Conventional chickens gain weight faster due to their genetic predisposition. This weight is causing them to suffer for various reasons. The effect of genetic selection for high muscle-to-bone ratio and high caloric intake is to cause significant mortality through the risk of vascular disease1. In addition, rapid growth caused by high nutrient intake can lead to severe lameness, skeletal deformities, and bone disorders.
One of the most serious welfare problems in broiler production is the high incidence of skeletal disorders, particularly those leading to reduced mobility or lameness2. The development of many of these problems is caused by selection for rapid growth. These disorders are rarely seen in higher welfare chicken breeds, but are very common in fast growing commercial broilers such as the Ross 5003. Skeletal disorders are not only welfare problems; moreover, they are costly for the industry. They are by far the most costly diseases for poultry producers in terms of lost production, wasted resources, treatment and prevention costs3. The development of large muscle mass on an immature skeleton leads to locomotor problems in fast-growing chickens4, (highest levels of lameness seen in the fastest growing birds5).
Poor leg health can be caused by a number of different factors, including bacterial chondronecrosis and osteomyelitis (BCO), also known as femoral head necrosis5. In severe cases, birds lose the ability to walk so that they cannot even access basic food and water supplies, which can lead to starvation and slow, painful death if not killed. Broilers are usually reared in barns with a stocking density of about 39 – 42 kg/m² (19.5 – 21 birds/m² when killed at 2 kg). Such high stocking densities adversely affect their walking ability6. Fast-growing broilers with poor foot health spend a greater amount of time shedding and long periods spent on poor litter, which can lead to the painful condition foot dermatitis (inflammation and necrotic lesions on the plantar foot) as well as foot litter burns and blisters on the breast7. The ability to walk is absolutely necessary to avoid pecking from other aggressive birds in the flock1.