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

TYPE

It is the outward confirmation of individuals i.e. the relative proportion, length, breadth and size of different parts of the body that include colour, size and shape of horns, ears etc.

Selection depend on type is inevitable when

• Reliable records of production are not available.
• Selection is to be made early in life before the availability of production records in order to reduce the cost of culling.
• When records are available in only one sex as milk yield, males have to be selected only as type.
• When production records come after the death of the individual e.g. Meat animals.
• Where productivity is not easily and completely measured as in works and speed.
• When market demands a particular type that is more profitable.
• Where beauty is the main consideration as in pet and fancy stock.

Production

• This needs accurate production records for all animals under selection. But the actual records available are varying when comparable with one another. E.g. In dairy cattle milk production in lactation is significantly correlated with lactation period and age of the cow. Dairy cows gradually increase their yield till 6^th or 7^th lactation and then decline.
• Similarly in sows, they produce more piglets than gilts, do owing to an age effect on fertility. Hence, it is necessary to standardize all the records to a uniform comparable basis. E.g. In cows – milk production should be adjusted to 305 days, 6 years and 4% fat.
• Similarly in sows all furrowing records should be adjusted to an equal gilt basis by correction factors. Then only the figures will be comparable. Average of many records will reduce the environmental variations in production. While using record all the available record should be used and not the selected ones. No records should be omitted except when circumstances are so definite that no doubt exists e.g. Foot and mouth disease, abortion etc. A poor setback in health should not be omitted as in itself is an indicator of poor genetic constitution and conducive for high production. Incomplete record should not be considered. Constitution, longevity, disease resistance, adaptation to climate is the other factors that should be considered in production and selection.

Selection for qualitative traits

• Here the animals are kept or rejected for breeding purpose on the basis of its own phenotyp0e for that particular trait. The progress made in selection depends on how closely genotype is correlated with phenotype. Phenotype of the individuals is often used to estimate the breeding value for qualitative traits such as colour and horned or polled conditions. Selection for such traits based on mass or phenotype is more effective than others.
• For e.g. In Angus cattle the coat colour Red (rr) is recessive to dominant black (BB) colour. But it is practically difficult to distinguish and differentiate the genotype BB and Bb phenotypically. Thus selection on the basis of individuality will be useful but not always completely accurate.

Selection for quantitative traits

• Quantitative traits are controlled by many genes and various environmental factors. There is no sharp distinction among the phenotypes and affected by both additive and non-additive gene action. No trait is 100% heritable, because the environment always affects the phenotype to a certain extent. Therefore phenotype of an individual for quantitative traits is not the true indicator of genotype. The phenotypic merit of the individuals for quantitative traits is determined by comparing the individual’s own phenotype with that of the average of all the individuals within a group from which it is selected and is called trait ratio.

Trait ratio = Individual’s record for a trait / group average for the same trait x 100

• Accurate records are also required. The individual’s record is of little value unless it shows where the individual ranked relative to others under similar conditions. The environmental part of phenotypic superiority or inferiority will not be transmitted to the offspring. Therefore in general there is tendency for the average phenotype of the offspring of a phenotypically superior individual will tend to regress toward the average of the population, whereas the average phenotype of the offspring of a phenotypically inferior individuals will tend to rise toward the average of the population.