ANIMAL BREEDING. The aim of the breeder is to obtain and maintain generation by generation improvement in the sum of the desirable qualities exhibited by his stock and to elimi nate qualities regarded as undesirable. His task is that of produc ing true-breeding types which, under the circumstances in which they are to be maintained, shall flourish and fulfil the purposes for which they were bred. Before there can be improvement there must be a standard of the ideal. Often it is a simple matter to assess the value of an individual or type in relation to the purposes for which it has been bred: in the case of dairy cattle and sheep, for example, it is possible to determine the quantity and quality of milk and wool and to use such determinations as guides in breeding practices. In the case of meat-producing animals the value of the individual cannot be assessed at all accurately until after slaughter, and breeders have been obliged to rely largely on outward signs that are not always trustworthy indications of quality. In the case of work-animals it is exceedingly difficult to assess potential usefulness, since no final test can be reached until the end of the working life, and here too breeders have been forced to adopt somewhat unsatisfactory empirical standards. It is a general rule that the simpler and more direct the test of utility is, the easier and the more rapid will improvement be.
In the task of securing breed improvement the breeder seeks to control (I) the environment of his stock, and (2) the forces of heredity and reproduction. With regard to the first, conspicu ous improvement has followed directly upon advances in knowl edge of animal nutrition and disease and of the general biology of the animals concerned. Such improvement is but temporary, how ever, and can only be maintained by ceaseless care. There is too a limit to such advance, and when this is reached further prog ress can only be made by deliberate control of the inborn qualities of the stock.
When the great pioneers first undertook the improvement of British livestock about the middle of the 18th century (see AGRI CULTURE), the farm animals were of very mixed type and generally of poor quality. Certain local types were distinguished by peculi arities of size, prevailing colour and productiveness, but there were no breeds in the modern sense, and in all these local types there was great variability in respect of all characters. The mod ern breeds had thus their beginnings in the picture of an ideal type at which the breeder aimed, and in the fortunate and for tuitous appearance of one or more superior individual animals. Having this ideal in mind, the breeder chose from among the local stock individuals which exhibited one or more of the components of his ideal. The initial stages in the improvement of each breed was the collection, in as small a group of the local type as possible, of as many of the characters of the ideal as possible. The breeder has not the power of producing hereditary characters, desirable or undesirable, but only of creating any combination of such heritable characters as already exist separately, and of manipulat ing the environment so that these become more fully expressed.
Having started on a basis sufficiently wide to ensure the inclu sion of all the characters that were esteemed, the second stage in improvement has been the fixing of the desired characterization of the stock and the elimination of undesirable qualities. By the selection of individuals in each generation as parents of the next, the breeder has sought to weed out the undesirable characters and to combine the desirable, first in one or two individuals, and finally throughout a true-breeding strain. He has selected the animals for breeding and controlled their matings. Though this process of selection has never yet been carried to the point of complete and final success, it has played an all important role in all instances of breed improvement.
Selection.—The simplest form of selection for breeding pur poses is that which is termed "mass" selection, the choice of individuals on the simple basis of individual merit as judged by appearance or performance without reference to ancestry, pro geny, or to the degree of the relationship of the individuals con cerned. The application of this kind of selection has given widely different results according to the kind of characters in relation to which it has been employed. For the purpose of changing the average characterization of a stock, mass selection has usually been applied to such characters as show a quantitative variation. The breeder has selected continuously, generation after generation, such individuals as showed a slight variation in the desired direc tion, as increased size, improved shape or pattern. The breeder quickly found that certain characters yielded very rapidly to such selection up to a given point, after which the method proved ineffectual. In other instances such selection yielded no advance, and so it is that till quite recently there has been no general ex planation of the manner in which mass selection sometimes accomplishes its aims and in certain instances fails to do so. An entirely satisfactory explanation has been disclosed within the last 20 years. It has been shown that mass selection is only followed by a movement in the desired direction when applied to a population or stock that is heterozygous for many of its characters, and when the characters for which selection is be ing practised are inherited in a straightforward fashion, not being conditioned in their expression to any marked extent by the conditions of nurture (see MENDELISM and HEREDITY). In the dairy cow, for example, the characters of the milk yield and com position are conditioned in their expression not only by the effects of age, time of calving, and fegding, but also by a multitude of environmental agencies, the total influence of which cannot yet be assessed. Selection will not lead to so great advance in a case such as this as it will when applied to the case of a simple recessive character the expression of which is in no way influenced by envi ronmental agencies. It is a simple and rapid matter to establish a true-breeding strain of red cattle by selecting for breeding the occasional red that crops up in black breeds, but the same methods will not yield a true-breeding roan or blue if, as seems the case, these coat colours are peculiar to unfixable heterozygotes.
The result of such selection, when effectual, is the isolation of individuals homozygous and therefore true-breeding for the char acters for which selection is being practised, and so it is that when complete homozygosity has been attained, further selection is un availing. It is for this reason that mass selection is relatively ineffectual in changing the characters of well established breeds. That this method has been and still is so successful is explained by the fact that most domesticated animals are so heterozygous that selection practised on ordinary stock is bound to be followed by some advance in quality.
Selection can create nothing ; it merely sorts out individuals and types homozygous for the characters for which the breeder is aiming, and alters the average characterization of the stock by altering the proportions of the different hereditary constitutions within it. Commonly it is impossible to disentangle the effects of selection in the production of improved types from those conse quent upon advances in husbandry inevitably developed in the case of a stock which is becoming more and more valuable to its owner.
Though it is sound policy to use for breeding only those individ uals of manifest merit out of admirable and true-breeding parents, the real test of breeding worth is the progeny test. The breeding value of an individual is estimated by examination of a sample of its early progeny. The progeny test is applied most readily to males because of the greater number of offspring which may be obtained from a single individual and because the flock or herd can be improved more rapidly in this way than through the selec tion of superior females, in the case of which half the breeding life must be over before a sufficient number of offspring has been secured. Those males which beget the best progeny are retained, the others, even though in general appearance better specimens, are discarded. The test of a dairy bull, for example, is the aver age milk-production of a number of his daughters compared with that of their dams, assuming, of course, that the conditions of husbandry are similar in the two cases. To compare the merits of different sires in the bacon pig, it is necessary to rear, under care fully controlled conditions, groups of at least four offspring of each sire and finally to convert them into bacon.
In the early days of breed improvement, the problem of decid ing which individuals should be mated did not exist. The improver found it increasingly difficult to find in other stocks animals which approached his ideal as nearly as did animals in his own, and so he was forced by circumstances to inbreed. So it was that Robert Bakewell (q.v.), when he found that he could mate closely related animals without disadvantageous results, resorted, as his blood lines became more and more intermingled, to more and more in tensive inbreeding. Since his time all efforts towards the produc tion of uniform and valuable breeds, have involved, sooner or later, the deliberate mating of closely related individuals.
Inbreeding.—Inbreeding (q.v.), in practice, is the term re stricted to describe the matings of close relatives, e.g., brother and sister, parent and offspring, and matings between comparatively unrelated or only distantly related individuals are described as outbreeding. Matings involving individuals of different breeds or varieties are examples of crossbreeding. Line-breeding involves matings between relatives in an attempt to increase or concentrate in one line of descent the hereditary constitution of one or a few individuals. All these systems can be grouped under the two headings of inbreeding and outbreeding, these differing one from the other only in the degree of relationship of the individuals concerned. Inbreeding is not necessarily harmful but can be definitely advantageous, leading to the development of a uniform and true-breeding stock. It is equally true, however, that benefit does not always follow this practice, for in certain cases there has been, and under experimental conditions still is, disappointing regression, diminution of vigour, lowered powers of resistance, de creased fertility, even reduction in size. In human societies this disagreement has resulted in laws and customs as diverse as those which forbid marriages outside a restricted group of relatives and those which forbid matings between relatives altogether. Though the facts vary, however, there has grown up amongst students of heredity a considerable degree of unanimity concerning the prin ciples involved and a general interpretation has been promulgated which explains the conflicting results scientifically. The effects of inbreeding depend, not on any pernicious attribute of this system of mating, but upon the hereditary constitutions of the individuals involved. Inbreeding has but one demonstrable effect upon the stock subjected to its action—the isolation of homozygous types. It can be stated confidently that no individual is homozygous for all the characters which it exhibits (see MENDELISM).
In the case of an undesirable character that is dominant, it will be eliminated either by its effects upon the individual or by artificial selection as practised by the breeder. If the undesirable character is a recessive, however, it will remain hidden in the stock until two individuals, each heterozygous in respect of it, are mated. If only one animal in ten carries the factor for such a recessive character and mating is at random, one individual in every four hundred of the next generation may be expected to exhibit this undesirable character, and even though there are 20 factors for different undesirable recessive characters among the stock and these are distributed in the same proportion, still 95% of the progeny will not exhibit these characters. But when mating ceases to be at random and related individuals are used, the probability that the two parents each carry the factor for some undesirable character is greatly increased and the proportion of the progeny exhibiting the undesirable character is also raised.
Inbreeding leads to a rapid increase in homozygosity, and when this state has been achieved, stability and uniformity will be reached. But as this process proceeds, there is an inevitable re shuffling of all factors, and so it is that individuals will appear which are homozygous for recessive characters which are definitely deleterious to their possessor. The weak, sterile, and abnormal individuals which appear amongst the offspring of an inbred line are such as have received during this process of the reshuffling of the hereditary factors those which correspond to undesirable recessive characters, these factors ultimately coming to be present in the duplex state. Such individuals will be eliminated, whilst, on the other hand, others which, thanks to this same process, have come to possess the factors for desirable factors in the duplex state, will show improvement over their ancestors. Rigorous selection from amongst these improved types will isolate strains that will compare very favourably with the original stock.
If inbreeding results in disappointment, all that has happened is that disadvantageous characters, the ingredients of which were previously hidden and unexpressed, have been brought to light. Inbreeding thus purifies a stock. This system of mating may be disastrously expensive if the ingredients of hereditary combina tions which result in undesirable or non-viable types pre-exist. But, on the other hand, such individuals as have been made homo zygous for the desirable characters will be far more valuable material in the hands of the breeder than the stock with which he started, for in virtue of their hereditary constitution they must now breed true. The principal effects of inbreeding are (I) a re duction of variability in the expression of inherited characters within inbred lines or families, and (2) a usual, but not inevitable, decline in general vigour. Line-breeding and family-breeding are merely inbreeding in a less intensive form and may therefore be expected to yield the same results in a longer period of time.
Since the earliest times, animal husbandmen have been familiar with the observed fact that the first crosses of different breeds or races of the same kind of farm animals commonly exhibit a re markable sturdiness. This "hybrid vigour" is manifested in dif ferent ways. Frequently the crossbred offspring are stronger, attain a larger size, mature in a shorter time than either parental stock, and not uncommonly possess notable powers of resistance to unfavourable conditions. Livestock breeders are well acquaint ed with this superiority of the first cross for many commercial purposes. As an example, the breeder of cattle will doubtless quote the case of the blue-grey, frosty-coated cattle of Scotland, a com bination of Black Galloway or Aberdeen-Angus with white Short horn, which have long been deservedly esteemed for their large size, goodly proportions and early marketability. The shepherd will extol the qualities of the Blackf ace with Border Leices ter crosses so popular on account of their quick growth and general usefulness. As an example of a wide outcross deliberately made, the case of the mule may be cited. It is better fitted for certain purposes, in virtue of the hybrid vigour that it exhibits, than either parent.
From critical experimental work undertaken to examine the conclusions of practical men, the following facts have emerged: (I) Crossbreds do not invariably exhibit this hybrid vigour. Any and all crosses will not produce offspring of greater excellence than their parental stocks.
(2) In those cases in which hybrid vigour is exhibited by the first cross, further crossing of the hybrids results in a manifest decrease of this vigour in subsequent generations. Hybrid vigour is the peculiar possession of the first cross.
(3) In order to obtain hybrid vigour in any degree, it is essential that the two parental stocks shall be unrelated, pure bred, and that each shall itself be possessed of qualities that, in relation to the object of the crossing, are excellent.
The explanation of these demonstrable facts concerning hybrid vigour is simple. It will be agreed that in general there is a ten dency for the qualities of the parents to be expressed in the off spring. This is the basis for an understanding of the vigour derived from crossing. Every individual is a mixture of desirable and undesirable qualities. In crossing, the qualities of the two parents are pooled ; the deficiencies of one parent can be made good by the excellencies of the other, the good of one can be re inforced by the good of the other. If an individual is deficient in any way in its hereditary constitution, there is a good chance that its needs may be supplied when it is crossed with other indi viduals, since it may well be that their deficiencies are not of the same kind : a pooling of the hereditary resources may easily yield a combination that is better than either of the ingredients alone. Hybrid vigour is based on heterozygosity.
If this interpretation is correct, then it follows that undesirable qualities are inherited as well as desirable, and that in order to secure this hybrid vigour, it is necessary that the individuals used for the production of the first cross shall be as excellent as may be and that the good qualities of the two shall be, as far as is possible, complementary. The qualities that are concerned are not necessarily those of form and structure but are physiological, concerned with the kind and rate of functional activity, and so it is that hybrid vigour, as great as that which results from the crossing of different species or breeds, can follow the crossing of different families or strains of one and the same variety or breed. The breeder who keeps his family lines distinct can, by appro priate matings, secure all the hybrid vigour he seeks, without call ing on the aid of other stocks.
Since for the production of hybrid vigour the qualities of the two parties must be compensatory and complementary, it follows that all matings cannot be expected to yield it. Experimentation alone can decide whether a particular mating will be attended by hybrid vigour in the offspring. Since it is the pooling of qualities that leads to hybrid vigour, and since breeding implies the assort ment and redistribution of these qualities among the offspring, it follows that the further breeding of the first cross can lead to reduction in general sturdiness. The mule has kept its reputation because in the great majority of cases it is infecund, incapable of producing functional ova or sperm. Without the purebred there cannot be the crossbred of any worth. The first cross, deliberately bred for a definite commercial purpose, must not be used for further breeding.
Thus it appears that the principles of animal breeding are as follow : (I) The definition of the ideal type in relation to habitat and destiny.
(2) The definition of the shortcomings of already existing types in relation to the ideal.
(3) The improvement of the already existing types by contin ued selection in the direction of the ideal.
(4) The selection of animals for breeding as far as possible on the basis of the progeny test ; individual merit and pedigree also to be considered.
(5) The system of inbreeding to be followed to as great an extent as is practicable.
(6) The system of outcrossing to be practised for the produc tion of special types for purposes other than for further breeding.
(See HEREDITY; VARIATION IN NATURE; SELECTION, ARTI FICIAL; INBREEDING; REPRODUCTION ; CATTLE; HORSE, etc.) BIBLIOGRAPHY.-J. A. S. Watson and J. A. More, Agriculture: The Bibliography.-J. A. S. Watson and J. A. More, Agriculture: The Science and Practice of British Farming (1924); F. A. E. Crew, Animal Genetics: An Introduction to the Science of Animal Breeding (1925, bibl.). (F. A. E. C.)