|
Broiler chickens (''Gallus gallus domesticus''), or broilers, are a gallinaceous domesticated fowl, bred and raised specifically for meat production. They are a hybrid of the egg-laying chicken, both being a subspecies of the red junglefowl (''Gallus gallus''). Typical broilers have white feathers and yellowish skin. Most commercial broilers reach slaughter-weight at between five to seven weeks of age, although slower growing breeds reach slaughter-weight at approximately 14 weeks of age. Because the meat broilers are this young at slaughter, their behaviour and physiology are that of an immature bird. Due to artificial 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. The breeding stock (broiler-breeders) grow to maturity and beyond but also have welfare issues related to frustration of a high feeding motivation and beak trimming. Broilers are usually grown as mixed-sex flocks in large sheds under intensive conditions, but some breeds can be grown as free-range flocks. Chickens are one of the most common and widespread domestic animals, and with a population of 19 billion in 2011, there are more chickens in the world than any other species of bird. ==Origins and domestication== The domestic chicken is descended primarily from the red junglefowl (''Gallus gallus'') and is classified as a sub-species (''Gallus gallus domesticus'') of that species. As such, it can and does freely interbreed with populations of red jungle fowl.〔(A genetic variation map for chicken with 2.8 million single-nucleotide polymorphisms ). International Chicken Polymorphism Map Consortium (GK Wong et al.) 2004. ''Nature'' 432, 717-722| PMID 15592405〕 The traditional poultry farming view is stated in ''Encyclopædia Britannica'' (2007): "Humans first domesticated chickens of Indian origin for the purpose of cockfighting in Asia, Africa, and Europe. Very little formal attention was given to egg or meat production... ". Genetic studies point to multiple maternal origins, with the clade found in the Americas, Europe, Middle East, and Africa, originating from the Indian subcontinent, where a large number of unique haplotypes occur. It is postulated that the jungle fowl, known as the "bamboo fowl" in many Southeast Asian languages, is a pheasant well adapted to take advantage of the large amounts of fruits that are produced during the end of the 50 year bamboo seeding cycle to boost its own reproduction. It has been suggested that in domesticating the chicken, humans took advantage of this prolific reproduction of the jungle fowl when exposed to large amount of food. Similarly, it is speculated that the flexibility and adaptability inherited from the chicken's red junglefowl ancestor allows them to cope with the "unnatural and intense conditions" of modern production.〔 Recent genetic analysis has revealed that at least the gene for yellow skin was incorporated into domestic birds through hybridization with the grey junglefowl (''G. sonneratii'').〔Eriksson J, Larson G, Gunnarsson U, Bed'hom B, Tixier-Boichard M, ''et al.'' (2008) ''Identification of the Yellow Skin Gene Reveals a Hybrid Origin of the Domestic Chicken.'' PLoS Genet January 23, 2008 (Genetics.plosjournals.org )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Broiler」の詳細全文を読む スポンサード リンク
|