Understanding $Index Values

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$Index Values are calculated using selection indexes derived using BREEDOBJECT software, developed by the Animal Genetics and Breeding Unit (see acknowledgements).

In an ideal situation it would be desirable to select animals for breeding that excel in all traits.But, in reality it is always necessary to make some compromises in balancing the strengths and weaknesses among the animals available for selection. A selection index is a formal method of combining Estimated Breeding Values (EBVs) for different traits into a single $Index Value.

BREEDOBJECT uses estimates of the genetic relationships between EBVs and the various traits in the breeding objective for each example to determine a set of index weighting factors to apply to each EBV.The individual EBVs are multiplied by these weighting factors and summed to produce an overall $Index Value for each animal.The weighting factors take account of the relative economic importance of the traits in the breeding objective and the capacity to change them through selection on EBVs.

Variation in $Index Values between sires indicate differences in the expected net profitability per cow joined.

The composition of any selection index is sensitive to the assumptions used in the BREEDOBJECT analysis.

$Index Values have been calculated for different production systems and markets as described below. For each $Index, typical production parameters, prices and production costs were assumed for a beef enterprise operating in the past three years. It was also assumed that the enterprise obtains benefits from improved carcase performance (eg. via retained ownership to slaughter) in addition to improved herd productivity. In addition, it was assumed that feed was a limiting resource for a large part of the year and any increase in herd feed requirements has a cost. In the self-replacing herd cases an average calving rate of 85% was assumed, with a moderate concern for calving difficulty.

Angus Australia has developed four new BreedObject $indexes. These are:

• Long Fed / CAAB Index
• Heavy Grass Fed Steer Index
• Short Fed Domestic Index
• Terminal Index

As these are 4 new indexes, it is not valid to compare old and new $index values. Detailed explanation on interpreting the new $indexes

BREEDOBJECT can be used to establish customised selection indexes for particular situations. A questionnaire needs to be completed to provide information on production costs, performance levels and market targets.This information is used to derive the relative economic values of different traits, and the index weighting factors to be applied to the EBVs for potential candidate animals for selection.

Contact Carel Teseling (Ph 02 6772 3011) for more information.

Acknowledgements:
The BREEDOBJECT program was developed by the Animal Genetics and Breeding Unit (AGBU), a joint institute of NSW Agriculture and The University of New England. Ongoing research and development at AGBU is supported by funding provided by Meat and Livestock Australia. The assistance of Dr S. Barwick in the development of the indexes described above is gratefully acknowledged.