Comparison of Two Serum Calcium Cut points (8 vs. 8.6 mg/dL) on Metabolic, Productive, and Reproductive Performance of Dairy Cows on Day 4 Postpartum

Introduction: Subclinical hypocalcemia is one of the most common metabolic disorders in early postpartum dairy cows and is associated with impaired metabolic regulation, oxidative imbalance, and reduced productive performance. A major challenge in the diagnosis of this condition is the lack of agreement on the optimal serum calcium threshold. Previous studies have applied different cut points, most commonly 8.0 and 8.6 mg/dL, which has led to inconsistent interpretations. Therefore, the objective of the present study was to compare these two commonly used serum calcium cut points and evaluate their ability to distinguish differences in metabolic status, oxidative stress indicators, milk production, milk composition, and selected reproductive health outcomes in Holstein dairy cows during early lactation.
Materials and Methods: The study was conducted on 100 Holstein cows from four commercial dairy herds located in Isfahan province. The farms included Goldasht, FKA, Namfar, and Baharan Agriculture-Industrial and Livestock. Blood samples were collected on day 4 postpartum to determine serum calcium and a comprehensive panel of metabolic and oxidative stress parameters, including glucose, total cholesterol, total antioxidant capacity (TAC), albumin, globulin, total bilirubin, total protein, malondialdehyde (MDA), β hydroxybutyrate (BHB), and non esterified fatty acids (NEFA). Cows were classified into normocalcemic and hypocalcemic groups using two diagnostic thresholds: 8.0 mg/dL (NC1 vs. HC1) and 8.6 mg/dL (NC2 vs. HC2). Monthly test day milk yield, milk fat and protein percentages, and somatic cell count (SCC) were obtained from the Dairy Herd Improvement (DHI) system. Cumulative yields at 100, 200, and 305 days in milk were estimated using a multiple trait procedure based on the Wilmink model. Data were analyzed using mixed models, and least squares means ± standard errors were reported (P ≤ 0.05).
Results and discussion: The results indicated that neither serum calcium threshold showed a significant association with the incidence of metritis or conception rate at first artificial insemination. At the 8.0 mg/dL threshold, the incidence of metritis in normocalcemic (NC1) and hypocalcemic (HC1) cows was 14.7% and 13.0%, respectively, with no significant difference observed between the groups (P = 0.77). Similarly, at the 8.6 mg/dL threshold, metritis incidence did not differ significantly between NC2 (14.0%) and HC2 (13.6%) groups (P = 0.96). Regarding reproductive performance, the conception rate at the 8.0 mg/dL threshold was 49.5% for NC1 and 47.0% for HC1 (P = 0.84). At the 8.6 mg/dL threshold, although the conception rate was numerically higher in NC2 (52.9%) compared to HC2 (46.0%), this difference did not reach statistical significance (P = 0.56). The serum concentrations of glucose, NEFA, BHBA, total bilirubin, and globulin did not differ significantly between the groups at 4 DIM (P > 0.05). In contrast, several metabolic and oxidative parameters including MDA, albumin-to-globulin ratio, total protein, albumin, TAC, and total cholesterol exhibited significant statistical changes, which were strongly influenced by the selected serum calcium threshold. When subclinical hypocalcemia was defined using the 8.0 mg/dL cut-point, hypocalcemic cows exhibited significantly lower total antioxidant capacity (TAC; P = 0.05), reduced serum albumin and total protein concentrations (P < 0.0001 and P = 0.02, respectively), and decreased serum cholesterol levels (P = 0.01). Furthermore, these cows experienced a marked increase in oxidative stress, as evidenced by higher malondialdehyde (MDA) concentrations (P = 0.01). Using the 8.6 mg/dL cut-point revealed a comparable overall trend, though with fewer significant associations. Specifically, only albumin (P = 0.002) and total protein (P = 0.01) remained significantly lower in HC2 versus NC2 cows. Furthermore, a significant reduction in MDA concentrations was noted in the HC2 group compared to its normocalcemic counterpart (P = 0.02). Based on the results, cows in the HC1 group (defined by the 8.0 mg/dL threshold) exhibited a significant reduction in 305-day milk fat yield (P = 0.02), with a trend toward a decrease in milk protein yield (P = 0.06). Conversely, when the 8.6 mg/dL threshold was applied, none of the milk production or composition traits differed significantly between NC2 and HC2 groups at any stage of lactation. Furthermore, no significant differences in milk yield were observed between hypocalcemic and normocalcemic groups at 100, 200, or 305 DIM. Crucially, the 8.6 mg/dL threshold demonstrated superior sensitivity in identifying cows at risk of metabolic disturbances and reduced production, as it captured a broader spectrum of affected individuals than the conventional 8.0 mg/dL limit.
Conclusion: This study highlights that the 8.6 mg/dL serum calcium threshold serves as a more reliable and sensitive indicator for identifying metabolic risks in early-lactation Holstein cows compared to the 8.0 mg/dL limit. Although the latter is often used to define clinically significant hypocalcemia, our results suggest it may overlook a substantial number of animals with subclinical metabolic and oxidative imbalances. Given its higher sensitivity, we conclude that adopting the 8.6 mg/dL threshold allows for earlier identification of at-risk cows, thereby providing a more robust framework for optimizing metabolic health and productive performance during the day 4 postpartum.