Unravelling the Drivers of Early-Life Mortality in Yak Calves: A Retrospective Cohort Study Using Survival Analysis

Early-life calf mortality is a significant barrier to herd growth and overall productivity in yak husbandry. This retrospective study investigated the survival pattern and potential risk factors linked to yak calf mortality during the first 90 days of life. Analysis was performed on existing farm records from 428 yak calves born between 2013 and 2025 at a semi-intensive yak farm. The Kaplan-Meier method was used to estimate survival probability, and the log-rank test was used to compare survival probabilities; additionally, a multivariate Cox proportional hazards model was employed to identify independent risk factors. Overall, 70 (16.4%) calves died within the first 90 days after birth. The Kaplan-Meier analysis indicated significantly lower survival probability among low-birth-weight calves (≤12 kg; p = 0.0005) and calves born to primiparous dams (p = 0.0003). In contrast, survival did not differ according to calf sex (p = 0.620) and birth season (p = 0.658). In the multivariate Cox proportional hazards model, low birth weight (HR = 2.27, 95% CI: 1.24-4.16; p = 0.008) and primiparous dams (HR = 2.08, 95% CI: 1.28-3.38; p = 0.003) were independently associated with higher mortality risk. As a result, calf sex and birth season were not significant predictors. Diarrhea and respiratory disease were the predominant causes of calf deaths. The current study's results indicate that maternal and neonatal factors are the major determinants of early-life calf survival. This highlights the need for special care with optimum nutrition and regular monitoring of primiparous dams and low-birth-weight calves in order to lower pre-weaning mortality and improve overall herd productivity.