Selection in Poultry: Types and Principles of Selection

Selection in Poultry: Types and Principles of Selection

Selection is an important tool in poultry breeding for improving desirable traits and overall productivity in a population. It plays a key role in genetic improvement by determining which individuals contribute to subsequent generations. Selection may be natural or artificial and can be practiced using several approaches, including individual selection, family selection, progeny testing, within-family selection, and combined selection. The choice of selection method depends on factors such as trait heritability, available performance information, generation interval, and breeding objectives.

Selection means that individuals possessing desirable traits are allowed to produce the next generation. Thus, individuals of certain genotypes may be retained in larger numbers for reproduction than others. This causes an increase in the frequency of desirable genes and a decrease in the frequency of undesirable ones. An increase in the frequency of desirable genes also results in an increase in the number of individuals homozygous for the desirable genes.

Selection can be:

• Natural
• Artificial

Artificial selection is that practiced by humans, whereas natural selection is that carried out by nature through the death of less viable individuals or their lower reproductive rate.

Natural Selection

In nature, the main force responsible for selection is the survival of the fittest in a particular environment. Natural selection is a complicated process, and many factors, such as livability, sexual activity, and fertility, influence the proportion of individuals that reproduce.

Artificial Selection

Artificial selection is practiced by humans by selecting individuals with superior performance to produce superior offspring. In this effort to improve a character in a population, the frequency of genes or combinations of genes in the herd or flock responsible for that character is also increased.

Deliberate selection is one by which humans try to change the values of a population through selection. In the process of deliberate selection, natural selection also plays a role that is not under the control of the breeder. This is called automatic selection, which results from the fertility and viability of the selected individuals.

Individual Selection

Individuals are selected solely in accordance with their own phenotypic values. Individual selection can be applied to both qualitative and quantitative characters. This method is usually the simplest to operate and, in many circumstances, yields the most rapid response. Mass selection is a term often used for individual selection, especially when the selected individuals are put together en masse for mating, as, for example, in flock mating in poultry. The term “individual selection” is used more specifically when the matings are controlled or recorded. For example, pen mating with trap nesting in poultry.

Family Selection

Whole families are selected or rejected as units according to the mean phenotypic value of the family. In other words, within-family deviations are given zero weight. The families may consist of full sibs, half sibs, or individuals with a more distant relationship. The chief circumstance under which family selection is preferred is when the character being selected has low heritability. The efficacy of family selection rests on the fact that environmental deviations among individuals tend to cancel each other out in the mean value of the family. Thus, the phenotypic mean of the family comes close to being a measure of its genotypic mean, and the advantage gained is greater when environmental deviations constitute a large part of the phenotypic variance, or, in other words, when heritability is low. There are two practical difficulties in the application of family selection: high inbreeding and low selection intensity.

Selection on the Basis of Collateral Relatives

Collateral relatives are those not related to an individual as ancestors or progeny. Thus, they include the individual’s sisters, cousins, uncles, aunts, etc. Close relatives can provide more information about an individual than distant relatives. This can also be called family selection. When selection is based only on information from sibs, the selection is called sib selection. Sib selection may involve full-sib or half-sib family selection, depending on the relatives providing information for selection. The distinction in the use of the term sib selection is that the selected individuals are not measured, whereas in family selection, they are measured and included in the family mean.

When broiler parent stock is selected for a carcass trait, such as breast yield, individual selection is not possible because measuring this trait by slaughtering the individual itself renders it unavailable for selection. Therefore, family selection is the only possible way of making selection for such traits. Similarly, when selection is made for a sex-limited trait, information from relatives of the opposite sex, in which the character is expressed, is used. For example, selection of males for egg production, a trait that can only be measured in females, can be performed using sib selection, i.e., based on the egg production performance of females in their half-sib families.

Progeny Testing

The criterion of selection in this method is the mean value of an individual’s progeny. This is the ideal method of selection and the easiest to evaluate because the mean value of an individual’s offspring comes as close as we can get to a direct measure of its breeding value. In practice, however, it suffers from the serious drawback of a much-lengthened generation interval because the selection of the parents cannot be carried out until the offspring have been measured.

Within-family Selection

The criterion of selection is the deviation of each individual from the mean value of the family to which it belongs, with those that exceed their family mean by the greatest amount being regarded as the most desirable. This is the reverse of family selection, with family means being given zero weight. The chief condition under which this method has an advantage over the others is a large component of environmental variance common to members of a family, i.e., high environmental variation between families. This method of selection can provide a high effective population size from a minimum total population, thereby reducing housing space requirements. Variation in the population is largely maintained.

Combined Selection

The optimal procedure for selection uses all the information available about each individual’s breeding value, combined into an index that gives different weights to the information sources. However, when there are more than two sources of information—for example, information from the individual, its parents, full sibs, half sibs, and other relatives—a multi-source index can be constructed from these data as follows.

I = b1P1 + b2P2 + b3P3 + ……….. + bnPn. The aim, therefore, is to combine all the information into an index on the basis of which the individuals will be selected.

This method of selection is practiced for improving lowly heritable traits such as egg production. E.g., Osborne’s index selection.