Due to extensive breeding selection for rapid early growth and the husbandry used to sustain this, broilers are susceptible to several welfare concerns, particularly skeletal malformation and dysfunction, skin and eye lesions and congestive heart conditions. Management of ventilation, housing, stocking density and in-house procedures must be evaluated regularly to support good welfare of the flock. The breeding stock (broiler-breeders) do grow to maturity but also have their own welfare concerns related to the frustration of a high feeding motivation and beak trimming. Broilers are usually grown as mixed-sex flocks in large sheds under intensive conditions.
Before the development of modern commercial meat breeds, broilers were mostly young male chickens culled from farm flocks. Pedigree breeding began around 1916. Magazines for the poultry industry existed at this time. A crossbred variety of chicken was produced from a male of a naturally double-breasted Cornish strain, and a female of a tall, large-boned strain of white Plymouth Rocks. This first attempt at a meat crossbreed was introduced in the 1930s and became dominant in the 1960s. The original crossbreed was plagued by problems of low fertility, slow growth and disease susceptibility.
Modern broilers have become very different from the Cornish/Rock crossbreeds. As an example, Donald Shaver (originally a breeder of egg-production breeds) began gathering breeding stock for a broiler program in 1950. Besides the breeds normally favoured, Cornish Game, Plymouth Rock, New Hampshire, Langshans, Jersey Black Giant, and Brahmas were included. A white feathered female line was purchased from Cobb. A full-scale breeding program was commenced in 1958, with commercial shipments in Canada and the US in 1959 and in Europe in 1963. As a second example, colour sexing broilers was proposed by Shaver in 1973. The genetics were based on the company's breeding plan for egg layers, which had been developed in the mid-1960s. A difficulty facing the breeders of the colour-sexed broiler is that the chicken must be white-feathered by slaughter age. After 12 years, accurate colour sexing without compromising economic traits was achieved.
Modern commercial broilers, for example, Cornish crosses and Cornish-Rocks, are artificially selected and bred for large-scale, efficient meat production. They are noted for having very fast growth rates, a high feed conversion ratio, and low levels of activity. Modern commercial broilers are bred to reach a slaughter-weight of about 2 kg in only 5 to 7 weeks. As a consequence, the behaviour and physiology of broilers reared for meat are those of immature birds, rather than adults. Slow growing free-range and organic strains have been developed which reach slaughter-weight at 12 to 16 weeks of age.
Typical broilers have white feathers and yellowish skin. Recent genetic analysis has revealed that the gene for yellow skin was incorporated into domestic birds through hybridization with the grey junglefowl (G. sonneratii). Modern crosses are also favorable for meat production because they lack the typical "hair" which many breeds have that must be removed by singeing after plucking the carcass.
Chickens are omnivores and modern broilers are given access to a special diet of high protein feed, usually delivered via an automated feeding system. This is combined with artificial lighting conditions to stimulate eating and growth and thus the desired body weight.
In the U.S., the average feed conversion ratio (FCR) of a broiler was 1.91 kilograms of feed per kilograms of liveweight in 2011, an improvement from 4.70 in 1925. Canada has a typical FCR of 1.72. New Zealand commercial broiler farms have recorded the world's best broiler chicken FCR at 1.38.
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.
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% to 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.
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.
Compassion in World Farming wrote on the incidence of leg problems in broilers: .mw-parser-output .templatequoteoverflow:hidden;margin:1em 0;padding:0 40px.mw-parser-output .templatequote .templatequoteciteline-height:1.5em;text-align:left;padding-left:1.6em;margin-top:0
...there is evidence that, far from improving, leg problems may have deteriorated further during the 1990s. Large and representative surveys of commercial broiler flocks in Denmark (1999) and Sweden (2002) found that in Denmark, 75% of the chickens had some walking abnormality and 30.1% were very lame (gait score greater than 2). In Sweden, over 72% of the chickens had some walking abnormality and around 20% were very lame. 36.9% of the chickens surveyed in Denmark and around half (46.4% and 52.6%, depending on strain) of the chickens surveyed in Sweden had leg deformities (varus/valgus). 57% of the chickens surveyed in Denmark and around half of the chickens surveyed in Sweden showed some evidence of tibial dychondroplasia (Sanotra, Berg and Lund, 2003).
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.
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.
Once the broilers have reached the target live-weight, they are caught, usually by hand, and packed live into crates for transport to the slaughterhouse. They are usually deprived of food and water for several hours before catching until slaughter. The process of catching, loading, transport and unloading causes serious stress, injury and even death to a large number of broilers.
The number of broilers that died in the EU in 2005 during the process of catching, packing and transport was estimated to be as high as 18 to 35 million. In the UK, of broilers that were found to be 'dead on arrival' at the slaughterhouse in 2005, it was estimated that up to 40% may have died from thermal stress or suffocation due to crowding on the transporter. 041b061a72