|Clinic:||University of California Davis|
|City, State, ZIP:||Davis, CA 95618|
A. Valldecabres, DVM
N. Silva-del-Río, DVM, PhD
1Veterinary Medicine Teaching and Research Center, University of California Davis
The most appropriate blood calcium (Ca) threshold to define subclinical hypocalcemia is still under study. Currently, there is a wide range of suggested definitions, and variability on its reported associations with productive, reproductive and health outcomes, which have not been described in Jersey cows. Our aim was to evaluate the associations of serum Ca concentration and subclinical hypocalcemia at calving with: subsequent lactation milk and energy-corrected milk yield, fat%, protein%, somatic cells count linear score, mastitis, herd removal, and pregnancy at 1st service and by 150 days in milk (DIM).
A total of 609 multiparous Jersey cows from 2 commercial herds fed acidifying prepartum diet were enrolled in the study. Blood samples for total serum Ca concentration determination were collected from the coccygeal vessels at 3 h 10 min (±2 h 17 min) after calving. Monthly test milk yield, fat%, protein% and somatic cells count information up to the 10th test was obtained from the Dairy Herd Improvement Association. Additional information was obtained from herd records.
Statistical analyses were conducted with multiple linear, Poisson, log-binomial, and Cox’s proportional hazards regression using SAS (version 9.4). Considered explanatory variables for all outcomes were: parity, herd, previous lactation length and 305-days mature equivalent milk yield, dry period length, calving body condition and locomotion scores, calving easiness, and oral Ca supplementation. Additional variables considered were: somatic cells count linear score at test for milk yield, fat%, and protein%; milk yield at test for fat%, protein% and somatic cells count linear score; and DIM at 1st service and breeding code (timed artificial insemination and heat breeding) for pregnancy at 1st service. Serum Ca thresholds among 1.80 and 2.20 mmol/L (7.2 and 8.8 mg/dL) at 0.02 mmol/L (0.08 mg/dL) intervals were used to define subclinical hypocalcemia when serum Ca concentration (in a continuous scale) was associated with the outcome. Serum Ca thresholds that better predicted the outcome (continuous outcomes: smallest P-value and most extreme estimate; categorical outcomes: maximized sensitivity and specificity on receiver operating characteristic curve analyses) were chosen to define subclinical hypocalcemia.
Subclinical hypocalcemia (Ca ≤2.18 mmol/L; 8.7 mg/dL) was associated with 1.52 and 1.88 kg/d more of milk and energy-corrected milk yield, respectively (P < 0.001). Milk fat% was 0.12 units of percentage higher and milk protein% was 0.06 units of percentage lower per day for cows with subclinical hypocalcemia (Ca ≤1.96 and ≤1.80 mmol/L, respectively; 7.9 and 7.2 mg/dL, respectively), compared to normocalcemic cows during the subsequent lactation (P = 0.01 and P = 0.03, respectively). Subclinical hypocalcemia was associated with lower 1st service pregnancy risk [Ca ≤2.08 mmol/L (8.3 mg/dL); risk ratio = 0.70; P = 0.03] and hazard of pregnancy by 150 DIM [Ca ≤1.90 mmol/L (7.6 mg/dL); hazard ratio = 0.50; P < 0.001]. No association was observed among serum Ca concentration, mastitis and herd removal. Similar effects were observed for additional thresholds evaluated.
Establishing a single serum Ca threshold for subclinical hypocalcemia definition based on productive, reproductive and health outcomes doesn’t seem feasible. Further studies are needed to elucidate the applicability of a subclinical hypocalcemia definition.