The goal of the experiment was to determine the appropriate time of calcium bolus (CB) administration on performance and health of primiparous fresh cows. Two - hundred fourteen primiparous cows were randomly assigned to one of four treatments immediately after calving: 1) control group without receiving CB (CON); 2) CB immediately after calving (BL0); 3) CB 24 h after calving (BL24); 4) two CBs immediately and 24 h after calving (BL0,24). Milk production and milk composition were recorded monthly (based on scheduled farm milking recording) and there were six records for each cow. Serum total calcium (tCa) concentrations was determined at 0, 24, 48, 72, 96 and 168 h after calving. The prevalence of diseases and reproductive performance were recorded for 210 DIM. Total calcium concentration was significantly lower in total period in BL24 treatment (P < 0.01). Also serum total calcium concentration in BL0 and BL0,24 groups increased significantly at 24 h after calving (P = 0.03), but was lower for BL24 cows after 72 h of calving (P < 0.01) compared with CON and BL0 treatments. Serum ß-hydroxybutyrate concentration in control group tended to be greater (P = 0.06) at 7 DIM compared to all treatments. Average milk yields were not different among treatment groups but treatment by time interaction was significant (P = 0.03), thereby in the first milk record BL0 and BL0,24 cows had greater milk production (P = 0.03) and tended to produce more at third record (P = 0.09). Treatments had no effect on milk compositions, reproductive performance and diseases incidence. Only, the days to the first artificial insemination tended to be reduced for BL24 cows compared with others (P = 0.08) and pregnancy at second artificial inseminations tended to be less for control compared with CB supplemented cows (P = 0.09). The subclinical hypocalcemia incidence at 24 h after calving was lower in BL0 treatment (P = 0.03). Based on the results of this study, supplementing at least one CB immediately after calving in primiparous cows could increase the blood calcium concentration and cause them to peak earlier.
