Poultry farming is the raising of domesticated birds such as chickens, turkeys, ducks, and geese, for the purpose of farming meat or eggs for food. Poultry are farmed in great numbers with chickens being the most numerous. More than 50 billion chickens are raised annually as a source of food, for both their meat and their eggs. Chickens raised for eggs are usually called layers whilst chickens raised for meat are often called broilers. In total, the UK alone consumes over 29 million eggs per day. In the US, the national organization overseeing poultry production is the Food and Drug Administration (FDA). In the UK, the national organisation is the Department for Environment, Food and Rural Affairs (Defra).
Intensive and alternative poultry farming 
According to the Worldwatch Institute, 74 percent of the world's poultry meat, and 68 percent of eggs are produced in ways that are described as 'intensive'. One alternative to intensive poultry farming is free-range farming, however, this method of husbandry also uses large flock sizes in high stocking densities. Friction between supporters of these two main methods of poultry farming has led to long-term issues of ethical consumerism. Opponents of intensive farming argue that it harms the environment and creates health risks, as well as abusing the animals. Advocates of intensive farming say that their highly efficient systems save land and food resources due to increased productivity, stating that the animals are looked after in state-of-the-art environmentally controlled facilities. The most intensive poultry farming methods are very efficient and allow meat and eggs to be available to the consumer in all seasons at a lower cost than free-range production. Poultry producers routinely use nationally approved medications, such as antibiotics, in feed or drinking water, to treat disease or to prevent disease outbreaks. Some FDA-approved medications are also approved for improved feed utilization.
Egg-laying chickens - husbandry systems 
Commercial hens usually begin laying eggs at 16–20 weeks of age, although production gradually declines soon after from approximately 25 weeks of age. This means that in many countries, by approximately 72 weeks of age, flocks are considered economically unviable and are slaughtered after approximately 12 months of egg production, although chickens will naturally live for 6 or more years. In some countries, hens are force moulted to re-invigorate egg-laying.
Environmental conditions are often automatically controlled in egg-laying systems. For example, the duration of the light phase is initially increased to prompt the beginning of egg-laying at 16–20 weeks of age and then mimics summer daylength which stimulates the hens to continue laying all year round; normally, egg production occurs only in the warmer months. Some commercial breeds of hen can produce over 300 eggs a year. Critics argue that year-round egg production stresses the birds more than normal seasonal production.
Free-range laying hens 
Free-range poultry farming allows the birds to roam freely for a period of the day, although they are usually confined in sheds at night to protect them from predators or kept indoors if the weather is particularly bad. In the UK, the Department for Environment, Food and Rural Affairs (Defra) states that a free-range chicken must have daytime access to open-air runs during at least half of its life. Unlike in the United States, this definition also applies to egg laying hens. The European Union regulates marketing standards for egg farming which specifies a minimum condition for free-range eggs that "hens have continuous daytime access to open-air runs, except in the case of temporary restrictions imposed by veterinary authorities". The RSPCA "Welfare standards for laying hens and pullets" indicates that the stocking rate must not exceed 1,000 birds per hectare (10 m2 per hen) of range available and a minimum area of overhead shade/shelter of 8 m2 per 1,000 hens must be provided.
Free-range farming of egg-laying hens is increasing its share of the market. Defra figures indicate that 45% of eggs produced in the UK throughout 2010 were free-range, 5% were produced in barn systems and 50% from cages. This compares with 41% being free-range in 2009.
Finding suitable land with adequate drainage to minimise worms and coccidial oocysts, suitable protection from prevailing winds, good ventilation, access and protection from predators can be difficult. Excess heat, cold or damp can have a harmful effect on the animals and their productivity. Unlike cage and barn systems, free-range farmers have little control over the food their animals come across, which can lead to unreliable productivity. In some farms, the manure from free-range poultry can be used to benefit crops.
The benefits of free-range poultry farming for laying hens include opportunities for natural behaviours such as pecking, scratching, foraging and exercise outdoors.
Free-range farming has come under criticism concerning animal welfare. Cannibalism, feather pecking and vent pecking, although not limited to free-range systems, can be common requiring beak trimming as a preventative measure. Diseases can be common and the animals are vulnerable to predators. In South-East Asia, a lack of disease control in free range farming has been associated with outbreaks of Avian influenza.
Organic laying hens 
In organic egg-laying systems, chickens are also free-range. Organic systems are based upon restrictions on the routine use of synthetic yolk colourants, in-feed or in-water medications, other food additives and synthetic amino acids, and a lower stocking density and smaller group sizes. The Soil Association standards used to certify organic flocks in the UK, indicate a maximum outdoors stocking density of 1,000 birds per hectare and a maximum of 2,000 hens in each poultry house. In the UK, organic laying hens are not routinely beak-trimmed.
Yarding for laying hens 
While often confused with free-range farming, yarding is actually a separate method of poultry culture by which chickens and cows are raised together. The distinction is that free-range poultry are either totally unfenced, or the fence is so distant that it has little influence on their freedom of movement. Yarding is common technique used by small farms in the Northeastern US. The birds are released daily from hutches or coops. The hens usually lay eggs either on the floor of the coop or in baskets if provided by the farmer. This husbandry technique can be complicated if used with roosters, mostly because of aggressive behaviour.
Battery cages for laying hens 
The majority of hens in many countries are reared in battery cages, although the European Union Council Directive 1999/74/EC has banned the conventional battery cage in EU states from January 2012. These are small cages, usually made of metal in modern systems, housing 3 to 8 hens. The walls are made of either solid metal or mesh, and the floor is sloped wire mesh to allow the faeces to drop through and eggs to roll onto an egg-collecting conveyor belt. Water is usually provided by overhead nipple systems, and food in a trough along the front of the cage replenished at regular intervals by a mechanical chain. The cages are arranged in long rows as multiple tiers, often with cages back-to-back (hence the term 'battery cage'). Within a single shed, there may be several floors containg battery cages meaning that a single shed may contain many tens of thousands of hens. Light intensity is often kept low (e.g. 10 lux) to reduce feather pecking and vent pecking. Benefits of battery cages include easier care for the birds, floor eggs which are expensive to collect are eliminated, eggs are cleaner, capture at the end of lay is expedited, generally less feed is required to produce eggs, broodiness is eliminated, more hens may be housed in a given house floor space, internal parasites are more easily treated, and labor requirements are generally much reduced.
In farms using cages for egg production, there are more birds per unit area; this allows for greater productivity and lower food costs. Floor space ranges upwards from 300 cm2 per hen. EU standards in 2003 called for at least 550 cm2 per hen. In the US, the current recommendation by the United Egg Producers is 67 to 86 in2 (430 to 560 cm2) per bird. The space available to battery hens has often been described as less than the size of a piece of A4 paper. Animal welfare scientists have been critical of battery cages because they do not provide hens with sufficient space to stand, walk, flap their wings, perch, or make a nest, and it is widely considered that hens suffer through boredom and frustration through being unable to perform these behaviours. This can lead to a wide range of abnormal behaviours, some of which are injurious to the hens or their cagemates.
Furnished cages for laying hens 
In 1999, the European Union Council Directive 1999/74/EC banned conventional battery cages for laying hens throughout the European Union from January 1, 2012; they were banned previously in other countries including Switzerland. In response to these bans, development of prototype commercial furnished cage systems began in the 1980s. Furnished cages, sometimes called ‘enriched’ or ‘modified’ cages, are cages for egg laying hens which have been designed to overcome some of the welfare concerns of battery cages whilst retaining their economic and husbandry advantages, and also provide some of the welfare advantages of non-cage systems. Many design features of furnished cages have been incorporated because research in animal welfare science has shown them to be of benefit to the hens. In the UK, the Defra "Code for the Welfare of Laying Hens" states furnished cages should provide at least 750 cm2 of cage area per hen, 600 cm2 of which should be usable; the height of the cage other than that above the usable area should be at least 20 cm at every point and no cage should have a total area that is less than 2000 cm2. In addition, furnished cages should provide a nest, litter such that pecking and scratching are possible, appropriate perches allowing at least 15 cm per hen, a claw-shortening device, and a feed trough which may be used without restriction providing 12 cm per hen.
Modern egg laying breeds often suffer from osteoporosis which results in the chicken's skeletal system being weakened. During egg production, large amounts of calcium are transferred from bones to create egg-shell. Although dietary calcium levels are adequate, absorption of dietary calcium is not always sufficient, given the intensity of production, to fully replenish bone calcium. This can lead to increases in bone breakages, particularly when the hens are being removed from cages at the end of laying.
Meat-producing chickens - husbandry systems 
Indoor broilers 
Meat chickens, commonly called broilers, are floor-raised on litter such as wood shavings or rice hulls, indoors in climate-controlled housing. Under modern farming methods, meat chickens reared indoors reach slaughter weight at 5 to 6 weeks of age.
Broilers are not raised in cages. They are raised in large, open structures known as growout houses. These houses are equipped with mechanical systems to deliver feed and water to the birds. They have ventilation systems and heaters that function as needed. The floor of the house is covered with bedding material consisting of wood chips, rice hulls, or peanut shells. Because dry bedding helps maintain flock health, most growout houses have enclosed watering systems (“nipple drinkers”) which reduce spillage.
Keeping birds inside a house protects them from predators such as hawks and foxes. Some houses are equipped with curtain walls, which can be rolled up in good weather to admit natural light and fresh air. Most growout houses built in recent years feature “tunnel ventilation,” in which a bank of fans draws fresh air through the house.
Traditionally, a flock of broilers consist of about 20,000 birds in a growout house that measures 400 feet long and 40 feet wide, thus providing about eight-tenths of a square foot per bird. The Council for Agricultural Science and Technology (CAST) states that the minimum space is one-half square foot per bird. More modern houses are often larger and contain more birds, but the floor space allotment still meets the needs of the birds.
Because broilers are relatively young and have not reached sexual maturity, they exhibit very little aggressive conduct.
Issues with indoor husbandry 
In intensive broiler sheds, the air can become highly polluted with ammonia from the droppings. This can damage the chickens’ eyes and respiratory systems and can cause painful burns on their legs (called hock burns) and feet. Broilers bred for fast growth have a high rate of leg deformities because the large breast muscles causes distortions of the developing legs and pelvis, and the birds cannot support their increased body weight. Because they cannot move easily, the chickens are not able to adjust their environment to avoid heat, cold or dirt as they would in natural conditions. The added weight and overcrowding also puts a strain on their hearts and lungs and Ascites can develop. In the UK, up to 19 million broilers die in their sheds from heart failure each year.
Indoor with higher welfare 
Chickens are kept indoors but with more space (around 12 to 14 birds per square metre). They have a richer environment for example with natural light or straw bales that encourage foraging and perching. The chickens grow more slowly and live for up to two weeks longer than intensively farmed birds. The benefits of higher welfare indoor systems are the reduced growth rate, less crowding and more opportunities for natural behaviour.
Free-range broilers 
Free-range broilers are reared under similar conditions to free-range egg laying hens. The breeds grow more slowly than those used for indoor rearing and usually reach slaughter weight at approximately 8 weeks of age. In the EU, each chicken must have one square metre of outdoor space. The benefits of free-range poultry farming include opportunities for natural behaviours such as pecking, scratching, foraging and exercise outdoors. Because they grow slower and have opportunities for exercise, free-range broilers often have better leg and heart health.
Organic broilers 
Organic broiler chickens are reared under similar conditions to free-range broilers but with restrictions on the routine use of in-feed or in-water medications, other food additives and synthetic amino acids. The breeds used are slower growing, more traditional breeds and typically reach slaughter weight at around 12 weeks of age. They have a larger space allowance outside (at least 2 square metres and sometimes up to 10 square metres per bird). The Soil Association standards indicate a maximum outdoors stocking density of 2,500 birds per hectare and a maximum of 1,000 broilers per poultry house.
Issues with poultry farming 
Humane treatment 
Animal welfare groups have frequently criticized the poultry industry for engaging in practices which they believe to be inhumane. Many animal rights advocates object to killing chickens for food, the "factory farm conditions" under which they are raised, methods of transport, and slaughter. Compassion Over Killing and other groups have repeatedly conducted undercover investigations at chicken farms and slaughterhouses which they allege confirm their claims of cruelty. Conditions in chicken farms may be unsanitary, allowing the proliferation of diseases such as salmonella and E. coli. Chickens may be raised in very low light intensities, sometimes total darkness, to reduce injurious pecking. Concerns have been raised that companies growing single varieties of birds for eggs or meat are increasing their susceptibility to disease. Rough handling, crowded transport during various weather conditions and the failure of existing stunning systems to render the birds unconscious before slaughter, have also been cited as welfare concerns.
Another welfare concern for broilers is the use of selective breeding to create heavy, large-breasted birds, which can lead to crippling leg disorders and heart failure for some of the birds.
A common practice among hatcheries for egg-laying hens is the culling of newly-hatched male chicks since they don't lay eggs and do not grow fast enough to be profitable for meat.
Beak trimming 
Laying hens are routinely beak-trimmed at 1 day of age to reduce the damaging effects of aggression, feather pecking and cannibalism. Scientific studies (see below) have shown that beak trimming is likely to cause both acute and chronic pain.
The beak is a complex, functional organ with an extensive nervous supply including nociceptors that sense pain and noxious stimuli. These would almost certainly be stimulated during beak trimming, indicating strongly that acute pain would be experienced. Behavioural evidence of pain after beak trimming in layer hen chicks has been based on the observed reduction in pecking behavior, reduced activity and social behavior, and increased sleep duration. Severe beak trimming, or beak trimming birds at an older age, may cause chronic pain. Following beak trimming of older or adult hens, the nociceptors in the beak stump show abnormal patterns of neural discharge, which indicate acute pain. Neuromas, tangled masses of swollen regenerating axon sprouts, are found in the healed stumps of birds beak trimmed at 5 weeks of age or older and in severely beak trimmed birds. Neuromas have been associated with phantom pain in human amputees and have therefore been linked to chronic pain in beak trimmed birds. If beak trimming is severe because of improper procedure or done in older birds, the neuromas will persist which suggests that beak trimmed older birds experience chronic pain, although this has been debated.
Beak-trimmed chicks will initially peck less than non-trimmed chickens, which animal behavioralist Temple Grandin attributes to guarding against pain. The animal rights activist, Peter Singer, claims this procedure is bad because beaks are sensitive, and the usual practice of trimming them without anaesthesia is considered inhumane by some. Some within the chicken industry claim that beak-trimming is not painful whereas others argue that the procedure causes chronic pain and discomfort, and decreases the ability to eat or drink.
Antibiotics have been used on poultry in large quantities since the 1940s, when it was found that the byproducts of antibiotic production, fed because the antibiotic-producing mold had a high level of vitamin B12 after the antibiotics were removed, produced higher growth than could be accounted for by the vitamin B12 alone. Eventually it was discovered that the trace amounts of antibiotics remaining in the byproducts accounted for this growth.
The mechanism is apparently the adjustment of intestinal flora, favoring "good" bacteria while suppressing "bad" bacteria that provoke inflammation of the gut mucosa. So, the goal of antibiotics as a growth promoter is the same as for probiotics. Because the antibiotics used are not absorbed by the gut, they do not put antibiotics into the meat or eggs.
Antibiotics are used routinely in poultry for this reason, and also to prevent and treat disease. Many contend that this puts humans at risk as bacterial strains develop stronger and stronger resistances. Critics point out that, after six decades of heavy agricultural use of antibiotics, opponents of antibiotics must still make arguments about theoretical risks, since actual examples are hard to come by. All antibiotic-resistant strains of human diseases whose origin is known originated in hospitals rather than farms.
A proposed bill in the United States Congress would make the use of antibiotics in animal feed legal only for therapeutic (rather than preventative) use, but it has not been passed. However, this may present the risk of slaughtered chickens harboring pathogenic bacteria and passing them on to humans that consume them.
In October 2000, the U.S. Food and Drug Administration (FDA) discovered that two antibiotics were no longer effective in treating diseases found in factory-farmed chickens; one antibiotic was swiftly pulled from the market, but the other, Baytril, was not. Bayer, the company which produced it, contested the claim and as a result, Baytril remained in use until July 2005.
To prevent any residues of antibiotics in chicken meat, any given antibiotics are required to have a "withdrawal" period before they can be slaughtered. Samples of poultry at slaughter are randomly tested by the FSIS, and shows a very low percentage of residue violations 
Poultry feed can also include roxarsone or nitarsone, arsenical antimicrobial drugs that also promote growth. Roxarsone was used as a broiler starter by about 70% of the broiler growers between 1995 to 2000. The drugs have generated controversy because it contains arsenic, which is highly toxic to humans. This arsenic could be transmitted through run-off from the poultry yards. A 2004 study by the U.S. magazine Consumer Reports reported "no detectable arsenic in our samples of muscle" but found "A few of our chicken-liver samples has an amount that according to EPA standards could cause neurological problems in a child who ate 2 ounces of cooked liver per week or in an adult who ate 5.5 ounces per week." The U.S. Food and Drug Administration (FDA), however, is the organization responsible for the regulation of foods in America, and all samples tested were "far less than the... amount allowed in a food product."
Growth hormones 
Hormone use in poultry production is illegal in the United States. Similarly, no chicken meat for sale in Australia is fed hormones. Several scientific studies have documented the fact that chickens grow rapidly because they are bred to do so, not because of growth hormones. A small producer of natural and organic chickens confirmed this assumption:
|“||Using hormones to boost egg production was a brief fad in the Forties, but was abandoned because it didn't work. Using hormones to produce soft-meated roasters lasted into the Fifties, but the improved growth rates of normal, untreated broilers made the practice irrelevant--the broilers got as big as anyone wanted without chemicals. The only hormone that was ever used in any quantity on poultry (DES) was banned in 1959, and everyone but a few die-hard farmers had given up hormones by then, anyway. Hormones are now illegal in poultry and eggs.||”|
E. coli 
According to Consumer Reports, "1.1 million or more Americans [are] sickened each year by undercooked, tainted chicken." A USDA study discovered E. coli (Biotype I) in 99% of supermarket chicken, the result of chicken butchering not being a sterile process. However, the same study also shows that the strain of E. coli found was always a non-lethal form, and no chicken had any of the pathenogenic O157:H7 serotype. Many of these chickens, furthermore, had relatively low levels of contamination. Feces tend to leak from the carcass until the evisceration stage, and the evisceration stage itself gives an opportunity for the interior of the carcass to receive intestinal bacteria. (So does the skin of the carcass, but the skin presents a better barrier to bacteria and reaches higher temperatures during cooking). Before 1950, this was contained largely by not eviscerating the carcass at the time of butchering, deferring this until the time of retail sale or in the home. This gave the intestinal bacteria less opportunity to colonize the edible meat. The development of the "ready-to-cook broiler" in the 1950s added convenience while introducing risk, under the assumption that end-to-end refrigeration and thorough cooking would provide adequate protection. E. coli can be killed by proper cooking times, but there is still some risk associated with it, and its near-ubiquity in commercially farmed chicken is troubling to some. Irradiation has been proposed as a means of sterilizing chicken meat after butchering.
Avian influenza 
There is also a risk that crowded conditions in chicken farms will allow avian influenza (bird flu) to spread quickly. A United Nations press release states: "Governments, local authorities and international agencies need to take a greatly increased role in combating the role of factory-farming, commerce in live poultry, and wildlife markets which provide ideal conditions for the virus to spread and mutate into a more dangerous form..."
Farming of chickens on an industrial scale relies largely on high protein feeds derived from soyabeans; in the European Union the soybean dominates the protein supply for animal feed, and the poultry industry is the largest consumer of such feed. Two kilograms of grain must be fed to poultry to produce 1 kg of weight gain, much less than that required for pork or beef. However, for every gram of protein consumed, chickens yield only 0.33 g of edible protein however the protein produced by the chicken is a complete protein containing all the essential amino acids unlike the protein that has been feed to the chickens.
Economic factors 
Changes in commodity prices for poultry feed have a direct effect on the cost of doing business in the poultry industry. For instance, a significant rise in the price of corn in the United States can put significant economic pressure on large industrial chicken farming operations.
World chicken population 
The Food and Agriculture Organization of the United Nations estimated that in 2002 there were nearly sixteen billion chickens in the world, counting a total population of 15,853,900,000. The figures from the Global Livestock Production and Health Atlas for 2004 were as follows:
- China (3,860,000,000)
- United States (1,970,000,000)
- Indonesia (1,200,000,000)
- Brazil (1,100,000,000)
- India (648,830,000)
- Mexico (540,000,000)
- Russia (340,000,000)
- Japan (286,000,000)
- Iran (280,000,000)
- Turkey (250,000,000)
- Bangladesh (172,630,000)
- Nigeria (143,500,000)
In 2009 the annual chicken population in factory farms was estimated at 50 billion. With 6 billion raised in the European Union, over 9 billion raised in the United States and more than 7 billion in China.
See also 
- Abnormal behaviour of birds in captivity
- Controlled-atmosphere killing (CAK)
- Environmental issues with agriculture
- European Union Council Directive 1999/74/EC
- Poultry farming in the United States
- "Compassion in World Farming - Poultry". Ciwf.org.uk. Retrieved 2011-08-26.
- State of the World 2006 Worldwatch Institute, p. 26
- Avery, Dennis. "Big Hog Farms Help the Environment," Des Moines Register, December 7, 1997, cited in Scully, Matthew. Dominion, St. Martin's Griffin, p. 30.
- "Performance Records of Hy-Line Grey|Accessdate 18 November 2011".
- "Compassion in World Farming - Egg laying hens". Ciwf.org.uk. Retrieved 2011-08-26.
- "European Union Regulation for marketing standards for eggs - page 25". Retrieved 2011-08-26.
- DEFRA The welfare of hens in free range systems retrieved July 6, 2007
- Chicken feed: Grass-Fed Chickens & Pastured Poultry retrieved July 6, 2007
- "Compassion in World Farming - Poultry - Higher welfare alternatives". Ciwf.org.uk. Retrieved 2011-08-26.
- Sherwin, C.M., Richards, G.J and Nicol, C.J. 2010. A comparison of the welfare of layer hens in four housing systems in the UK. British Poultry Science, 51(4): 488-499
- WSPA International 'Free-range farming and avian flu in Asia retrieved July 6, 2007
- "Soil Association Standards". Retrieved 5 December 2011.
- "European Union Council Directive 1999/74/EC". Retrieved 15 November 2011.
- VEGA Laying hens, free range and bird flu retrieved July 6, 2007
- Chickens: Layer Housing, Michael C. Appleby, Encyclopedia of Animal Science. doi:10.1081/E-EAS-120019534
- Housing, space, feed and water United Egg Producers
- "Animal Pragmatism: Compassion Over Killing Wants to Make the Anti-Meat Message a Little More Palatable". Washington Post. 2003-09-03. Retrieved 2009-07-30.
- Appleby, M.C.; J.A. Mench and B.O. Hughes (2004). Poultry Behaviour and Welfare. Wallingford and Cambridge MA: CABI Publishing. ISBN 0-85199-667-1.
- "Defra Code For The Welfare Of Laying Hens|Accessdate 5 December2011".
- "Animal Welfare For Broiler Chickens". National Chicken Council. Retrieved June 21, 2012.
- "Poultry Industry Frequently Asked Questions". U.S Poultry & Egg Association. Retrieved June 21, 2012.
- "Compassion in World Farming - Meat chickens - Welfare issues". Ciwf.org.uk. Retrieved 2011-08-26.
- "Compassion in World Farming - Meat chickens". Ciwf.org.uk. Retrieved 2011-08-26.
- "Undercover Investigations :: Compassion Over Killing Investigation". Kentucky Fried Cruelty. Retrieved 2011-08-26.
- Breward, J., (1984). Cutaneous nociceptors in the chicken beak. Proceedings of the Journal of Physiology, London 346: 56
- Gentle, M.J., (1992). Pain in birds. Animal Welfare, 1: 235-247
- Gentle M.J., Hughes B.O. and Hubrecht R.C., (1982). The effect of beak-trimming on food-intake, feeding behaviour and body weight in adult hens. Applied Animal Ethology, 8: 147–157
- Duncan I.J.H., Slee G.S., Seawright E. and Breward J., (1989). Behavioural consequences of partial beak amputation (beak trimming) in poultry. British Poultry Science, 30: 479–488
- Gentle M.J., Hunter L.N. and Waddington D., (1991). The onset of pain related behaviours following partial beak amputation in the chicken. Neuroscience Letters, 128: 113–116
- Gentle, M.J., Hughes, B.O., Fox, A. and Waddington, D., (1997). Behavioural and anatomical consequences of two beak trimming methods in 1- and 10-d-old domestic chicks. British Poultry Science, 38: 453-463
- Breward, J., (1985). An Electrophysiological Investigation of the Effects of Beak Trimming in the Domestic Fowl (Gallus gallus domesticus). Ph.D. thesis, University of Edinburgh.
- Gentle, M.J., (1986). Beak trimming in poultry. World’s Poultry Science Journal', 42: 268-275
- Breward, L. and Gentle, M.J., (1985). Neuroma formation and abnormal afferent nerve discharges after partial break amputation (beak trimming) in poultry. Experientia, 41: 1132-1134. doi:10.1007/BF01951693
- Devor, M. and Rappaport, Z.H., (1990). Pain Syndromes in Neurology., edited by H.L. Fields, Butterworths, London, p. 47.
- Lunam, C.A., Glatz, P.C. and Hsu, Y-J., (1996). The absence of neuromas in beaks of adult hens after conservative trimming at hatch. Australian Veterinary Journal, 74: 46-49
- Kuenzel, W.J. (2001). Neurobiological basis of sensory perception: welfare implications of beak trimming. Poultry Science, 86: 1273-1282
- Grandin, Temple; Johnson, Catherine (2005). Animals in Translation. New York, NY: Scribner. p. 183. ISBN 0-7432-4769-8.
- Singer, Peter (2006). In Defense of Animals. Wiley-Blackwell. p. 176. ISBN 1-4051-1941-1.
- Hernandez, Nelson (2005-09-19). "Advocates Challenge Humane-Care Label on Md. Eggs". Washington Post. Retrieved 2009-07-30.
- "Md. Egg Farm Accused of Cruelty". Washington Post. 2001-06-06. Retrieved 2009-07-30.
- Ewing, Poultry Nutrition, 5th ed., 1963, p. 1283.
- Ewing, Poultry Nutrition, 5th ed., 1963, p. 1284.
- "UNL.edu". Retrieved 2011-08-26.
- "Chicken: Arsenic and antibiotics". ConsumerReports.org. Retrieved 2009-03-24.
- Baytril: FDA Bans Bayer Antibiotic for Poultry Use Randy Fabi / Reuters, July 29, 2005[dead link]
- "Chicken from Farm to Table | USDA Food Safety and Inspection Service". Fsis.usda.gov. 2011-04-06. Retrieved 2011-08-26.
- Jones, F. T. (2007). "A Broad View of Arsenic". Poultry Science 86 (1): 2–14. PMID 17179408.
- "The Use Of Steroid Hormones For Growth Promotion In Food-Producing Animals"
- "Chicken from Farm to Table | USDA Food Safety and Inspection Service". Fsis.usda.gov. 2011-04-06. Retrieved 2011-08-26.
- "Landline - 5/05/2002: Challenging food safety myths . Australian Broadcasting Corp". Abc.net.au. 2002-05-05. Retrieved 2011-08-26.
- Havenstein GB, Ferket PR, Qureshi MA (October 2003). "Carcass composition and yield of 1957 versus 2001 broilers when fed representative 1957 and 2001 broiler diets". Poult. Sci. 82 (10): 1509–18. PMID 14601726.
- Havenstein GB, Ferket PR, Scheideler SE, Rives DV (December 1994). "Carcass composition and yield of 1991 vs 1957 broilers when fed "typical" 1957 and 1991 broiler diets". Poult. Sci. 73 (12): 1795–804. doi:10.3382/ps.0731795. PMID 7877935.
- Robert Plamondon. "Chicken Myths and Scams". Plamondon.com. Retrieved 2008-11-24.
- "Nationwide Broiler Chicken Microbiological Baseline Data Collection Program July 1994 - June 1995". Retrieved 6 November 2012.
- "Revised Young Chicken Baseline" (PDF). Retrieved 2011-08-26.
- "UN task forces battle misconceptions of avian flu, mount Indonesian campaign". UN News Center. Retrieved 24 July 2009.
- "Protein Sources For The Animal Feed Industry". Fao.org. 2002-05-03. Retrieved 2011-08-26.
- Lester R. Brown (2003). "Chapter 8. Raising Land Productivity: Raising protein efficiency". Plan B: Rescuing a Planet Under Stress and a Civilization in Trouble. NY: W.W. Norton & Co. ISBN 0-393-05859-X.
- Adler, Jerry; Lawler, Andrew (2012-06). "How the Chicken Conquered the World". Smithsonian. Retrieved May 27, 2012.
- Tom Lovell (1998). Nutrition and feeding of fish. Springer. p. 9. ISBN 978-0-412-07701-2.
- Jonathan Starkey (9 April 2011). "Delaware business: Chicken companies feeling pinch as corn prices soar". News Journal (Gannett). DelawareOnline. OCLC 38962480. Retrieved 10 April 2011
- "Chicken population". Fao.org. Retrieved 2011-08-26.
- Foer, Jonathan Safran (2009). "Eating Animals", Page 136. Little, Brown and Company, USA. ISBN 978-0-316-06990-8