The breeding and raising of broiler chickens has created health and animal welfare issues, such as cardiovascular and skeletal dysfunction.
Meat birds
Artificial selection has led to a great increase in the speed with which broilers develop and reach slaughter-weight. The time required to reach 1.5 kg (3 lb 5 oz) live-weight decreased from 120 days to 30 days between 1925 and 2005. Selection for fast early growth-rate, and feeding and management procedures to support such growth, have led to various welfare problems in modern broiler strains. Welfare of broilers is of particular concern given the large number of individuals that are produced; for example, the U.S. in 2011 produced approximately 9 billion broiler chickens.
Cardiovascular dysfunction Selection and husbandry for very fast growth means there is a genetically induced mismatch between the energy-supplying organs of the broiler and its energy-consuming organs. Rapid growth can lead to metabolic disorders such as sudden death syndrome (SDS) and ascites. SDS is an acute heart failure disease that affects mainly male fast-growing broilers which appear to be in good condition. Affected birds suddenly start to flap their wings, lose their balance, sometimes cry out and then fall on their backs or sides and die, usually all within a minute. In 1993, U.K. broiler producers reported an incidence of 0.8%. In 2000, SDS has a death rate of 0.1–3% in Europe. Ascites is characterised by hypertrophy and dilatation of the heart, changes in liver function, pulmonary insufficiency, hypoxaemia and accumulation of large amounts of fluid in the abdominal cavity. Ascites develops gradually and the birds suffer for an extended period before they die. In the UK, up to 19 million broilers die in their sheds from heart failure each year.
Skeletal dysfunction Breeding for increased breast muscle means that the broilers' centre of gravity has moved forward and their breasts are broader compared with their ancestors, which affects the way they walk and puts additional stresses on their hips and legs. There is a high frequency of skeletal problems in broilers, mainly in the locomotory system, including varus and valgus deformities, osteodystrophy, dyschondroplasia and femoral head necrosis. These leg abnormalities impair the locomotor abilities of the birds, and lame birds spend more time lying and sleeping. The behavioural activities of broilers decrease rapidly from 14 days of age onwards. Reduced locomotion also decreases ossification of the bones and results in skeletal abnormalities; these are reduced when broilers have been exercised under experimental conditions. Most broilers find walking painful, as indicated by studies using analgesic and anti-inflammatory drugs. In one experiment, healthy birds took 11 seconds to negotiate an obstacle course, whereas lame birds took 34 seconds. After the birds had been treated with carprofen, there was no effect on the speed of the healthy birds, however, the lame birds now took only 18 seconds to negotiate the course, indicating that the pain of lameness is relieved by the drug. In self-selection experiments, lame birds select more drugged feed than non-lame birds leading to the suggestion that leg problems in broilers are painful. Several research groups have developed "gait scores" (GS) to objectively rank the walking ability and lameness of broilers. In one example of these scales, GS=0 indicates normal walking ability, GS=3 indicates an obvious gait abnormality which affects the bird's ability to move about and GS=5 indicates a bird that cannot walk at all. GS=5 birds tried to use their wings to help them walking, or crawled along on their shanks. In one study, almost 26% of the birds examined were rated as GS=3 or above and can therefore be considered to have suffered from painful lameness.
The video recordings below are examples of broilers attempting to walk with increasing levels of gait abnormalities and therefore increasing gait scores.
Integument lesions Sitting and lying behaviours in fast growing strains increase with age from 75% in the first seven days to 90% at 35 days of age. This increased inactivity is linked with an increase in dermatitis caused by a greater amount of time in contact with ammonia in the litter. This contact dermatitis is characterised by hyperkeratosis and necrosis of the epidermis at the affected sites; it can take forms such as hock burns, breast blisters and foot pad lesions.
Stocking density
Broilers are usually kept at high stocking densities which vary considerably between countries. Typical stocking densities in Europe range between about 22–42 kg/m2 (5–9 lb/sq ft) or between about 11 to 25 birds per square metre (1.0 to 2.3/sq ft). There is a reduction of feed intake and reduced growth rate when stocking density exceeds approximately 30 kg/m2 (6 lb/sq ft) under deep litter conditions. The reduced growth rate is likely due to a reduced capacity to lose heat generated by metabolism. Higher stocking densities are associated with increased dermatitis including foot pad lesions, breast blisters and soiled plumage. In a large-scale experiment with commercial farms, it was shown that the management conditions (litter quality, temperature and humidity) were more important than stocking density.
Ocular dysfunction In attempts to improve or maintain fast growth, broilers are kept under a range of lighting conditions. These include continuous light (fluorescent and incandescent), continuous darkness, or under dim light; chickens kept under these light conditions develop eye abnormalities such as macrophthalmos, avian glaucoma, ocular enlargement and shallow anterior chambers.
Ammonia The litter in broiler pens can become highly polluted from the nitrogenous feces of the birds and produce ammonia. Ammonia has been shown to cause increased susceptibility to disease and other health-related problems such as Newcastle disease, airsaculitis and keratoconjunctivitis. The respiratory epithelium in birds is damaged by ammonia concentrations in the air exceeding 75 parts per million (ppm). Ammonia concentrations at 25 to 50 ppm induce eye lesions in broiler chicks after seven days of exposure.
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