controlled propagation of domestic animals. Its aim is the improvement of qualities considered desirable by humans. Breeding procedures involve the application of several basic sciences, chiefly reproductive physiology, genetics, and statistics. This article deals with the practical application of scientific principles to the selection of superior animals and the planning of mating combinations. The fundamental biological principles underlying animal breeding are discussed in the articles heredity and reproductive system, animal. Animals are bred for utility, sport, pleasure, and research. Dogs, for example, have been bred to serve as watchdogs, police dogs, hunters, sheep dogs, and pets. Many species of small mammals, especially rats, mice, rabbits, and guinea pigs, are bred for research, chiefly in genetics, physiology, and medicine. The basic principles of breeding are the same no matter what the animal species or the purpose in breeding may be, but the practical approach to the problem may differ in several respects, depending on such considerations as the mode and rate of reproduction and the relative effects of genetic and environmental factors on the traits of greatest interest. The term population is used in this article to denote a group of interbreeding individuals; i.e., a breed, or strain within a breed, which in some respects is genetically different from other breeds, or strains, of the same species. The word purebred is used here in its sense of referring to animals registered in the herdbook maintained for a certain breed, or to animals eligible for such registration, and the mating of purebred animals is called pure breeding. It is to be understood that genetically pure breeds (homozygous for all traits) do not exist. The objectives of animal breeding vary with regard to species, local conditions, and time. Early in history horses were bred mainly for riding or loading purposes; later they were bred for traction; and nowadays, to a large extent, for sport (racing and hunting). In North America and western Europe, cattle populations are specialized for beef or milk production, or bred for a combination of both. In southern Europe and in many parts of Africa and Asia, oxen are still produced for pulling plows or carts. Some breeds of sheep are specialized for wool production, some for meat, and one breed, the Karakul sheep, is bred for fur production (Persian lamb). Pigs always are bred for meat production, but they may be specialized to produce a certain type of meat, either pork or bacon. At one time, chickens were bred for the combined production of eggs and meat, but in the Western world there is now a pronounced specialization of breeds and crosses to produce either eggs or meat. Evaluation of animalsIn breeding farm animals for utility, selection must be based as far as possible on objective measurements of traits that are decisive for the economy of production. Judging animals on the basis of appearance alone has not become obsolete but its importance has been reduced. Certain traits, of course, are difficult or impossible to measure objectively. In all kinds of horses, for example, the legs should be well proportioned in relation to the body and free from faults and weaknesses; this judgment is difficult to render quantitatively. Similarly, the strength of the legs is important in cattle and pigs, but difficult to measure. In dairy cows, the attachment of the udder to the body is difficult to measure; but it is significant, and it can be judged and scored. In mink breeding, a judging of the animals with regard to fur quality and shade of colour is necessary and cannot, so far, be substituted by laboratory tests. In the breeding of pet animals, such as dogs and cats, judging the animals for conformity with breed standards is usually decisive in determining their market value. Actual measurement of an individual animal's performance is a fairly recent innovation in animal breeding, except with regard to Thoroughbred horses, the selection of which for centuries has been based on speed at standard racing distances. Systematic recording of milk production in dairy cows started in Denmark in 1895, and the movement spread rapidly in northwestern Europe and North America. In several countries, young beef and dairy bulls are tested at special stations for rate of growth and muscle development. In sheep breeding, the number of lambs borne by each ewe is recorded, as is the weight of the lambs at five months of age. The recording of growth rate, feed consumption, and carcass quality of pigs began in 1907. Progeny testing and performance testing (see below Methods for estimating breeding values) has subsequently been added to judge young males (bulls, rams, and boars) intended for breeding. The productivity of sows is measured by recording the number of pigs in each litter at birth and their weight at three weeks of age. An ultrasonic technique measures the thickness of backfat on live pigs. Special nests with hinged doors (trapnests) were employed to record the egg production of individual hens from the end of the 19th century until the introduction of individual laying cages made trapnests unnecessary. The recording of individual performance in breeding populations of farm animals developed with great rapidity after World War II, in terms of both frequency of use and refinement of methods. It may be considered to be the very foundation of progress in breeding programs. Genetic and environmental variation in animal traitsThe traits of animals in any population may be classified roughly into two groups; viz., qualitative and quantitative traits. Qualitative traits show discontinuous variation; for example, coat colour, presence or absence of horns, certain blood characteristics (e.g., blood types), presence or absence of particular enzymes, and existence of metabolic defects and congenital malformations (e.g., bleeding disease [hemophilia] in dogs). In general, the inheritance of qualitative traits is relatively simple, in accordance with the laws of heredity, and environment plays only a minor role in their variation. The quantitative traits show continuous variation between the extreme variants, the mean type being, as a rule, most frequent. Growth rate, live weight, body measurements at mature age, milk yield and composition in cows; body length and backfat thickness in pigs; wool yield and quality in sheep; and egg production in fowl are typical examples of quantitative traits. As a rule, such traits are influenced by many genes (polygenes), each gene exerting relatively small effect, and environmental factors are responsible for a considerable part of the variation. There is, in fact, no distinct borderline between qualitative and quantitative traits. A congenital malformation, for example, may be determined by a major gene, as well as a number of polygenes, and the latter may cause a considerable variation in the expression of the trait. Similarly, variation in the expression of a trait regulated by a single gene may be caused by environmental factors. Nevertheless, this classification is useful because typical qualitative traits can be analyzed with regard to a single gene and its behaviour; whereas quantitative traits are best studied by statistical methods that permit one to proceed without knowledge of the number of genes involved or their interrelations. By using appropriate statistical methods, for example, it is possible to estimate the fraction of the total variation in the population that is caused by the additive effect of the genes; this fraction is termed the heritability. A heritability of 1.00 indicates that all the variation observed in the population of the trait in question is genetically determined; a heritability of 0.00 indicates that the variation is wholly environmental in cause; and when the heritability is 0.50 the genetic and environmental factors are equally responsible for the observed variation of the trait. A few estimates of heritability of traits are given to illustrate typical values. In horses: pulling power, 0.25. In dairy cows: height at withers at three years of age, 0.45; milk yield at first lactation, 0.30; fat and protein content of milk, 0.55; resistance to mastitis, 0.30. In beef cattle; daily weight gain (from weaning to slaughter), 0.40. In sheep (Merinos): yield of clean wool, 0.45. In pigs: daily weight gain (from weaning to slaughter) in group feeding, 0.30, in individual feeding, 0.60; backfat thickness, 0.50; resistance to atrophic rhinitis, 0.25. In chickens: egg production (in first laying year), 0.30; egg size, 0.50; resistance to leukosis, 0.10. In general, heritability is relatively low for such traits as fertility and resistance to infectious diseases, and it is high for growth rate, body size at mature age, and composition of cows' milk. The magnitude of environmental effects, and to some extent also the additive genetic effects, may vary considerably between different populations of the same species; therefore, heritability estimates also vary and cannot be considered as constants for the various traits. For example, as noted above, when the heritability of growth rate of pigs is estimated from individual-feeding data, it is twice as high as when it is estimated from group-feeding data. In spite of these limitations, heritability estimates for quantitative traits have proved to be of great value in planning animal improvement programs. Methods for estimating breeding valuesThe breeding value of an animal depends on the genes it passes on to its offspring. Each individual offspring receives a random sample of one-half of the genes from each of the parents. Concerning qualitative traits with known inheritance patterns, the situation is relatively simple. A dominant gene manifests its presence in the physical character (phenotype) of the individual even when the gene it is paired with is different—that is, in the heterozygous condition. A recessive gene also can be recognized but only when it is present in duplicate (the homozygous state). In both cases, of course, there must be no complications caused by modifying polygenes or environmental factors. The breeder can then select for or against the trait according to his wishes. To recognize heterozygous carriers of recessive genes by visual inspection of the animals is impossible in most cases, but it can be done by mating the suspected carrier to a certain number of known heterozygotes or to its own offspring. The breeding value of a quantitative trait may be assessed on the basis of the merits of: (1) the individual himself, (2) his ancestors, (3) his collateral relatives, (4) his progeny, or (5) a combination of any two or more of the above. The relative value of these different approaches depends on the heritability of the trait and the rate of reproduction. With regard to quantitative traits that are sensitive to environmental influence, it is often advisable to use the deviation of the individual's record from the mean record of the herd, rather than the individual's record itself. This procedure eliminates, or at least reduces, the effect of nongenetic differences between herds. Each of the above methods of assessing quantitative traits has its own uses (as discussed below in Methods of selection). Individual meritIf it is assumed, for example, that a cow in her first lactation has produced 10,000 pounds (4,500 kilograms) of milk, and that her contemporary herd mates in the corresponding lactation have produced 8,900 pounds (4,000 kilograms) on average, then the phenotypic merit of this cow is 1,100 pounds (500 kilograms) above that of her contemporaries. By this procedure, environmental effect is minimized. When cows of different age (or undergoing different lactation periods) are compared, it is necessary to correct the yield of each individual to a standard age, because the yield increases until the fourth or fifth lactation. The fat content of the milk is much less influenced by environment and by individual age; therefore, no age corrections are needed, and individuals can be compared directly on their actual records. The same principles apply to other traits and other species of animals. Ancestor meritThe merit of ancestors is usually the first available information on the breeding value of an individual, and such pedigree information, therefore, is valuable as a rule. With each earlier generation in the pedigree, however, the value of this information is halved. Furthermore, since a grandparent can pass his genes to a grandson or a granddaughter only through one of its parents, the more information known about this parent, the less valuable is the information known about the grandparent. For example, if a reliable progeny test (see below) shows that the sire of an animal has a high breeding value, there is no need to consider the parents of that sire. In many cases, especially in horse and dog breeding, the importance of long pedigrees has been greatly exaggerated. Merit of collateral relativesThe genetic similarity between an individual and a randomly chosen full sib (brother or sister) is the same, on average, as that between the individual and one of his parents; and an individual's genetic similiarity with a half-sib is the same as that with one of his grandparents. An individual, however, can have many more full sibs and half-sibs than parents or grandparents, and the sibs, therefore, may be of much greater value than parents or grandparents for estimating the breeding value of the individual. In pigs, rabbits, dogs, and fowl, the number of full sibs can be fairly large, and in artificial insemination of cattle the number of half-sibs can be very large. Progeny meritProgeny tests yield the final information on an animal's breeding value. The relative importance of such tests increases with decreasing heritability of the trait, and it is especially valuable for sex-limited traits, as, for example, in testing a bull's breeding value with respect to the milk yield of his daughters. The lower the heritability of a given trait, the larger a progeny group is needed for a reliable test of the individual. Combined methodsInformation from two or more of these methods can be combined into a single estimate of the individual's breeding value. In such circumstances, the different criteria should be weighted according to their expected contribution to the accuracy of the final estimate. The breeding programThe genetic improvement of a herd or a breed requires careful planning with regard to the choice of animals for breeding and the mating combinations that are carried out. Methods of selectionSelection of breeding animals can be carried out in different ways. Among the more important are mass selection, pedigree selection, family selection, and progeny selection. 1. Mass selection is based solely on individual merit. Applied to traits with high heritability and about equal manifestation in both sexes, mass selection can be expected to give good results. With decreasing heritability the efficiency decreases, and for sex-limited traits (and heritability considerably below 0.5) it is always inefficient. 2. Pedigree selection depends on the merits of the ancestors. It is valuable in the first selection among young animals, especially when the heritability of the traits is high. Relying solely on pedigree selection, however, results in very slow progress. 3. Family selection is based on the merits of collateral relatives, such as full sibs or half-sibs, and it is used mainly as an aid to individual selection. It is especially valuable with regard to sex-limited traits, for traits with low heritability, or when some animals have to be slaughtered, as for determining the carcass quality. In the selection of young males for breeding, for example, no data may be available on their individual performance; e.g., egg production in the fowl. When sib groups of pullets start laying early in the autumn, the cockerels may be selected for breeding on the laying records of their full sibs and half-sibs. Similarly, young bulls may be selected mainly on the milk records of their paternal half-sibs; that is, on the progeny tests of their sires. Individual pigs may be selected on the basis of carcass tests made on their sib groups. With regard to traits that can be recorded for all the animals alike, males and females, selection of individuals can be based simply on their family average or on an index that combines the individual's own performance and the average for the rest of the family. A simple procedure, applicable in dog and pig breeding, would be to select the best individual from the best litter. One difficulty in the application of family selection is that systematic environmental differences may occur, especially between full-sib groups, and these tend to mask the genetic differences. When inherited defects appear in sib groups, there is a certain risk that some of the healthy animals carry a hidden gene for the defect. 4. Progeny selection has been applied with great success in dairy cattle breeding, and in general it is valuable in all types of livestock when applied to sex-limited traits and traits with low heritability. Early progeny testing of males on a sufficient number of offspring and an effective selection among those tested are very important. The disadvantage of selection of sires on progeny testing is that it means increased length of the generation interval and thereby tends to slow down the rate of genetic improvement. In general, it is not necessary to include all the traits used for selection in an overall index; for example, all animals used for breeding should possess normal fertility, and those that do not should be excluded from the breeding program. Also, as a rule, any animal known to be a carrier of a gene for a serious metabolic or morphological defect should be eliminated even if the merits for some other traits are fairly high. Usually selection is made in a stepwise fashion. With regard to dairy bulls, for example, selection on the basis of pedigree can be made soon after birth; a second selection can be made later based on growth rate during the first year of life and fertility in the first series of inseminations; and finally a third selection can depend on the results of progeny testing, when offspring are old enough to be judged. Mating systemsRandom matingRandom mating implies that each possible mating in a population has the same probability of occurrence. Only artificial selection by the breeder can really be eliminated; however, a certain amount of natural selection always takes place. Random mating often is used in breeding experiments to minimize genetic changes in a control population with which selected populations are compared. InbreedingInbreeding may be defined as mating of individuals more closely related than the average of the population. It increases the homozygosity and decreases the heterozygosity of the inbred animals. The so-called inbreeding coefficient is a measure of the loss of heterozygosity due to inbreeding, and it is expressed as a fraction, or percentage, of the amount of heterozygosity present when inbreeding started. After one generation of mating between full sibs or mating of sire with daughter or dam with son, the heterozygosity of the offspring is reduced by 25 percent (or the inbreeding coefficient is 25 percent). In the mating of half-sibs or double first cousins the inbreeding coefficient is 12.5 percent. Mating of single first cousins gives an inbreeding coefficient of 6.25 percent, and that of half first cousins 3.12 percent. Mating between full sibs in two successive generations decreases the heterozygosity by 37.5 percent, and in three generations by 50 percent. So-called inbred lines are produced by continuous consanguineous matings in several generations; genetic variation decreases within each line and increases between separate lines. In experiments with mice, rats, and guinea pigs, full-sib matings have been continued through many generations. Farm animals and birds are much more sensitive to inbreeding, and usually full-sib matings can be continued for a few generations only because of a marked decrease in viability and fertility. Breeding within small populations, such as a herd or flock, without infusion of new animals from outside, leads automatically to a certain amount of inbreeding. In farm mammals, each male is invariably used to serve a large number of females, and under such conditions the increase of the inbreeding coefficient per generation in a closed breeding unit can be estimated simply by dividing 100 by eight times the number of males used. Thus, when only one homebred sire is used, the decrease in heterozygosity in each generation is 12.5 percent. When artificial insemination is used, a small number of sires again are used to serve a large number of females. Nevertheless, the reduction in heterozygosity may be relatively small in such cases, because many young sires are used for progeny testing, and several tested sires enter into service each year. The modern trend in artificial insemination is to use the tested sires for only relatively short periods of time, after which they are replaced with younger sires, which presumably have made even better records. Such rapid turnover in sires serves also to reduce the length of the generation interval in breeding programs. Inbreeding increases the homozygosity of unfavourable as well as favourable genes. As a result there is a segregation of various kinds of congenital defects and, more important, a general decline in fertility and viability of the inbred animals. The latter finding has been demonstrated in numerous experiments with farm animals, especially pigs and poultry. In experiments with laboratory animals it has been shown also that the sensitivity to unfavourable environmental influences increases with inbreeding. Although the decrease in fitness resulting from inbreeding is a general phenomenon, the amount of decrease is dependent on the genetic constitution of the animals used for inbreeding. Linebreeding is a form of mild inbreeding designed to concentrate the genes of a certain ancestor in a strain of animals. The most intensive form of linebreeding is repeated backcrossing to a particular parent, but usually a more distant relationship is preferred; for example, a female may be mated to her grandsire or uncle. OutbreedingOutbreeding is defined as mating individuals less closely related than the average of the population. The degree of outbreeding can vary, just as that of inbreeding can. In some cases, it is even possible to make crosses between species, as in the crossing of the horse and ass for the production of mules. The term crossbreeding, however, usually refers to crosses between breeds within the same species. Crosses often are made between more or less inbred strains or lines of the same breed (called, respectively, strain and line crosses). Crossbreeding has been practiced for a long time, and it also has been subjected to experimental research in the United States and Europe. Various methods have been developed and tested, with generally favourable results. The main function of crossbreeding is to increase the heterozygosity of the offspring. One of the breeds selected may be superior in certain traits and the other breed may excel in other traits. It can be expected, then, that the first-generation crossbred animals will be about intermediate to the parental breeds with respect to both traits. In some cases, however, the first-generation animals are somewhat superior to the better parental breed with regard to total merit. When the average quality of the first generation exceeds the average of the two parental breeds, the phenomenon is called heterosis, or hybrid vigour. Heterosis is displayed mainly in the so-called fitness traits, fertility and viability. It is thought to result from interaction of different forms (alleles) of a given gene (a phenomenon called overdominance) or from interactions of quite separate genes (epistasis). Crossbreeding thus is a way of utilizing nonadditive gene effects that cannot be exploited by selection within the separate breeds. When the heterozygosity is increased, the number of different genes is increased, a result that probably makes the animals better able to adapt to environmental stress. Backcrosses (crosses of crossbred offspring to one of the parental breeds) and successive three-breed crosses seem to have an advantage when maternal influence on the offspring is important; crossbred sows, for example, generally take better care of their young than purebred sows do. Rotational crossbreeding, usually involving three breeds, is a favoured method in commercial pig breeding since it necessitates purchase of males only. The first cross is made between breeds A and B, for instance, and the female offspring are mated to a boar of breed C; the females resulting from this cross are then mated to a boar of breed A, and in the next generation a boar of breed B is used, and so on. Many experiments have been carried out on the crossing of inbred lines of the same breed or of different breeds. Generally, fertility and viability are restored even in the first crossbred generation. It has been possible with this technique, for example, to produce commercial “hybrid chicks” with superior egg-laying performance. Because the individual inbred lines are poor producers, the hybrid chicks are usually developed by a four-way cross. This is carried out by mating the offspring from crossing of lines A and B with the offspring from crosses between lines C and D, producing in effect a “double” hybrid. In order to obtain the best possible result, a large number of lines are tested in various crosses for combining ability—that is, ability to produce desirable results from crossing. Most of the eggs marketed in North America and western Europe are produced by hybrid chicks. Similar methods of breeding have been tried also on pigs and cattle, but the results have been less favourable. With these animals the market value of the individual is so high that it is impossible for breeders to keep the costs of developing and testing inbred lines within reasonable limits. Artificial insemination and egg transplantationThe practical application of artificial insemination in horses, cattle, sheep, and pigs was developed in Russia during the first decades of the 20th century. Semen was collected in an artificial vagina when the male mounted a female, or a dummy, and methods were developed for dilution of semen and its preservation for several days outside the body. More rapid progress became possible in 1950, when it was shown that bull semen could be deep-frozen at −79° C with solid carbon dioxide and later thawed without serious effect on fertility, provided that a certain amount of glycerol was added before freezing. Later, the use of liquid nitrogen made it possible to store semen at about −196° C. Calves have been produced from semen that has been frozen for more than 10 years, and it seems possible to increase the period of storage indefinitely. Theoretically, it should be possible to produce more than 100,000 calves per bull annually; 10,000 or more actually have been produced. In Denmark about 95 percent of the dairy cows are artificially inseminated; in England and Wales, about 70 percent; and in the United States, about 50 percent. The greatest advantage of artificial insemination of dairy cattle from a genetic point of view is that the bulls can be progeny tested with much greater accuracy than in natural breeding, not only because the number of daughters is greater but also because the daughters are spread over many herds with different environmental conditions. A progeny test carried out in any one herd is valid for the conditions in that herd only. Furthermore, by artificial insemination the progeny test can be completed at least one year earlier, on average, than if the bulls were used in natural service. Artificial insemination is practiced also on beef cattle, horses, sheep, pigs, and poultry, although on a small scale compared to its use with dairy cattle. The application of artificial insemination in pig breeding was delayed by difficulties in deep-freezing boar semen, but these difficulties have been overcome. By hormone treatment, production of many ova (polyovulation) can be induced in females. After artificial insemination, the fertilized eggs can be collected from the oviducts or uterus of the donor and transferred to the uteri of recipient females for development into normal fetuses. At the time of transfer, the recipient must be synchronized with the donor with regard to the sexual cycle. Many successful transplantations of fertilized eggs have been made in sheep, and some have been carried out in cattle and pigs. Theoretically, it is possible to harvest thousands of eggs from an individual female within a few years' time and to distribute these eggs to a large number of recipients, thereby multiplying the possibilities for propagation of superior females. To secure the eggs from the donor, however, and transfer them to other females is laborious and costly, even if it can be done without major surgery. Methods of freezing and storing eggs without harmful effects must be developed before the transplantation technique can be applied as a routine procedure in the breeding of farm animals. The rate of genetic improvementThe rate at which genetic improvement can be carried out within a certain population depends on three factors: 1. The accuracy in estimating the breeding value of the individuals. This accuracy itself depends on the heritability of the trait or traits on which the selection is based. The value used may be based on the animal's own performance, as, for example, the milk yield of a cow, or it may be an index formed by a combination of information on breeding value derived from various sources such as pedigree, collateral relatives, and progeny. 2. The selection differential, which is defined as the difference between the index of the animals selected for breeding and the mean of the entire population or group to which the selected animals belong. 3. The length of the generation interval, defined as the average interval (in years) between the birth of the parents and the birth of their offspring used for breeding. On average, this interval for horses is about nine years, for cattle five, for sheep three and a half, for pigs two and a half, and for chickens one and a half. It is possible, however, to reduce these intervals quite considerably by planned breeding. The overall rate of genetic improvement, or response to selection, is equal to the accuracy in selection multiplied by the selection differential and divided by the generation interval. In equation form this is expressed as: Re = HS/Y, in which Re is the response to selection, H is the heritability, S the selection differential, and Y the generation interval. From this formula, it is evident that the rate of genetic improvement of the population can be increased by increasing the accuracy in choice of breeding animals, by increasing the selection differential, and/or by decreasing the length of the generation interval. The possibility of increasing the selection differential depends on the rate of reproduction. Pigs have a much higher reproductive rate than cattle, and therefore fewer animals are needed for breeding and a greater selection differential is possible. In dairy cattle it may be necessary to raise about 60 percent of all heifer calves for breeding, merely to keep the size of the herd constant. For bulls, however, the selection differential can be very high, especially when artificial insemination is used. The size of the population subjected to selection is also important. It has been calculated, for example, that when the number of performance-tested dairy cows in an artificial insemination unit increases from 2,000 to 20,000, it should be possible to increase the rate of genetic improvement for milk yield about 50 percent by means of efficient progeny testing and selection. In most Western countries there was a pronounced improvement in many economically important traits of farm animals after World War II. In the United States, for example, the average yield of milk from milk-recorded cows increased from 9,425 pounds (4,275 kilograms) in 1955 to 12,209 pounds (5,538 kilograms) in 1967, an increase of 29.5 percent, or 2.27 percent per year; by 1980 an official test reported an average yield of 14,960 pounds (6,786 kilograms). For the Swedish red and white breed, the average milk yield increased from 9,473 pounds (4,297 kilograms) in 1960 to 11,094 pounds (5,032 kilograms) in 1969; i.e., by 17.1 percent, or 1.71 percent per year. It has been estimated that the genetic improvement of this breed during the same period corresponded to about 1 percent per year, or more than half the actual rise in milk yield. It seems probable that the major part of the genetic improvement in this instance was due to a more efficient progeny-testing and selection program that had been started among the tested bulls in the mid-1950s. Similarly, in Danish pig-testing stations, the average daily gain in live weight increased from 23.9 to 24.6 ounces (678 to 697 grams) from 1955 to 1962, the length of carcasses increased from 36.9 to 37.8 inches (93.8 to 95.9 centimetres), and the backfat thickness decreased from 1.28 to 1.10 inches (3.26 to 2.80 centimetres). The modern broiler chick is an example of the success obtained by crosses between breeds that have been specialized for different lines of production without close inbreeding. When there is a considerable amount of hereditary variation, it is possible to change a breeding population considerably in about five to 10 generations of intense selection. Sooner or later, the response to selection decreases, and ultimately a selection limit is reached. This may be due to an exhaustion of the genetic variation or, more likely, to a disturbed gene balance, especially if the selection is concentrated on one, or a very few, traits only, without considering the fertility and viability of the animals. The remedy in such a situation is either a cautious introduction of new genes from another population or deliberate crossbreeding to increase the genetic variation. In either case, selection in the gene-enriched population can start again. Another possibility is to relax the selection for a number of generations, giving the population time to recover, but this is a rather time-consuming process. Cooperative and governmental promotion of animal breedingIn most countries in which animal breeding has reached a fairly high level, associations were formed long ago (beginning in Great Britain in the late 18th century) with the aim of promoting the development of existing breeds. At first the primary objectives of breeders' associations were to publish herdbooks and to lay down rules for registration of “purebred” animals, to arrange shows and fairs, and to work for the dissemination of the breeds at home and abroad. Later, these associations started performance testing or worked in close cooperation with other organizations developed for this purpose. In European cattle breeding there is a trend toward concentration into one organization of the various activities, such as performance and progeny testing, artificial insemination, and other services, among which herdbook registration may be only a minor detail. A similar trend has appeared also in the breeding of sheep, pigs, and fur animals. With regard to the breeding of animals for sport or as pets, traditional breeders' associations are, and probably will continue to be, of great importance. Scientific research has been the foundation, and education the impelling force, in the accelerated development of animal breeding in the 20th century. Specialized research institutes and experimental stations, with cooperating agricultural colleges and extension divisions, have arisen in almost every country in which animal production is an important enterprise. Most information based on laboratory research and breeding experiments has arisen in the United States and Great Britain, but Denmark seems to have been the first country to organize an agricultural extension service that reached all levels of farming and animal husbandry. Organizations have been established in several countries to promote animal science and production; for example, in the United States, the American Society of Animal Science and the American Dairy Science Association; in Great Britain, the British Society of Animal Production; and in Germany, Deutsche Gesellschaft für Züchtungskunde. In 1966 these national organizations formed the World Association for Animal Production, the major objective of which is to arrange periodic world conferences for the exchange and dissemination of knowledge in the field of animal science. |