What are Australian Sheep Breeding Values (ASBV)?
Genetic improvement programs aim to improve specific economically important traits in a flock through breeding. The most effective support tool to generate change is to make use of ASBV.
ASBV describe a particular sheep’s breeding value for a certain trait, for example fleece weight, body weight or fibre diameter, and express the relative breeding value of sheep across different breeding flocks of that breed.
ASBV are available for many traits such as growth rate, wool, reproduction, internal parasite (worm) resistance, dags and body conformational traits such as wrinkle and breech cover. ASBV are directly comparable across flocks within a breed.
The first step before using ASBV is to define the breeding objective — where genetic change is to be directed. This will determine the traits that are to be improved and their relative importance.
Within any group of sheep, considerable differences can be observed from one animal to the next. Some animals are larger than others, some have more wool and some have finer wool. These differences are called variation. Variation between sheep is the result of:
- the animal's individual genes
- the environment in which the animal was raised and produced.
These two sources of variation form the total variation which is called the 'phenotypic variation'. The phenotype of an animal is thus the sum of all genetic and environmental factors that impacted on a trait.
Genetic factors
Only identical twins have identical sets of genes, and all other animals have different sets of genes which impact on different traits. Animal geneticists don’t presently have a clear idea as to which genes influences which trait, but in future it will be possible to determine this. Currently technology has not advanced that far to know which genes influences which trait and by how much. Thousands of genes may be implicated for a trait and this is referred to as the genotype of an animal, and the sum effect of all the genes affecting a specific trait is measured by the ASBV value.
It is important to note that each parent passes half of their genes on to their progeny. However, the half that is passed on is not the same for different siblings. Therefore, siblings inherit different sets of genes from their parents and that is why their ASBV can be different.
As siblings will have more of their genes in common than unrelated animals, this information is used to increase the accuracy of ASBV. Some sources of information that will increase the accuracy of breeding values include:
- individual performance records
- several records on the same individual
- ancestors’ records
- half-sibling records.
Environmental factors
Different genotypes result in genetic variation between sheep, but these genetic differences can be obscured by large environmental effects such as feeding levels, management and a multitude of other environmental factors. To reduce the effect of these large environmental factors, it is important that the sheep to be compared are managed the same and in one group to ensure that all receive the same feeding and management as much as possible.
In addition, other factors that can have a major effect on an animal’s performance should also be measured. The most important factors are:
- rearing type (reared as a single or twin)
- birth date (to determine its age at measurement)
- age of its dam (older ewes lambs perform better than that of maiden ewes).
Having this information will allow corrections to be made to the different measurements to get a more accurate indication of its genotype.