Modern broiler chickens have been selectively bred for growth rates so extreme that skeletal, cardiovascular, and respiratory systems cannot keep pace, causing systemic suffering at industrial scale.
Fast-growth broiler genetics create a welfare crisis at the individual level: birds that grow so quickly that their hearts, lungs, and legs cannot support them. The majority of the 70+ billion broilers raised annually experience some degree of pain and incapacitation related to growth-related pathology. The existence of commercially viable slower-growing breeds makes this suffering avoidable.