“Poultry Selection: Basis and Breeding Methods”
For highly heritable traits like body weight and egg weight selection of individuals is adjudged best on the basis of their phenotypic performance; it yields satisfactory results. This is known as individual selection. Selection on the basis of family averages, i.e. family selection, is advocated for improving lowly heritable traits Larger the family size, better is the estimation of breeding value and hence response is better. Sire family selection is preferred over dam family selection for traits of low heritability like egg production. Dam family selection may have advantage over sire family selection only when the reproductive rate is high. Combining information on the individual phenotypic value and its sire and dam family averages and other relatives with appropriate weights attached to each of the components increases the accuracy of selection, and has been found to be superior to individual as well as family selection as it combines the advantages of both individual and family selection. This method of selection is known as combined selection. For sex limited traits the selection of the individuals of the sex not expressing the trait is done based on the performance of the relatives like full or halfsibs of other sex. This is called sib selection.
Selection of parents based on average performance of progeny, called progeny testing, results in greater response than the selection methods presented above. Progeny testing has been effectively used for the development of high producing commercial egg laying strains. It is not very popular at present since it increases the generation interval and reduces the progress per unit of time compared to family selection.
Selection Methods
Three selection methods, viz. tandem selection, independent culling level selection and Index selection have been described in the literature for simultaneous improvement of several traits.
Index selection has been shown to be superior to tandem and independent culling level selection for simultaneous selection of several traits to optimise genetic gains. Each of the three selection methods or a combination of them however has often been used. Culling level selection in which culling level is fixed for each trait is easier to practice and requires less rigid assumption than index method. Culling level selection also permits selection at different stages for different characters and helps in the reduction of breeding costs. Selection on the basis of an index however is a more balanced approach since it combines information on various traits on the basis of their economic importance. Efficiency of the index selection increases as the number of traits to be selected increases but the response in the individual traits becomes less. Tandem selection which involves selection for different traits at different times is least efficient among the three methods.
Selection of individuals within a population to reproduce the next generation using one or combination of the methods presented above is known as intrapopulation or closed flock selection. Two or more such populations are crossed to produce the cross progeny for commercial egg or meat production. Selection in pure strains results in concomitant improvement in crosses is the underlying principle of closed flock selection to improve cross performance.
Selection on cross performance (either on the cross progeny performance or crossbred collateral relatives) is often advocated for development of elite crosses. This method of selection, known as inter-population or interline selection although not a widely used method, has been used successfully for developing high productive strains. Of the two interline selection methods reciprocal recurrent selection is preferred over recurrent selection. Reciprocal recurrent selection increases the frequency of both additive and non-additive genes, hence improves both pure as well as cross-line performance. For obtaining desirable results from reciprocal recurrent selection both populations should be open bred with history of nicking. Theoretical studies have shown beyond doubt the superiority of RRS when overdominance is important. Use of RRS would seem desirable for the populations which have ceased to respond to conventional closed flock selection methods. In recurrent selection, only one of the two populations is purebred and is improved upon in reference to other population which is called a tester or test population. The tester line is either an inbred line or cross of two inbred lines. The success of recurrent selection depends upon the test populations and is practised for utilizing non-additive genetic variance. Commercial crosses in this scheme of selection are produced using the two lines involved in the improvement programme.
In order that selection operates as desired, the population size should be fairly large, inbreeding should be kept to minimum and intensity of selection should be fairly high.