Modern broiler chickens have been selectively bred to grow so rapidly that welfare problems are endemic, making broiler welfare one of the most significant issues in animal agriculture.
The selective breeding of broilers for extreme growth rate is one of the most significant welfare interventions in the history of animal agriculture, though not in a positive sense. The birds have been engineered to suffer: their bodies grow faster than their skeletal and cardiovascular systems can support, causing chronic pain and premature organ failure in a substantial minority. Even the majority who do not develop clinical pathology live in conditions of severe space restriction, artificial lighting, and barren environments. The scale — 70 billion individuals annually — makes this arguably the largest single welfare problem on earth.