Can the assessment of skin injuries and keel bone damage at the slaughterhouse replace on-farm assessments?

Two major welfare problems in laying hen farming are keel bone damage (KBD) and cannibalism. Their assessment is time-consuming, needs well-trained assessors, and prevalence estimates are often imprecise due to small sample sizes. Here, the bottleneck slaughterhouse comes into focus where large numbers of animals can be inspected. However, this is only an option if the prevalences recorded at the slaughterhouse reasonably agree with on-farm assessments. The aim of this study was to compare the prevalence of KBD and skin injuries in 20 commercial laying hen flocks (i) before depopulation on-farm (ii) after transport and lairage time at arrival at the slaughterhouse, and (iii) at the slaughter line. Bland-Altman plots and equivalence tests were conducted. In addition, the consistency of welfare evaluations of the results according to a traffic light scheme was investigated. Cloacal injuries could not technically be recorded on the slaughter line. With an arbitrary precision of ± 2% for dorsal skin injuries and ± 5% for KBD, the results from farm and slaughter line did not reach equivalence. For dorsal skin injuries, the detected mean prevalence across all flocks examined increased numerically from the farm (15.1%) to the slaughter line (22.8%). In addition, the traffic light evaluations changed between farm and slaughter line in 80% of cases in different directions. Therefore, it cannot be recommended to derive evaluations of on-farm welfare from assessments of skin injuries at the slaughter line. In contrast, the mean detected prevalence of KBD across all flocks decreased consistently (r = 0.794) from the farm (56.0%) to the slaughter line (41.7%). It can be concluded that the assessment of KBD at the slaughter line consistently underestimates KBD prevalences compared to on-farm assessments, but this can be taken into account in the interpretation of the results. Slaughter line assessment of KBD may be a feasible option for monitoring severe welfare problems due to KBD in commercial practice.