Effects of broiler strain and stocking density on causes of death in broilers in Denmark

Broiler production industries are increasingly prioritizing animal welfare in line with the traditional focus on production traits such as rapid growth rate and feed efficiency. Broiler strain and stocking density are key welfare factors that are addressed in high-welfare systems. Still, it is unclear how slower-growing broilers or lower densities affect health, causes of death, and gross lesions, which are vital issues in welfare assessment. This study investigated the effects of broiler strain, stocking density, and age on causes of death and gross lesions, with particular attention to growth-related diseases in broilers that died between 7 days of age and slaughter under non-outbreak conditions. It was hypothesized that Rustic Gold broilers would have lower overall mortality and a lower prevalence of growth-related diseases (e.g., cardiovascular diseases and locomotory disorders). Stocking densities and slaughter ages included two Ross 308 broiler densities (33 and 41 kg/m², slaughtered at 34 days) and Rustic Gold broilers at 36 kg/m² (slaughtered at 41 days). The study used a repeated cross-sectional observational design with three rotations per farm/group (36 flocks in total, excluding three). Overall mortality proportions for both Ross 308 groups and Rustic Gold were nearly identical. The data showed that 81% of dead broilers on farms experienced cardiovascular collapse. No significant differences in the primary causes of death, including cardiovascular collapse, leg disorders, renal disorders, septicemia, trauma, polyserositis, cannibalism, and intestinal volvulus, were observed among the different production types. However, Ross 308 exhibited significantly higher prevalences of cardiac dilatation and ascites at both stocking densities than Rustic Gold, whereas the Rustic Gold birds showed higher prevalences of nephropathy and peritonitis. Age analysis showed lower occurrence of cardiovascular collapse and polyserositis in birds ≥28 days, but higher occurrence of leg disorders, renal disorders, and trauma in the same age group. Overall, from 7 days post-hatch to slaughter age, mortality and the distribution of causes of death in Rustic Gold broilers and Ross 308 at low densities were not significantly different from those in Ross 308 at high density.