The physiological effects of induced prepartum subclinical hypocalcemia in multiparous Holstein cows

Subclinical hypocalcemia (SCH) is prevalent during the periparturient period in dairy cows due to increased calcium (Ca) demand for milk and colostrum synthesis. Transient SCH has been shown to activate a negative feedback loop, thereby improving Ca homeostasis. Here, we investigate the impact of induced SCH during the prepartum period on Ca homeostasis pre- and postpartum, and milk production. Thirty multiparous Holstein cows were enrolled 21 d before the expected due date, in a randomized complete block design, and were assigned to receive either a 6-h continuous intravenous infusion of saline solution (CON) or 5% egtazic acid (EGTA) solution (n = 15/treatment) for a minimum of 3 d and a maximum of 14 d until parturition. Blood samples were collected daily from enrollment to the onset of infusion, and from 1 to 49 DIM (d 1 to 49). During infusions, blood samples were collected immediately before infusion, hourly during infusion, and 1-h post-infusion termination. Milk samples were collected weekly during the initial 3 wk of lactation, and milk weights were recorded daily from d 0 to 49. Results demonstrated that prepartum EGTA treatment led to decreased ionized Ca (iCa) and total Ca (tCa) concentrations compared with the control group during the infusion. Postpartum, EGTA-treated cows exhibited higher iCa on d 0 h 12 (12 h postpartum), d 1, and d 2, and higher tCa concentrations on d 0, d 0 h 12, d 1, d 2, d 3, and d 5. Additionally, EGTA-treated cows developed resistance to induced SCH, as indicated by an increased infusion rate (IR) on d -3, d -2, and d -1 compared with the first day of infusion (622.68 ± 40.19 mL/h). We observed a treatment × day × time interaction in parathyroid hormone (PTH) concentrations, with EGTA-treated cows having higher concentrations at d -2 h 0 and d -1 h 0 compared with CON-treated cows. During the postpartum period, EGTA-treated cows had increased PTH concentrations compared with CON-treated cows. Additionally, EGTA-treated cows tended to have higher serotonin concentrations during the prepartum period compared with CON-treated cows. A treatment × day interaction on milk production was observed, with EGTA-treated cows having decreased milk yield on d 16, and d 18 to 24; however, there were no significant differences in milk production over the entire lactation period. These findings indicate that prepartum SCH induction primes Ca homeostatic mechanisms, potentially mitigating postpartum SCH.