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

SELECTION BASED ON PROGENY TESTING

The idea of progeny testing is not new, having been advocated 2000 years ago by Roman Varro. Robert Bakewell is reported to have used in the eighteenth century by letting out bulls and rams on an annual basis. Then he could later use those which proved to be outstanding transmitters.

• Individuality tells us what an animal seems to be,
• his pedigree tells us what he ought to be,
• but the performance of his progeny tells us what he is.”

This progeny testing is used to rate a sire or dam’s breeding value. It attempts to evaluate the genotype of an individual on the basis of its progeny’s performance. It is the best way of determining the genetic make up of an individual. Each parent contributes sample halves of genes to each offspring. Thus an effort to evaluate an individual (usually a male) on the basis of one or a few offspring can be misleading. Chance at segregation may result in any one or a few offspring receiving a better or poorer than average sample of genes from the parents. Progeny testing is a technique generally used for males because they are responsible for more progenies in their lifetime than any one female. Use of progeny test is not a very practical preposition to establish the breeding value of females, since the number of offspring per female is small. When the individual produces sufficiently large number of offspring, the individual has already completed its productive life and the need for selection will be already over.

It is very important that all of the progeny and not just a selected sample of the progeny be included in the progeny test appraisal. Omitting the poor progeny is unfair and misleading because, similar poor progenies are just as likely to be produced among the next group of progeny. Progeny testing may be used in selection of traits expressed in both traits. When heritability is low, fewer progenies are required to make the progeny test. However the accuracy of progeny test is reduced when there is an environmental correlation among progenies due to non-genetic factors. This situation arises when several progeny tested sires are being compared, but their progeny had been tested at different locations. Feeding and management also influences the progeny group differences. These will reduce the accuracy of progeny testing. Progeny testing are conducted to compare the performance of progeny of two or more parents. Usually sires rather than dams are progeny tested because generally sires produce more progeny in a given season or year.

Use of progeny test depends upon

• Accuracy of the test.
• The number of sires to be tested during specified period of time.
• For greater accuracy greater numbers of offspring are needed. If more number of offspring has to be produced, then large numbers of females have to be mated, thereby reducing the number of bulls tested.
• Progeny testing is carried out based on the assumption that most of the inheritance in the livestock is due to additive genetic effects. If there are sizeable dominant and epistatic effects, then the following to be accounted i.e. whether the offspring’s performance is due to additive genetic effects alone or is due to dominant and epistatic effects.
• If some offspring of a male mated to certain set of females, perform better than offspring of the same male mated to another set of females. Then a male and female that produce better averages in the offspring will be chosen to exploit dominant and epistatic effects over and above additive effects.
• In livestock breeding, progeny test based on more than five unselected offspring usually reduces the chances of error considerably. With traits having very low heritability, large number of offspring (10 or more) has to be used to get a reliable progeny test. The rapid acceptance of artificial insemination and the advancement of techniques for the freezing and storage of bovine semen have greatly extended the use of outstanding progeny tested sires.

Points to be considered

• Test as many as sires possible (5 to 10 would be minimal)
• Make sure that dams are mated to sires at random, within age group is possible.
• Produce as many progeny per sire as possible (10 to 15 progenies of either sex for growth traits but up to 300 to 400 progeny is required for traits like calving difficulty and fertility).
• No progeny should be culled until the end of the test.
• Offspring that are being tested are not a select group.
• Performance of an adequate sample of an animal’s progeny under normal environmental conditions will give a true indication of its genotype than any knowledge of individuality or pedigree.

Precautions to be taken to make progeny tests more accurate

• Dams mated to all sires on a given progeny test should be selected randomly.
• Feed all animals the same ration and in same manner to avoid bias.
• Compare different parental groups raised in as nearly the same environment as possible.
• Compare the parent groups born during the same year or same season of the year when possible.
• Include all healthy progeny of a particular parent in the test, if possible whether they are inferior or superior. This tends to average the Mendelian and environmental errors for each sire group.
• Pens should be rotated among progeny groups to reduce the pen effects.
• Larger the number of progeny tested per parent, within limits, the more accurate the estimate of that parent’s probable breeding value.
• Errors like effects of year, season and location should be eliminated as far as possible.

The accuracy of selection that is the correlation of the genotype of the parent with the average genotype of its progeny may be calculated as:

PBV = h / 2 Ö n /1 + (n-1) t

Where,

h – square root of heritability

n – number of progeny per parent used in the average

t – ¼ h² if progeny group is composed of half sibs and there is no environmental correlations between sibs.

Testing of progeny at several locations using artificial insemination and adoption of comparison of performance with contemporary animals can increase the accuracy.

Advantages

• For selecting sex limited traits.
• For selecting traits require sacrifice of the animal (carcass traits)
• For selecting traits expressed late in life
• For traits having low heritability value.
• For selection of animals that nick or combine well.
• For testing animals for recessive traits.

Limitations

• More number of animals must be progeny tested.
• It prolongs the generation interval. Hence it is time consuming and expensive
• Use of superior animals extensively once they have been located and errors due to environment that are not standard for the progeny are more serious limitations.
• Sires can be selected only when the progenies come for production and by the time the sire may become old and useless. Therefore, the annual rate of genetic gain is lowered.
• Hence it is time consuming and expensive

In conclusion, Progeny testing is estimating the breeding value of a sire based on the average performance of its offspring. Each offspring receives a sample half of genes from the sire. Therefore, the performance of large number of daughters will indicate the breeding value of sire on progeny testing. Progeny testing is usually conducted for males as more number of progenies can be produced for males and also proven bulls can be extensively used for production of more number of progenies. The primary selection of the bulls is based on the sibs’ average. The bulls with highest averages are selected and included in the progeny testing. Then the bulls are used on many females to produce many progenies. The performances of progenies are then studied to estimate the breeding value of each bull. It is the best way of determining the genetic makeup of an individual. The genetic principle behind progeny testing is that the more the number of progeny are tested the greater the accuracy of assessment of the parents, since the errors in sampling are reduced.