Modern commercial broiler chickens have been selectively bred for extremely rapid growth, resulting in welfare problems from the mismatch between growth rate and musculoskeletal development.
Fast-growing broiler chickens experience welfare compromise directly caused by their extreme growth rates. Leg weakness and lameness are endemic in commercial flocks, with birds spending increasing proportions of their time lying down as they approach slaughter weight. Lameness causes chronic pain that affects behavior, feed intake, and general welfare. Skeletal abnormalities including tibial dyschondroplasia cause permanent structural weakness that compounds with the birds' heavy weight. Ascites causes fluid accumulation around the heart and lungs, leading to respiratory distress and death. The welfare problems of fast-growing broilers are inherent to the breeds themselves and cannot be fully managed away by changes in housing or management alone.