Summary
This review examines the welfare issues arising from the decades-long genetic selection of chickens raised for meat (i.e., broiler chickens). To reduce costs and increase profits, broilers have been increasingly bred for rapid growth while simultaneously requiring fewer resources (e.g., feed).
Broiler chickens are at a higher risk of cardiovascular disease due to a variety of risk factors resulting from their rapid growth, body composition (i.e., high muscle to bone ratio) and food intake. In particular, they need more oxygen, have disproportionately small hearts and lungs, and insufficient blood oxygen levels.These factors can contribute to ascites (accumulation of large amounts of fluid in the abdominal cavity) and sudden death syndrome.
Regarding musculoskeletal disorders, broilers most commonly experience
reduced walking ability associated with pain, increased risk of injury, and difficulty accessing food and water and performing natural behaviors. Bacterial chondronecrosis with osteomyelitis (BCO), brought about by a bacterial infection of the bone, is reportedly the most common cause of severe leg disorders in broilers and shows a genetic predisposition. There also appears to be a genetic link for bone deformities. Contact dermatitis, inflammation or lesions on the skin, is associated with broiler genotype and reduced mobility. These lesions can be painful and increase the risk of infection and limb problems.
Finally, because higher body weight decreases reproductive efficiency, hens that produce eggs are raised differently than broiler chickens raised meat. Specifically, breeder hens are subjected to long-term and severe feed restrictions to reduce their high rates of mortality and mobility issues.
The review also discusses consumers’ growing awareness of farmed animal welfare and the sustainability of the industry.
The genetic selection of broilers over the past 60 years has focused narrowly and intensely on production traits, namely growth rate and feed efficiency. This has led to significant welfare problems in birds grown for meat, including leg disorders, cardiovascular diseases, and resulting high mortality rates, while the breeder birds are subjected to severe feed restriction. Bone problems such as bacterial chondronecrosis and tibia dyschondroplasia are prevalent, and recent studies have reported the prevalence of birds with moderate to severe gait impairment to be between 5.5 and 48.8%. Worldwide, over 66 billion broilers are slaughtered annually. This huge scale of meat chicken production means that welfare problems are widespread and are likely to increase in severity due to the increasing global human population, increasing demand for meat, and a continued focus on efficiency of production in the agricultural sector. The commercial broiler industry therefore represents some of the most serious animal welfare issues in agriculture. There is an urgent need to address these problems by making welfare traits high priorities in breeding programmes and integrating these with other breeding goals. Many studies recommend the use of slower-growing breeds that do not have the same welfare problems. Addressing these welfare issues is essential to improve bird welfare and for social acceptability and sustainability of the broiler industry worldwide.