Risk factors associated with postpartum subclinical hypocalcemia in dairy cows.

Our study objectives were to evaluate the association of prepartum plasma Mg concentrations with subclinical hypocalcemia (SCH) classification at parturition and to evaluate the association of other cow-level risk factors with SCH classification at calving or at 2 d in milk (DIM). A total of 301 animals from 2 dairy herds located in New York were enrolled in a cohort study. Blood samples were collected at approximately 1 wk before the expected calving date, within 4 h of calving, and at 2 DIM. Prepartum samples had plasma macromineral concentrations (Ca, K, Mg, P), albumin, and β-hydroxybutyrate analyzed. Samples collected at calving were analyzed for Ca only, and samples from 2 DIM had macromineral and albumin concentrations determined. Postpartum SCH was defined as Ca concentrations ≤2.1 mmol/L. The prevalence of SCH at calving was 2, 40, and 66% for first, second, and third or greater parities, respectively. Only 4% of cows could be classified with prepartum subclinical hypomagnesemia (Mg concentrations <0.8 mmol/L), which did not provide enough power to appropriately determine the association of plasma Mg with postpartum Ca concentrations and its effect on SCH classification. Multiparous cows with Ca concentrations ≤2.4 mmol/L in the prepartum period and third or greater parity cows had a higher risk of being categorized as SCH at calving [relative risk (RR) = 1.4 and 1.7, respectively]. The risk of SCH at 2 DIM was associated with the interaction of Ca status at calving and lameness score. Nonlame cows with Ca concentrations ≤2.1 mmol/L (RR = 3.2) and normocalcemic lame cows at parturition (RR = 3.4) were more likely to be SCH at 2 DIM compared with nonlame normocalcemic cows. In conclusion, we identified a prepartum Ca cut-point for identification of cows that are more likely to be classified as SCH at calving. Different risk factors were associated with SCH depending on the timing of diagnosis relative to parturition.