Breeding Values are tools used for assessing the relative genetic worth of different animals in the same flock or herd. They are always expressed in terms like kilograms of liveweight or fleece weight, or microns of fibre diameter. They incorporate all the relevant genetic information about an animal, compressing it into a single, easily understood, easily applied figure. It places a very powerful selection tool in breeders’ hands.
The Breeding Value (BV) predicts ‘the value of an animal as a parent’. Thus a BV describes how good an animal is in terms of the likely merit of the animal’s progeny. Be aware that a BV is primarily a predictor of progeny performance, using all available sources of information, rather than a direct measure of the animal’s own performance.
Two components are used in calculating Breeding Values. Direct performance for the trait in question and indirect performance for genetically related traits. Both use performance figures on the animal and its relatives which have been adjusted to remove environmental factors. These components are combined using appropriate weighting factors which modify the predictors. All BVs can be expressed in a simple equation: BV= Sum (Predictor x appropriate Weighting factor).
These measure how good the animal is compared to its fellows. The value used for this measurement is the amount of deviation from the average. This is derived by first calculating the average of a particular trait for the group, then calculating the difference between each record and this average.
The amount of variation within a flock or herd fora particular trait may also be taken into account. This involves calculating the standard deviations, comparing them with the standard value for that flock or herd, and if necessary providing a further adjustment to the record.
2. The weighting factor
This is also referred to as a reliability factor and is a measure of how much you can rely on a predictor as an estimate of the animal’s By. Animalplan, and any good recording programme, draws on as much information as possible to make the weighting factor accurate. Accordingly several different types of information can be used in calculating weighting factors. They are:
• Genetic correlation
• Number of records on relatives
• Phenotypic correlation
Values for these different factors are drawn from relevant national research data, and continuing analysis of Animalplan data.
Both the direct influence of a trait on an animal’s performance and the indirect influences arising from genetic correlations are taken into account in calculating weighting factors. Genetic correlations occur where the same genes affect two different traits. If the traits are positively correlated both improve together. If they are negatively correlated, one will improve as the other deteriorates. Fortunately most economically important traits are generally favourably correlated. For example hogget liveweight and hogget fleece weight.
Heritabilities of different traits are taken into account when deciding how much emphasis to give to their direct and indirect influence. For example fertility is weakly inherited, but it has a good genetic correlation with hogget liveweight which is strongly inherited. Because fertility is so economically important, in calculating the Breeding Value for it, hogget liveweight is added into the equation, thus using indirect selection to speed up progress.
Information on the performance of relatives, if it is available, will further improve the calculation of BVs. When a recording programme first begins obviously there are no relatives to draw on, but as the years of records accumulate, they provide the data to help make selection decisions more accurate.