A longitudinal study of causes of mortality in Danish commercial laying hens in non-cage housing systems

Mortality is a widely accepted indicator of health and welfare in commercial laying hens with economic and environmental implications. Rising demand for “cage-free” eggs is driving a transition towards non-cage systems in many parts of the world. However, little is known about the causes of mortality in laying hens in non-cage systems, and better understanding thereof may ease the transition. The objective of this study was to investigate causes of mortality and risk periods for health management in laying hens housed in barn and organic systems in Denmark. A longitudinal study was designed with sampling of the first 10 dead hens (euthanized or found dead) every second week from placement to slaughter in seven commercial layer flocks (three organic, four barn) from six farms in Denmark in 2020–2022. A total of 1801 carcases were necropsied; bacterial culture was performed on indication (609 bacterial isolates obtained). Overall, non-infectious causes of mortality (63.9%) were more prevalent than infectious causes (37.6%). The five most prevalent causes of mortality were salpingitis-peritonitis, cannibalism, egg binding, pecking, and uraemia, which accounted for the death of 71.1% of the hens. Risk periods could not be determined due to flock variation. Focus on improving control of these few conditions may support reductions in mortality in non-cage housing systems to the benefit of welfare, health, production economics and environmental sustainability. More research is needed to better understand the pathogeneses and risk factors of the identified causes of mortality, and interactions between them, to enable development of effective preventative strategies. Five causes of mortality accounted for 71.1% of total mortality of laying hens.Non-infectious causes of mortality (63.9%) more prevalent than infectious (37.6%).Risk periods were inconclusive due to flock variation.Egg binding can cause mortality, and may be linked to above normal body condition. Five causes of mortality accounted for 71.1% of total mortality of laying hens. Non-infectious causes of mortality (63.9%) more prevalent than infectious (37.6%). Risk periods were inconclusive due to flock variation. Egg binding can cause mortality, and may be linked to above normal body condition.