Effects of dietary calcium carbonate on blood ionized calcium, acid–base status and plasma metabolites in lactating Holstein cows with reference to mid-lactation milk fever

With reference to mid-lactation milk fever, the aim of this study was to investigate basal variability in blood ionized calcium as well as related urinary and blood plasma variables and to investigate the effects of feeding calcium carbonate. Feeding calcium carbonate to dairy cows induced increased absorption and excretion of non-metabolizable base. No diurnal variation in plasma total calcium concentrations was observed but calcium activity as well as concentrations of bicarbonate, potassium, sodium, glucose, L-lactate, D-3-hydroxybutyrate, magnesium and urea were affected by sampling time. Feeding calcium carbonate above requirements did not increase diurnal variability in calcium activity, less variability was observed with the highest calcium intake. Late lactating cows are not at high risk of mid-lactation milk fever in their ordinary circadian rhythm, however, blood calcium activity dropped systematically in the morning when light was switched on and the cows were milked.