Determinants of mortality from bovine respiratory disease in Australian feedlot cattle: A survival analysis.

Bovine respiratory disease (BRD) is the leading cause of morbidity and mortality in Australian feedlot cattle, yet large-scale quantification of risk factors for BRD-specific mortality using time-to-event methods remains limited. A retrospective cohort survival analysis was conducted on 1,040,253 cattle from 7825 lots across three Australian feedlots inducted between 1 January 2018 and 5 December 2024 to characterise temporal patterns, identify risk factors, and assess attenuation pathways for BRD-specific mortality within 90 days on feed. The survival analysis was carried out using Cox proportional hazards and time-stratified models, Fine-Gray competing-risks regression, and sequential attenuation decomposition. BRD accounted for 37.6% (5813/15,446) of all feedlot deaths. The BRD-specific hazard peaked at 35.1 days on feed (95% confidence interval [CI]: 34.0-40.0), and 69% of BRD deaths occurred within the first 60 days. The vendor property identification code (PIC) risk score, a standardised measure of historical BRD mortality at source-property level, showed the strongest association with BRD-specific mortality (cause-specific hazard ratio [csHR] = 4.307, 95% CI: 4.149-4.471). Pre-entry vaccination against infectious bovine rhinotracheitis and Mannheimia haemolytica was associated with lower BRD mortality (csHR = 0.646, 95% CI: 0.503-0.829; subdistribution hazard ratio [SHR] = 0.749, 95% CI: 0.600-0.935). Bos indicus and crossbred cattle had lower hazards than temperate breeds, and saleyard-sourced cattle had a higher hazard than paddock-sourced cattle. Seasonal associations were significantly attenuated after accounting for thermal stress and source-property clustering. These findings support the use of integrated BRD control strategies that prioritise source-property risk profiling, timely pre-arrival vaccination, and intensified surveillance during the early post-induction period.