT HYPOCALCEMIA IN DAIRY-COWS - EFFECTS OF DIETARY ACIDITY ONPLASMA MINERALS AND CALCIOTROPHIC HORMONES

Mature Friesian COWS were given silage plus either an alkaline or an a cidic concentrate mix during the last 28 days of pregnancy in two expe riments. There mere no significant differences in plasma calcium, phos phorus or magnesium concentrations before parturition but the blood io nised calcium and plasma chloride concentrations were significantly in creased, together with a lower blood pH and acid-base excess in the co ws on the acid diet. The mean 1,25-dihydroxyvitamin D concentrations t ended to increase before parturition in the cows on the acid diet comp ared with the alkali diet in experiment 1 and were significantly highe r in experiment 2. At parturition, the acid-fed cows had higher mean p lasma calcium concentrations and significantly more of them had values >2.0 mmol litre(-1) than the alkali-fed groups, and the plasma 1,25-d ihydroxyvitamin D and parathyroid hormone concentrations were related to these different plasma calcium concentrations. The acidic diets the refore appeared to prevent the onset of hypocalcaemia at parturition b y increasing the plasma 1,25-dihydroxyvitamin D concentration before p arturition.