BY PAUL WILLIAMS TBTS TECHNICAL OFFICER
TECHTALK JUNE 2013DOWNLOAD ARTICLE HERE
An important step when making selection decisions using BREEDPLAN Estimated Breeding Values (EBV) is the consideration of EBV accuracy. The following information provides a guide to understanding and utilising EBV accuracy in selection decisions.
By definition, an EBV is an estimate of an animal's true breeding value. The "accuracy" figure produced with each EBV provides an indication of the amount of information that has been used in the calculation of that EBV. The higher the accuracy, the more likely the EBV is to predict the animal's true breeding value and the lower the likelihood of change in the animal's EBV as more information is analysed for that animal, its progeny or its relatives.
The following guide is recommended when interpreting accuracy:
A range of factors influence the accuracy of an EBV including:
The maximum likely change to EBVs at different accuracy levels is described by the confidence range (also known in statistical circles as the standard error of estimate). The size of this value decreases as the accuracies increase. Statistically, there is a 67 % chance that an animal's true breeding value will be within 1 standard error of its EBV, and a 96% chance that it will be within 2 standard errors of its EBV. As an example, table 1 shows the Trans-Tasman Angus BREEDPLAN confidence ranges associated with different accuracy levels for various traits.
For example, a 600 Day Wt EBV with an accuracy of 90% will have a confidence range of ± 8.5 kg. If an animal's EBV is +100 then, with the addition of further information (e.g. progeny or sibling records), the EBV would be expected to still fall within the range of +91.5 kg to +108.5 kg (i.e.100 ± 8.5kg) 67% of the time; and, within the range of +83 kg to +117 kg (i.e.100 ± (2 X 8.5)kg) 96% of the time.
To assist with the understanding of confidence ranges, a graph has been developed when viewing animals within the EBV Enquiry facility on Internet Solutions. This is available for implementation by Breed Societies that have upgraded to ABRI's new generation of breed registry software known as ILR2.
Known as the EBV Standard Error graph, it depicts in graphical form the possible change in an animal's EBVs for each trait. The horizontal bar for each trait displays one standard error either side of the current EBV value, meaning that statistically, there is a 67% chance that the true breeding value for this trait will be within this range.
Figure 2 shows an example Standard Error graph for a young animal of lower accuracy, compared to a proven sire of higher accuracy.
While many beef producers look at EBV accuracy in relation to an individual animal, it is also worthwhile considering how accurate the EBVs are in describing the genetics of groups of animals within a breeding program.
In conducting a breeding program, it is normal practice for multiple animals to influence the genetics of the breeding herd rather than just an individual. It is therefore important to understand the accuracy of the EBVs describing the breeding value for a group or team of animals.
For example, the EBV accuracy for the team of females being flushed in an embryo transfer program, or the team of bulls being joined. This may be a group of bulls used in a specific joining (e.g. Spring/Summer 2013) or a group of bulls used over subsequent joinings (e.g. all bulls used over last 3 years).
Figure 3 illustrates the EBV accuracy for a group (or team) of animals with an average EBV accuracy of 30% for a trait. While individual bulls within the team may have "low" EBV accuracy, the accuracy of the EBVs describing the breeding value for the entire team of animals will be considerably higher. Put in practical terms, while individually some bulls within the team will perform above expectation, some will perform below expectation and some will perform exactly as expected, across the entire team, the EBVs will describe the breeding value of the team of bulls with considerably higher accuracy.
For example, an individual bull with an EBV of relatively low accuracy of 30% has some level of risk attached as the EBV could change significantly as more information is analysed. If there is a bull team of two, averaging 30% accuracy for the EBV, the EBV accuracy for the bull team is considerably higher at 58% for a team of full sibs, 68% for half sibs and 74% for unrelated bulls.
Similarly, if there is a bull team of ten, averaging 30% accuracy for the EBV, the EBV accuracy for the bull team is 74% for a team of full sibs, 88% for half sibs and 95% for unrelated bulls. For the same situation, but all with 60% accuracy for the EBV, the EBV accuracy for the bull team is 90% for a team of full sibs, 93% for half sibs and 97% for unrelated bulls.
The reason higher bull team EBV accuracy is observed for unrelated bulls, compared to half sib or full sibs, is due to there being less chance of a bias affecting the EBVs for all bulls in the team.
This shows that the risk of using bulls with relatively low individual EBV accuracy (e.g. yearlings) can be overcome by considering them is a bull team context, rather than individuals. This is a result of "spreading-the-risk" across the team, rather than "putting-all-your-eggs-one-basket" approach. This also suggests that a team of younger "unproven" bulls can be competitive, in terms of EBV accuracy, to an individual "proven" sire.
Although the accuracy of an EBV should be considered, animals should generally be compared on EBVs regardless of accuracy as they are still the best estimate of an animal's breeding value. In the case where animals have similar EBVs, the animal with the higher accuracy would be preferable because the results can be predicted with more confidence (i.e. less risk).
If seedstock producers prefer to minimise risk through the use of animals with higher accuracy EBVs, consideration could be given to:
Lohuis, M.M and Smith, C. (1994). Risk associated with teams of bulls. Proceedings of the 5th World Congress of Genetics Applied to Livestock Production 18:298-301