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Debeaking, also called beak trimming, is the partial removal of the beak of poultry, especially layer hens and turkeys although it may also be performed on quail and ducks. Most commonly, the beak is shortened permanently, although regrowth can occur. The trimmed lower beak is somewhat longer than the upper beak. USA's UEP guidelines suggest that in egg laying strains of chickens, the length of the upper beak distal from the nostrils which remains following trimming, should be 2 to 3 mm. In the UK, the Farm Animal Welfare Council stated: "The accepted procedure is to remove not more than one third of the upper and lower beaks or not more than one third of the upper beak only" but went on to recommend: "Where beak trimming is carried out, it should, wherever possible, be restricted to beak tipping; that is the blunting of the beak to remove the sharp point which can be the cause of the most severe damage to other birds." The term "debeaking" implies that the entire beak is removed during the trimming process, though in reality only half or less of the beak is removed.
Beak trimming is most common in egg-laying strains of chickens. In some countries such as the United States, turkeys are routinely beak trimmed, however, in the UK, only 10% of turkeys are beak trimmed. Beak trimming is a preventive measure to reduce damage caused by injurious pecking such as cannibalism, feather pecking and vent pecking, and thereby improve livability. Commercial broiler chickens are not routinely beak trimmed as they reach slaughter weight at approximately 6 weeks of age, i.e. before injurious pecking usually begins. However, broiler breeding stock may be trimmed to prevent damage during mating. In some countries, beak trimming is done as a last resort where alternatives are considered not to be possible or appropriate. Beak trimming has been banned in Switzerland since 1992.
In close confinement, cannibalism, feather pecking and aggression are common among turkeys, ducks, pheasants, quail, and egg laying strains of chickens of many breeds (including both heritage breeds and modern hybrids). The tendency to cannibalism and feather pecking varies among different strains of chickens, but does not manifest itself consistently. Some flocks of the same breed may be entirely free from cannibalism, while others, under the same management, may have a serious outbreak. Mortalities mainly due to cannibalism can be up to 15% in egg laying flocks housed in aviaries, straw yards, and free-range systems.
Because egg laying strains of chickens can be kept in smaller group sizes in caged systems, cannibalism is reduced  leading to a lowered trend in mortality as compared to non-cage systems. Cannibalism among flocks is highly variable and when it is not problematic, then mortalities among production systems are similar. 
Beak trimming was developed at the Ohio Experiment Station in the 1930s. The original technique was temporary, cutting approximately 6 mm (1/4 inch) off the beak. It was thought that the tip of the beak had no blood supply and presumably no sensation. The procedure was performed by hand with a sharp knife, either when deaths due to cannibalism became excessive, or when the problem was anticipated because of a history of cannibalism in the particular strain of chicken.
Cannibalism is a serious management problem dating back to the periods before intensive housing of poultry became popular. Poultry books written before vertical integration of the poultry industry describe the abnormal pecking of poultry:
Chicks and adult birds' picking at each other until blood shows and then destroying one another by further picking is a source of great loss in many flocks, especially when kept in confinement .... The recommendation of the Ohio Experiment Station of cutting back the tip of the upper beak has been found to be effective until the beak grows out again.
Cannibalism has two peaks in the life of a chicken; during the brooding period and at the onset of egg laying. The point-of-lay cannibalism is generally the most damaging and gets most of the attention. The temporary beak trimming developed at the Ohio Experiment Station assumed that cannibalism was a phase, and that blunting the beak temporarily would be adequate.
In recent years, the aim has been to develop more permanent beak trimming (although repeat trimming may be required), using electrically heated blades in a beak trimming machine, to provide a self-cauterizing cut. There are currently (2012) four widely used methods of beak trimming: hot blade, cold blade (including scissors or secateurs), electrical (the Bio-beaker) and infrared. The latter two methods usually remove only the tip of the beak and do not leave an open wound, therefore they may offer improvements in welfare. Other approaches such as the use of lasers, freeze drying and chemical retardation have been investigated but are not in widespread use. The infrared method directs a strong source of heat into the inner tissue of the beak and after a few weeks, the tip of the upper and lower beak dies and drops off making the beak shorter with blunt tips. The Bio-beaker, which uses an electric current to burn a small hole in the upper beak, is the preferred method for trimming the beaks of turkeys. The Farm Animal Welfare Council (FAWC) wrote regarding beak trimming of turkeys that cold cutting was the most accurate method, but that substantial re-growth of the beak occurred; although the Bio-beaker limited beak re-growth, it was less accurate. It was considered that the hot cut was the most distressing procedure for turkeys.
In the UK, beak trimming of layer hens normally occurs at 1-day of age at the same time as the chick is being sexed and vaccinated.
The costs of beak trimming relate primarily to welfare concerns. These include acute stress, and acute, possibly chronic, pain following trimming. A bird’s ability to consume feed is impaired following beak trimming because of the new beak shape and pain. Most studies report reduced body weights and feed intake following beak trimming, however, by sexual maturity or peak egg production, growth rates are usually normal. Weight losses were reduced in chicks that were beak trimmed by infrared compared with chicks trimmed by a hot-blade.
Whether beak trimming causes pain is a hotly debated concern. It is a complex issue as it may involve acute and/or chronic pain, and depends on the age it is performed, the method of trimming and the length of beak that is removed. Beak trimming in the poultry industry usually occurs without anaesthetic at 1-day of age or when the chicks are very young, but can occur at a later age if an outbreak of feather pecking occurs, and in some cases, birds may be beak trimmed on repeated occasions.
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. In Japanese quail, beak-trimming by cauterization caused lower body weights and feed intake in the period just after beak trimming. Beak trimmed Muscovy ducks spent less time engaging in beak-related behaviours (preening, feeding, drinking, exploratory pecking) and more time resting than non-trimmed ducks in the days immediately post-trim. These differences disappeared by 1 week post-trim. At 1 week post-trim the trimmed ducks weighed less than non-trimmed ducks, but this difference disappeared by 2 weeks post-trim. It is, however, unclear if the above changes in behaviour arise from pain or from a loss of sensitivity in the beak. Pecking force has been found to decrease after beak trimming in adult hens possibly indicating that hens are protecting a painful area from further stimulation. However, pecking force did not differ between chicks with or without minor beak-trims at 2 to 9 days of age, suggesting that chicks with minor beak-trims do not experience pain from the beak.
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.
The benefits of beak trimming are mainly welfare advantages, some of which directly relate to increases (or reduced decreases) in production. These include reduced feather pecking and cannibalism, better feather condition, less fearfulness and nervousness, less chronic stress, and decreased mortality.
A range of options have been proposed as possible alternatives to beak trimming including modifying the genetics of domesticated poultry to reduce cannibalistic tendencies. For confined housing where light control is possible, lowering light intensity so that birds cannot see each other as easily reduces antagonistic encounters and aggressive behaviour. Enrichment devices, introduced at an early age, as simple objects hung in a habitat can reduce aggressive behavior. Dividing the population into smaller group sizes reduces cannibalism. Proper body weight management that avoids underweight pullets reduces the probability of underweight pullets with uterine prolapse that leads to cloacal cannibalism.