A randomized controlled trial evaluating postpartum delayed milking strategy and oral calcium bolus administration on production and blood metabolites of Holstein cows.

Postpartum hypocalcemia has substantial economic impacts on dairy production systems. Our objectives were to investigate the effects of delayed milking and Ca bolus supplementation on lactation performance and blood metabolites of dairy cows. A total of 699 Holstein cows (175 primiparous and 524 multiparous) were stratified by parity group and randomly assigned within each group to 6 treatments in a 2 × 3 factorial arrangement. Factor 1 was supplementation with 2 Ca boluses (BL; 31 g Ca/bolus), administered twice: once within 30 min and the second one 24 h after parturition, or no supplemental Ca (NBL). Factor 2 was the milking strategy whereby cows were milked 3 times daily (M0) or delay-milked for 24 (M24) or 48 h (M48) after the colostrum harvest. This resulted in 6 treatment groups: M0-BL (n = 125), M24-BL (n = 118), M48-BL (n = 111), M0-NBL (n = 118), M24-NBL (n = 114), and M48-NBL (n = 113). Blood was collected immediately before calving and again at 1, 2, 3, 5, and 11 d postpartum. Milk production was recorded daily until 12 DIM, then monthly until 90 DIM. Generalized linear mixed models were used for the analysis of data. The BL supplementation did not affect milk yield or measured blood metabolites. In multiparous cows, blood Ca concentration at d 2 postcalving was 2.59 mmol/L (95% CI = 2.49 to 2.69 mmol/L) for M48 cows and 2.32 mmol/L (95% CI = 2.26 to 2.48 mmol/L) for M0 and M24 cows. Blood P concentration at d 2 postcalving was 2.33 mmol/L(95% CI = 2.19 to 2.47 mmol/L) for M48 cows, 1.81 mmol/L (95% CI = 1.66 to 1.96 mmol/L) for M24 cows, and 1.65 mmol/L (95% CI = 1.49 to 1.81 mmol/L) for M0 cows. At d 3 postcalving, blood P concentration was 2.01 mmol/L(95% CI = 1.80 to 2.22 mmol/L) for M48 cows, 1.78 mmol/L (95% CI = 1.63 to 1.93 mmol/L) for M24 cows, and 1.66 mmol/L (95% CI = 1.50 to 1.82 mmol/L) for M0 cows. Multiparous cows in the M48 group also had greater blood glucose at d 3 and lower BHB at d 2 after calving than M0-treated cows. In multiparous cows, the delayed milking strategy resulted in a lower daily milk yield from 3 to 12 DIM as compared with the M0 group. The 3 monthly test-day records remained unaffected across the experimental groups. Overall, delayed milking increased blood Ca and P and decreased BHB concentration in M48-treated cows. The effects of BL supplementation on production performance and blood metabolites and the potential synergy with delayed milking were not evident in this experiment.