CONTENT
Selection Methods in Poultry: Pedigree, Combining Ability & Multi-Trait Selection
Selection is a fundamental component of poultry breeding used to improve economically important traits and enhance the genetic merit of breeding populations. Different selection methods are used depending on the trait being improved, its genetic basis, heritability, and the relative importance of other traits.
Major methods include pedigree selection, selection for specific combining ability, recurrent reciprocal selection, tandem selection, independent culling, and selection index methods.
Pedigree Selection
This type of selection is based on the phenotypic merit of the ancestors. The deviation of the phenotypic value of an ancestor from the average value of its contemporaries provides an indication of its genetic value for breeding. The record containing details of the ancestors of an individual, such as identification number, name, registration number (if any), and the phenotypic value of the trait of interest, is called a Pedigree Record.
These pedigree records are more useful if performance is also recorded in terms of trait ratios. This gives a more accurate picture of how well an ancestor performed when compared with its contemporaries.
Selection of an individual for a qualitative trait involves examination of its pedigree records for cleanliness. For example, when a flock is selected against a recessive character, a family devoid of this character is termed a ‘clean family,’ whereas one in which this particular trait is expressed in any ancestor is called a ‘dirty family,’ which should be culled and excluded from selection.
When pedigree selection is considered for a quantitative trait, records of the performance of ancestors make selection more accurate than when it is practiced based only on the individual’s own value. This information provides some indication of the relative merit of an individual’s ancestors compared with their contemporaries.
In this process, the phenotypic value of a close ancestor is given more weight than that of a distant ancestor. The probable breeding value of an individual subjected to this kind of selection is based on its own phenotypic value plus those of its relatives.
Selection for Specific Combining Ability
This kind of selection is considered for traits influenced substantially by nonadditive gene action; therefore, hybrid vigor is expected. Nonadditive genetic action may involve dominance and interaction (overdominance and epistasis).
If dominance is an important gene action in determining a trait, individual selection may be useful in improving the character. However, individual selection is less effective in the case of interaction gene action such as overdominance.
As discussed earlier, heterozygosity is better than homozygosity in the case of overdominance. However, heterozygous individuals do not breed true; therefore, selection cannot be made based on phenotype alone. The best option is therefore to develop two specialized lines in which the loci influencing that trait are made homozygous and fixed, but for opposite alleles.
Therefore, when crossed, the alleles at different loci complement each other and ‘nick’ well to produce heterozygosity in the hybrid. Parents that nick well can be identified only by cross-progeny performance. This kind of selection is called Recurrent Reciprocal Selection.
Recurrent Reciprocal Selection
Recurrent reciprocal selection is practiced to increase the combining ability of two or more lines or breeds, more commonly two lines that have already demonstrated through past crosses that they ‘nick’ or combine well.
The principle involved is to refine these two populations for homozygosity at most of the loci, but in opposite directions. Crossing the lines and selecting the individuals to reproduce each pure line based on the performance of their crossbred progeny will increase homozygosity, but in opposite directions in the two lines.
Methods of Selection for More than One Trait
Farm animals are often selected for more than one trait at a time. The traits of high economic importance in poultry include egg production in layers and body weight in broilers. However, egg-type chickens, apart from egg production, are also selected for egg weight, shell thickness, etc.
Similarly, broiler-type chickens are selected not only for body weight and growth rate, but also for conformation, carcass quality, reproductive ability, viability, etc.
1. Tandem Method
In this method, selection is practiced for only one trait at a time. After attaining satisfactory improvement in the first trait during the first phase, selection efforts are directed toward the improvement of the second trait, then the third trait, and so on in subsequent phases.
This is the least efficient of the three methods from the standpoint of the amount of genetic progress per unit time.
2. Independent Culling Method (ICL)
In this method, selection is practiced for two or more traits simultaneously, but a minimum value or culling level is fixed independently for each trait.
Only those animals that meet the minimum requirements for all the traits are selected for breeding purposes; the others are summarily rejected.
3. Selection Index Method
In this method, a total score or index value is determined for every individual animal based on the values for the various traits involved. I = b1X1 + b2X2 + b3X3 ………. + bnXn. The amount of weight (b1, b2 … bn) given to each trait depends on its relative economic value, since all traits are not equally important in this respect, as well as on the heritability of each trait and the genetic associations among the traits.
The influence of each trait on the final index is determined by how much weight that trait is given in relation to the other traits. Animals with the highest total scores are then retained for breeding purposes. The selection index is more efficient than the independent culling method because it allows individuals that are superior in some traits to be retained for breeding purposes even though they may be slightly deficient in one or more of the other traits.
If an index is properly constructed, taking all factors into consideration, it is a more efficient method of selection than the tandem and ICL methods because it should result in greater genetic improvement per unit time.
