Occurrence of keel bone lesions measured at the slaughterhouse in laying hens: analysis of risk factors for on farm animal welfare

Laying hens in conventional farms are exposed to several risks which can challenge their welfare. Different animal-based indicators (ABMs), i.e. keel bone lesions, breast blisters, and footpad lesions, can be collected at slaughtering for information about welfare on farm. Nevertheless, risk factors for ABMs and relationships with housing systems have not been fully elucidated yet. The present study evaluated the risk factors for keel bone (KB) lesions by collecting data in a commercial slaughterhouse for 15 months on 51 flocks (200 hens per flock): floor systems (16 flocks), enriched cages (16 flocks), aviary systems (19 flocks). The effects of slaughtering season, genetic line (brown vs white feathered), housing system, flock size, and length of the rearing cycle were evaluated by a multivariate logistic regression analysis; the risk factors were identified through a forward stepwise selection; the regression coefficients were expressed as odds ratio (OR) with 95% confidence interval. As for the odds of showing KB deviations, a lower value (OR: 0.70; P<0.001) was found in hens slaughtered in winter compared to spring; a higher value (OR: 1.26; P<0.001) was recorded in white compared to brown hens; the risk was lower in floor systems (OR: 0.78; P<0.001) and higher in enriched cages (OR: 1.41; P<0.001) compared to the aviary system; a higher odds ratio (OR: 1.50; P<0.001) was recorded in medium compared to small flocks. The odd ratio of showing KB deviations was lower in hens from flocks reared for extended cycles compared to those from standard ones (OR: 0.84; P<0.01). As for KB fractures, a higher odds ratio (OR: 2.04; P<0.001) was recorded in winter compared to spring, whereas a lower odds ratio (OR: 0.69; P<0.001) was recorded in white compared to brown hens. The odds ratio of showing KB fractures was 1.66 for enriched cages and 0.99 for floor systems (P<0.001) compared to aviary system. Higher odd ratios were obtained for KB fractures occurrence of medium and big flocks (OR: 1.69 and 1.83; P<0.001) compared to small flocks, whereas OR was lower in flocks reared for an extended compared to a standard cycle (OR: 0.69; P<0.001). In conclusion, changes in KB lesions according to slaughtering season, flock size, and rearing length require further insights. Brown and white hens have a different risk for KB deviations or fractures. Finally, hens in enriched cages are more exposed to risk for KB fracture compared to hens kept in aviary systems.