Inflammation and delayed oral calcium supplementation: Associations with milk yield, reproduction, and herd removal in multiparous Holstein dairy cows.

The transition from late gestation to early lactation is characterized by metabolic and immune challenges that predispose dairy cows to conditions such as dyscalcemia and excessive inflammation. Both conditions have been associated with impaired health and productivity, and recent evidence suggests they are physiologically linked. Oral calcium supplementation has been proposed as a strategy to support calcium homeorhesis, but results have been inconsistent, possibly due to cow-level factors such as inflammation. The objective of our observational study was to determine whether inflammatory status at 2 DIM, assessed by serum haptoglobin concentrations, modifies the response to oral calcium supplementation administered at 2 and 3 DIM in multiparous Holstein cows. Serum samples and data from 535 cows enrolled in a previously conducted randomized trial on 4 commercial farms in New York were analyzed. Outcomes included average daily milk yield through 15 wk of lactation, pregnancy to first service, time to pregnancy, and time to herd removal. Linear mixed-effects, quasi-Poisson, and Cox proportional hazards models were used to evaluate associations, with marginal means, risk ratios, and hazard ratios calculated at low (0.4 g/L) and high (1.1 g/L) haptoglobin concentrations. For cows with high haptoglobin concentrations at 2 DIM, oral calcium supplementation increased milk yield by 3.1 kg/d and the risk of pregnancy to first service (risk ratio = 1.6, 95% CI = 0.9 to 2.6) in parity ≥3 cows, and improved the hazard of pregnancy (hazard ratio = 1.3; 95% CI: 1.0 to 1.7) and reduced the hazard of herd removal (hazard ratio = 0.5; 95% CI: 0.3 to 1.0) for cows of all parities. No benefit of oral calcium supplementation was observed in cows with low haptoglobin concentrations at 2 DIM other than a 60% increase in risk of pregnancy to first service for parity 2 cows. Our findings suggest that inflammatory status may modify the response to oral calcium supplementation and highlight the potential for targeted strategies to improve postpartum performance.