WHAT is EBV, you are thinking, and, how can it help me!
In animal breeding we like to rank and select animals based on their pedigree.
This is the worth in terms of economic value, of an individual animal’s genotype,as judged by the average performance of its offspring. From the size of the breeding value,it is possible to choose suitable parents for future crosses.
This knowledge assists livestock farmers to generate good cattle stock breeds.
Given we cannot ‘see’ genes and breeding values, we must use observed phenotypes to obtain estimated breeding values (EBVs).
EBVs, which are underused, are an essential selection tool. It is a value which expresses the relative difference between an individual animal and the herd or breed benchmark to which the animal is being compared.
The values can be positive (+) or negative (–), depending on whether the value for an animal is under or over the ‘base’, which is constant and set at zero.
The current breed average is often a more important benchmark than the base. The most obvious piece of phenotypic information we can use to estimate an animal’s breeding value is the animal’s own phenotype. But information from relatives, such as the sire, the dam, siblings and progeny can also be used.
For example, individual’s cattle performance + performance of relative + linked traits X heritabililty = EBV.
Although this is a simplified description of what is involved in outlining the principle, the actual calculations are more complex and beyond what a farmer needs to know or understand.
Heritabilityis the term used to describe how strongly a characteristic is transferred from parents to progeny and is rarely considered. It is the proportion of an animal‘s production that comes from its genetics and varies between traits.
The higher the value, the faster genetic improvement can be made and positive benefits observed. EBVs are reported in terms of actual product, that is, days, kg of weight or mm of fat depth, among others.
It is important to remember that each parent contributes half the resulting calf’s genetic make-up.
Genetic change due to selection based on EBV will be lower than if selection had been on true breeding value; with the relative response being proportional to accuracy of EBV – usually between 0 and 1.
EBVs are not a waste of time and money but an estimation of how the progeny of an individual should perform and to bring all the cattle within a herd or breed onto a common platform so that those making selection decisions can know they are comparing like with like regardless of the system or location they are produced in. This performance estimation measures the parts of the animal that the eye cannot see. An animal’s physical appearance (phenotype) is determined by two components — its genetics (genotype) and environmental (non-genetic) influences, where it is bred.
The influences include grass quality, disease burden, parasites, supplementary feeding and management ability, among others. The environment component is a part that herd keepers have influence over and the ability to compensate for or enhance.
Despite the environmental influences, the genotype remains the same and parents do not pass on to their progeny the environmental effects that have influenced them. EBVs measure this genetic component and allow cattle within their respective breeds to be compared, excluding their management and environment.
Although what you see is not necessarily what you get, given that no two animals are the same, the more information that can be collected, the more one can account for the variation between individuals; so the overall risk of using them can be reduced. EBVs draw information from a number of sources in addition to the animal’s own performance. The basis is comparing individuals within a contemporary group, one to another, minimising non-genetic influences.
The system also compares contemporaries in other herds where genetic linkages exist through siblings and ancestors.
Linked traits are those which share common connections with each other.
EBV data must usually reach a certain level of accurateness before it can be reported to ensure the information is as relevant as possible and actually reflects the potential performance of the animal in question.
Some traits, such as growth, will not be reported until they reach an accuracy level of 40 percent, which is typically at the lower range of reported accuracies, which can be up to 99 percent. Traits which are more difficult to measure, such as carcass and fertility related, can be reported at much lower levels due to the limited information available, requiring the user to make a judgement decision.
The breeding component will determine how the non-genetic component is used. At low to average levels of performance, feeding and health will have more of a contribution to an animal’s overall performance, but for those trying to maximise returns, the right genetics are vital.
A typical example could be milking ability, where if the genetics for milk are poor, feeding will not make a significant difference, the cow will just get fatter.
Throughout a performance-recorded animal’s life, measurements are taken which are used to produce its EBVs.
These start with a birth weight and calving ease score.
These measurements are supplemented by weights at 200 days, 400 days and 600 days as well as scrotal circumference (bulls only) and ultrasonic measurement of back-fat, rump fat and muscle depth.
At weaning time, dam weights will also be collected to contribute to the mature weight EBV. In the future, genomic data may also be incorporated for the difficult-to-measure traits such as carcass and reproductive characteristics.
Disposition data is collected by some overseas breeds and a docility EBV produced to measure their temperament based on ‘flight time’.
A study carried out in Australia found that ‘flighty’ cattle were gaining 0,4 kg/day less than their docile contemporaries, a substantial amount in today’s challenging climate.
In addition to the information collected from an individual animal, the data from siblings and other family members also makes a valuable contribution to the overall data. There are many factors involved in the production of quality EBVs, but it is useful to mention a few of the factors that lead to poor quality EBVs as well.
The most important principle of performance recording is that of grouping the cattle together for the comparison.
This is the biggest challenge for the system to overcome and depends on accurate record keeping and submission of raw data. Where the same sire is used on the same cows year-after-year, or where there is only a small number in the management group, there is obviously a lack of genetic diversity. This limits the amount of variation that can be accounted for.
Most cattle houses and breeding farms overseas request an EBV when selling or buying cattle; such data is made easier these days with the advent of computerised records.
Dr Tony Monda holds a PhD in Art Theory and Philosophy and a DBA (Doctorate in Business Administration) and post-colonial heritage studies. He is a writer, lecturer, musician, art critic, practicing artist and corporate image consultant. He is also a specialist art consultant, post-colonial scholar, Zimbabwean socio-economic analyst and researcher. E-mail: tonym.MONDA@gmail.com
WHAT is EBV, you are thinking, and, how can it help me!