When reviewing the research selection indexes that have been developed, there are a number of important considerations to make.
While it is more sophisticated in practice, for the purposes of explanation, an economic index is calculated by multiplying each EBV by an economic value, and adding all the components together. This is illustrated in the formula below:
Index = (EBV1 x EV1) + (EBV2 x EV2) + (EBV3 x EV3) + ………. + (EBVn x EVn)
While the EBVs used when calculating the research selection indexes are the same, the considerable increase in beef prices, coupled with relatively stable costs, means the economic value of each trait is now considerably higher than what they were when the selection indexes published in the TransTasman Angus Cattle Evaluation were last updated in 2014.
The multiplication of the same EBVs by a higher economic value results in the selection index value having a higher value. Similarly, the same difference in EBVs between two animals now results in a larger difference in their selection index values.
The nature of an economic selection index means that animals with extremely favourable EBVs for traits of high economic importance will still rank highly on the index, even if they have EBVs for several other traits that many breeders would consider unfavourable. For example, an animal with an extremely high 400 or 600 Day Weight EBV may still rank highly even if its Calving Ease EBVs is below average.
This scenario does not reduce the value of the index as a selection tool, but highlights the importance of considering the index value in association with the individual EBVs when making selection decisions.
A common misconception is that the relative emphasis given to each trait in the calculation of the selection index equates to the relative change that will occur in that trait if selection decisions are based on the selection index.
In practice, the relative change that will occur in traits is influenced by not only the trait contribution, but also factors such as the animals that are available for selection and the genetic correlation that exists between traits.
A better representation of the response to selection is the selection advantage. The selection advantage is indicative of the long term direction and relativity of response that will occur in individual traits if selection is based on the selection index.