The aim of the present study was to estimate the effectiveness of administering MgSO₄ × 7H₂ O to cows as a treatment preventing the occurrence of hypocalcaemia, one of the most important health disturbances in high-yielding dairy cows during transition and early lactation period. MgSO₄ × 7H₂ O was administered to young (2nd-3rd lactation) and older (4th-6th lactation) cows at a dose of 100 g/day/head (decrease of DCAB to 45 mEq/kg DM) alone during 2 weeks a.p., or in combination with CaCO₃ at a dose of 100 g/day/head (increase of Ca concentration from 0,54% to 0,93% of DM) during the last 10 days a.p.. Moreover, the studies concerned the effect of this treatment on milk yield and milk composition as well as fertility and cows’ health. The experiment was conducted on 32 cows divided into age 2 groups: young and older cows (16 heads per group). All experimental animals received magnesium sulphate, but half of the cows from each group (8 heads) received additionally a higher amount of Ca in rations. Selected compounds and biochemical parameters including pH (blood, urine), Ca, P, Mg (serum, urine), Na, K, Cl (urine), glucose (blood), protein, urea (serum), pCO₂, pO₂, HCO₃, BE(E) (blood) were determined in urine, blood and blood serum. In the 8th week of lactation the milk yield and milk composition (fat, protein, lactose, dry matter, urea) were determined. In the 21st and 45th day after calving the level of progesterone was estimated. Moreover, fertility indices such as conception rate, insemination index and calving interval, as well as the occurrence and intensity of metabolic disorders were also determined. A decrease in DCAB, regardless of carbonate addition, prevented hypocalcaemia. On the other hand, an increase in Ca concentration caused a slight deterioration in milk yield, biochemical indices of urine and blood, as well as the health and fertility of young cows. The administration of calcium carbonate to older cows had no effect on the milk yield and biochemical indices of urine and blood. However, the fertility and health were improved. In view of these facts, calcium carbonate combined with the so-called “strong anions” ought to be provided before calving only to older cows.
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