Bovine respiratory disease (BRD) cause-specific and overall mortality in preweaned calves on California dairies: The BRD 10K study.

Mortality in preweaned dairy calves is a significant source of economic loss for dairy producers. In particular, bovine respiratory disease (BRD) is a leading cause of death in preweaned dairy calves. The objectives of this study were to investigate management practices and their effects on mortality, both that specifically attributed to BRD and overall mortality due to all causes, in preweaned dairy calves. Rates of failure of passive transfer of immunity (FPT) are also reported. The study consisted of a convenience sample of 5 dairies across California, selected based on management practices, calf records, location, and size. Trained study personnel performed comprehensive calf management surveys on every dairy at least once every season. Calves were enrolled in the study at birth and followed until weaning. Mixed-effect logistic regression models were specified for the outcomes all-cause mortality (any death before weaning) and mortality attributed to BRD. The 2 final models included a total of 11,470 calves that were born on the study dairies and followed until weaning. The study cohort's overall crude mortality was 2.8%, with crude mortality of individual dairies ranging from 1.7 to 7.2%. The proportion of mortality attributed to BRD was 19.3%, with a range of 0 to 27.1% on the study dairies. Increasing the frequency of changing maternity pen bedding was associated with a decreased risk of mortality due to BRD. Calves diagnosed with BRD in the spring had an increased risk of mortality compared with calves born in the summer; mortality in calves with fall and winter BRD diagnoses did not different significantly from that in summer. Season of mortality was not significant in either model. Feeding ≥5.7 L of milk per day per calf (vs. ≤3.7 L/d) decreased the risk of mortality in calves over 21 d of age. Twins had a 68% increased risk of all-cause mortality compared with calves born as singletons. Both mortality models showed an association between administration of a modified live vaccine in dams (targeting BRD pathogens) and a decreased risk of mortality in calves. Using a serum total protein cut-off of 5.2 g/dL, 16.8% of calves had FPT, with a mean serum total protein concentration of 5.94 ± 0.06 g/dL across all calves sampled.