upon that individual with regard to genetic contribution in the absence of inbreeding. Pedigree information is useful for the selection of young animals prior to the development of their individual mature phenotypes. Pedigrees are also a means of identifying superior lines or families providing good records are maintained. A major disadvantage of pedigree selection is that pedigrees are often popular because of fad and not because of the phenotypic merit of the individuals it contains.
Considerable livestock selection is practiced on the basis of information or records of collateral relatives. Collateral relatives are related because of common ancestry rather than direct descent. The closer the relationship, the more valuable the information to selection procedure. Full sibs and half sibs are collaterally related and may provide much useful information for the selection of young animals. Selection by progeny tests means that we evaluate breeding value, primarily among sires, of individuals on the basis of the merit of the traits possessed by their offspring. It is very important that these comparisons be fair and accurate by using random samples of progeny. Selection of this nature is necessary for traits which are lowly heritable. Considerable selection of this kind has prevailed for many years in the breeding of Thoroughbred horses.
Most selection practiced in animal breeding is accomplished through sire selection. Horse breeders select for several traits simultaneously. This limits the amount of selection pressure which can be exercised on a single trait. We should place primary emphasis on those traits which are of greatest economic importance and have the highest heritabilities. Selection may be based on several criteria. Most young animals are selected initially on the basis of pedigree and records of close collateral relatives. Individuality becomes important at which time phenotype becomes mature or is actually established. Following reproduction, progeny information becomes available and selection continues on the basis of progeny. Good records on animals will materially aid all aspects of selection.
ANIMAL BREEDING PLANS
Animal breeding plans are systems which may be employed to facilitate greater control of heredity than if selection alone is used. These plans or systems are based on the degree of relationship existing between individuals which are mated for reproduction. Discussion of these animal breeding plans will be restricted to a within-breed basis.
The two main types of animal breeding plans employed by breeders are inbreeding and outbreeding. Linebreeding is mild inbreeding.
The mating of animals that are more closely related than the average of the breed is termed inbreeding. The parents of an inbred individual will ordinarily possess common ancestors as indicated on pedigrees extended to four generations. The mating of animals that are less closely related than the average of the breed is termed outbreeding. The parents of an outbred individual will not possess common ancestors as indicated on four generation pedigrees. If breeders plan matings with no regard to relationship, the plan( or lack of plan) comprises what is known as random mating. This method exists in nature but is seldom exercised by breeders because they nearly always give some attention to animal relationship in planning matings.
Inbreeding includes full sib matings and parentoffspring matings. Usually, no special effort is made to retain a high degree relationship between the inbred and one or more of the common ancestors of its parents. The plan may continue beyond a single generation.
The primary genetic effect of inbreeding is that of increasing the chance that an animal will inherit the same genes from both of its parents. This facilitates genetic homozygosity and prepotency. The trend toward genetic homozygosity facilitates the " fixing of traits." The fixing of desirable or undesirable alleles is equal by chance under the influence of inbreeding. Selection can alter gene frequencies but inbreeding does not. Inbreeding accompanied by selection comprises one of the most powerful tools available to the animal breeder.
Rapid inbreeding over several generations is certain to cause some deterioration in size, vigor, and fertility. Many breeders avoid it, even to a slight degree. The success of an inbreeding program depends upon( 1) the genetic merit of the foundation animals;( 2) rate or intensity of inbreeding; and( 3) possibilities and efficiency of selection. Inbreeding should usually be restricted to superior stock or herds. While it is practiced, the herd or line is closed to outside sources of genetic material which limits the germ plasm available for breeding improvement. While inbreeding presents some hazards, it is a highly effective animal breeding plan and has been used by outstanding livestock breeders who have achieved true breeding improvement. This is true in horses as well as other classes of farm animals.
There are no precise criteria for evaluating the advisability and extent of inbreeding prior to its adoption. A breeder must make these decisions as a result of trial.
Linebreeding is inbreeding, usually to a mild degree, practiced with the intent of maintaining a high degree of relationship of descendants to one animal of merit. The animal of merit will appear as the common ancestor of the linebred ' s parents. The plan is accomplished by maintaining a high degree of relationship between each parent and the common ancestor but otherwise not related to each other.
Outbreeding is the most prevalent animal breeding plan for livestock in general. Breeders employ it with the intent of introducing superior germ plasm into their herd from unrelated outside sources. Outbreeding increases genetic heterozygosity in a herd
and prevents the formation of distinct lines within breeds. Breeders who outbreed tend to follow the " random drift " of the breed with reference to the genetic merit of their herd. Because of its genetic effects, outbreeding facilitates reproductive efficiency, increased growth rate, and increased vigor.
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VOICE of The Tennessee Walking Horse