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Culling is the process of segregating organisms from a group according to desired or undesired characteristics. In animal breeding, it is removing or segregating animals from a breeding stock based on a specific trait. This is done to exaggerate desirable characteristics, or to remove undesirable characteristics by altering the genetic makeup of the population. For livestock and wildlife, culling often refers to animal slaughter based on their characteristics, such as their sex or species membership, or as a means of preventing infectious disease transmission.
In and , culling is the sorting or segregation of fresh harvested produce into marketable lots, with the non-marketable lots being discarded or diverted into food processing or non-food processing activities. This usually happens at collection centres located at, or close to farms.
In the breeding of pedigreed animals, both desirable and undesirable traits are considered when choosing which animals to retain for breeding and which to place as pets. The process of culling starts with examination of the conformation standard of the animal and will often include additional qualities such as health, robustness, temperament, color preference, etc. The breeder takes all things into consideration when envisioning their ideal for the breed or goal of their breeding program. From that vision, selections are made as to which animals, when bred, have the best chance of producing the ideal for the breed.
Breeders of pedigreed animals cull based on many criteria. The first culling criterion should always be health and robustness. Secondary to health, temperament and conformation of the animal should be considered. The filtering process ends with the breeder's personal aesthetic preferences on pattern, color, etc.
For example, a breeder that is pleased with the muzzle length, muzzle shape, and eye placement in the breeding stock, but wishes to improve the eye shape of progeny produced may determine a minimum level of improvement in eye shape required for progeny to be returned into the breeding program. Progeny is first evaluated on the existing quality thresholds in place for muzzle length, muzzle shape, and eye placement with the additional criterion being improvement in eye shape. Any animal that does not meet this level of improvement in the eye shape while maintaining the other qualities is culled from the breeding program; i.e., that animal is not used for breeding, but is instead neutered and placed in a pet home.
This method measures several characteristics at once. Should progeny fall below the desired quality in any one characteristic being measured, it will not be used in the breeding program regardless of the level of excellence of other traits. With each successive generation of progeny, the minimum quality of each characteristic is raised thus ensuring improvement of these traits.
For example, a breeder has a view of what the minimum requirements for muzzle length, muzzle shape, eye placement, and eye shape they are breeding toward. The breeder will determine what the minimum acceptable quality for each of these traits will be for progeny to be folded back into their breeding program. Any animal that fails to meet the quality threshold for any one of these criteria is culled from the breeding program.
The total score method is the most flexible of the three. it allows for weighted improvement of multiple characteristics. It allows the breeder to make major gains in one aspect while moderate or lesser gains in others.
For example, a breeder is willing to make a smaller improvement in muzzle length and muzzle shape in order to have a moderate gain in improvement of eye placement and a more dramatic improvement in eye shape. Suppose the breeder determines that she would like to see 40% improvement in eye shape, 30% improvement in eye placement, and 15% improvement in both muzzle length and shape. The breeder would evaluate these characteristics on a scale of 1 to 10 and multiply by the weights. The formula would look something like: 15 (muzzle length) + 15 (muzzle shape) + 30 (eye placement) + 40 (eye shape) = total score for that animal. The breeder determines the lowest acceptable total score for an animal to be folded back into their breeding program. Animals that do not meet this minimum total score are culled from the breeding program.
Criteria for culling livestock and production animals can be based on population or production (milk or egg). In a domestic or farming situation, the culling process involves the selection and selling of surplus stock. The selection may be done to improve breeding stock—for example, for improved production of eggs or milk—or simply to control the group's population for environmental and species preservation. In order to increase the frequency of preferred Phenotype, agricultural practices typically involve using the most productive animals as breeding stock.
With dairy cattle, culling may be practised by Insemination cows—considered to be inferior—with beef breed semen and by selling the produced offspring for meat production.
Approximately half of the chicks of egg-laying chickens are males who would grow up to be roosters. These individuals have little use in an industrial egg-producing facility as they do not lay eggs, so the majority of male chicks are killed shortly after hatching.
Culling of farmed animals is considered a necessary practice to prevent the spread of damaging and fatal diseases such as foot-and-mouth disease, avian flu, Influenza A virus subtype H5N1 and bovine spongiform encephalopathy ("mad cow disease").
Populations of game animals such as elk may be informally culled if they begin to excessively eat winter food set out for domestic cattle herds. In such instances the rancher will inform hunters that they may hunt on their property in order to thin the wild herd to controllable levels. These efforts are aimed to counter excessive depletion of the winter feed supplies. Other managed culling instances involve extended issuance of extra hunting licenses, or the inclusion of additional "special hunting seasons" during harsh winters or overpopulation periods, governed by state fish and game agencies.
Culling for population control is common in wildlife management, particularly on and national parks. In the case of very large animals such as elephants, adults are often targeted. Their orphaned young, easily captured and transported, are then relocated to other reserves. Culling is controversial in many African countries, but reintroduction of the practice has been recommended in recent years for use at the Kruger National Park in South Africa, which has experienced a swell in its elephant population since culling was banned in 1995.
However, culling can act as a selection force intentionally implemented by humans to counteract the selection force of trophy hunting. Hunting typically enforces selection towards unfavorable phenotypic traits because of the strong hunting bias for specific traits, such as large antler size. Culling "low-quality" traits can counteract this force.
Animal rights activists argue that killing animals for any reason (including hunting) is cruel and unethical.
In 2014, a controversial policy was introduced by the Western Australian state government which became known as the Western Australian shark cull. Baited hooks known as drum lines were to be set over several consecutive summers to catch and kill otherwise protected great white sharks. The policy's objective was to protect users of the marine environment from fatal shark attack. Thousands of people protested against its implementation, claiming that it was indiscriminate, inhumane and worked against scientific advice the government had previously received. Seasonal setting of drum lines was abandoned in September 2014 after the program failed to catch any great white sharks, instead catching 172 other elasmobranchii, mostly tiger sharks.
The effectiveness of these deer culls has been debated and often criticized as only a temporary fix to the larger problem of deer overpopulation and argue that the use of culling will increase fertility of remaining deer by reducing competition. Those in favor of the culls argue that they can be used to combat the selection pressure that is imposed by hunting that creates smaller antler and body sizes in deer. People in favor of the culls recommend that they not be random and actively select for smaller individuals and bucks with smaller antlers, specifically "button bucks" or bucks with only spiked antler in their first year as opposed to forked antlers.
Culling of deer can also have benefits in the form of disease prevention and in places that the white-tailed deer is an invasive species such as New Zealand culling of deer has added benefits for native species. Diseases are density dependent factors and decreases in the density of the deer populations through culling causes diseases, such as chronic wasting disease and Lyme disease, to spread less quickly and effectively.
Zoos sometimes consider female animals to be more desirable than males. One reason for this is that while individual males can contribute to the birth of many young in a short period of time, females give birth to only a few young and are pregnant for a relatively long period of time. This makes it possible to keep many females with just one or two males, but not the reverse. Another reason is that the birth of some animal species increases public interest in the zoo.
Germany's Animal Welfare Act 1972 orders that zoo animals cannot be culled without verification by official veterinary institutes of the Landkreis or federated state.
In the UK, there is no general prohibition on animal euthanasia, which is allowed when overcrowding compromises the well-being of the animals.
Culling has been criticized on animal rights grounds as Speciesism—it has been argued that killing animals for any reason is cruel and unethical, and that animals have a right to live.
Some argue that culling is necessary when biodiversity is threatened. However, the protection of biodiversity argument has been questioned by some animal rights advocates who point out that the animal which most greatly threatens and damages biodiversity is humanity, so if we are not willing to cull our own species we cannot morally justify culling another.Brown, Les. 1988. Cruelty to Animals: The Moral Debt. London, UK: Palgrave Macmillan.Clarke, Paul A.B. and Linzey, Andrew (eds.). 1990. Political Theory and Animal Rights. Winchester, MA: Pluto Press.
Currently, wildlife contraceptives are largely in the experimental phase and include such products as Gonacon, an adjuvant vaccine which delivers a high dosage of a competitor ligand of the hormone GnRH to female mammals (e.g. whitetail deer). The complex formed of GnRH and the Gonacon molecule promotes production of antibodies against the animal's own GnRH, which themselves complex with GnRH. This encourages an extended duration of the drug's effects (namely, reduction of active/unbound GnRH in the animal's system). Though the endocrinology behind Gonacon is sound, the need for multiple lifetime doses for full efficacy make it a less-guaranteed and less-permanent solution for wildlife than lethal culls. Even among domestic animals in controlled conditions, Gonacon cannot ensure 100% reduction in the occurrence of pregnancies.
Reproductive inhibitors need not act on the parental individuals directly, instead damaging reproductive processes and/or developing offspring to reduce the number of viable offspring per mating pair. One such compound called Nicarbazin has been formulated into bait for consumption by Canada goose, and damages egg yolk formation to reduce the viability of clutches without harming the adult geese.
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