AGB 211: Livestock and Poultry Breeding (2+1)

PROBABLE BREEDING VALUE

• The accuracy of selection on the basis of collateral relatives depends upon the degree of heritability, closeness of the relationship ® of the sibs and individual being selected, number of sibs used to determine the sib average, degree of correlation (t) between the phenotypes of sibs.

Accuracy of selection = Rh Ö n / 1 + (n-1) t

• If environmental correlation among the phenotypes of the sibs are zero, then t = Rh²
• The accuracy of selection increases as the records on a large number of half sibs are considered and as the heritability increases. The accuracy of selection never exceeds 0.5, regardless of the number of half-sibs tested and the degree of heritability of that trait.
• Nearly 30 half sibs are required to give the same accuracy as information on the individual’s own record when heritability is as low as 0.10 and 100 or more when heritability is higher than 0.10. The addition of the record of another half sib is affected by the law of diminishing returns. However in instances where information cannot be obtained from the individual, such as sex limited traits can be used effectively in selection.
• Full sibs may be used in selection, but they have a similar maternal environment from conception to weaning lowers the accuracy of their use for such a trait. The selection on the basis of individuality is relatively more accurate than selection on the basis of full sib records when the trait is highly heritable. However, when heritability is low, and records on six or more full sibs are available then selection on the basis of full sibs is more accurate.
• The combination of records on the individual and its sibs for selection is more advantageous than records on the individual’s own performance when R and t are greatly different. It is more useful when difference between families are mainly due to environment possibly because different families have been treated differently.

Families can be broadly classified into three types:

• Sire families: These are progeny of one sire.
• Out of different dams – born in the same year (contemporaries)
• Out of different dams – born over a number of years
• Dam families
• By different sire – born in the same year i.e. by super ovulation before artificial insemination with mixed semen from number of sires and identification of sires by blood typing
• By different sires – born over a number of years
• Sire and dam families: These are progeny by one sire out of one dam.
• Family selection is more effective when the genetic relationship between members of the same family is large, and the phenotypic relationship between members is small. When heritability is low, the use of family data is most valuable as it reduces the chances of making wring decisions.

Indications

• For sex-limited traits,
• For carcass traits and
• For traits of low heritability.

Limitations

• If selection intensity is more, then there may be an increase in inbreeding and
• Increase in cost and space in raising larger population.

Precautions

• Number of progeny in each family should be large and
• There should not be common environment between sibs.