A Guide to Angus Selection Indexes

Ten selection indexes are published for animals within the TransTasman Angus Cattle Evaluation (TACE).

The selection indexes cater for a range of different breeding systems, target markets and production systems, and include: each animal to identify animals with genetics that are most aligned with the breeding objective for the given selection scenario.

Example Selection Index Calculation

Selection Index = b1T1 + b2T2 + b3T3 + …….. + bⁿTⁿ

Where b = weighting placed on the trait

T = trait

How are selection indexes calculated?

The selection indexes published within the TransTasman Angus Cattle Evaluation are economic selection indexes and are derived using BreedObject software, as developed by the Animal Genetics & Breeding Unit (AGBU) in Armidale, NSW.

What is a selection index?

Selection indexes are utilised by livestock breeders of many species around the world and are considered an essential part of any modern livestock breeding program. Selection indexes aid in the selection of animals for use within a breeding program where there are several traits of economic or functional importance by providing an overall “score” of an animal’s genetic value.

Selection indexes are derived for a specific breeding purpose and are calculated based on weightings placed on individual traits that are deemed to be important for that purpose.

The selection indexes assist in making “balanced” selection decisions, taking into account the relevant attributes of

A range of different selection indexes are published, with each individual selection index reflecting differences in net profitability, being income minus expenses, in a defined breeding system, target market and production system. For the breeding system, target market and production system relevant to each respective selection index, the BreedObject software firstly calculates the net economic value of changing each trait in the breeding objective by one unit, independent of any change in other traits. The BreedObject software then links the economic value of the traits in the breeding objective with the Estimated Breeding Value (EBV) traits published in the TransTasman Angus Cattle Evaluation, and places appropriate emphasis on each EBV based on its relative economic importance in the defined breeding system, target market and production system.

Australia

BreedObject

Why use an economic selection index?

Economic selection indexes assist in making balanced selection decisions across the suite of traits that influence net profitability in a commercial beef enterprise.

Importantly, the trait emphasis that is placed on each EBV is determined by science, not the breeding preferences of either individual breeders, researchers, or staff at Angus Australia.

Understanding economic selection indexes

The economic selection indexes published in the TransTasman Angus Cattle Evaluation are reported as genetic differences in units of net profit per cow joined ($) for the given selection scenario.

The selection indexes are developed to reflect the economics in commercial beef production systems, rather than in seedstock breeding enterprises, and consider both income and expenses.

The selection indexes reflect the short term profit generated by the animal, and where applicable, the longer term profit generated through the retention of their progeny in the breeding herd.

The selection indexes are “whole-chain” selection indexes, and consider traits influencing net profitability across the entire commercial supply chain from conception to slaughter.

Economic selection indexes not only consider the influence of each individual trait on net profitability in the scenario defined by the index, but also balance the genetic relationships that exist between traits, both antagonistic and favourable, to place the appropriate emphasis on each EBV.

By comparison to selection based on consideration of individual EBVs alone where each trait is considered sequentially, economic selection indexes consider all traits simultaneously, resulting in a balanced selection outcome across all the available EBVs.

Economic selection indexes form an important decision support tool to assist breeders in the selection of breeding animals.

Selection indexes do not take the selection decision out of the hands of the breeder, but rather should be used in association with consideration of individual EBVs and any other available information to identify animals that possess genetics that are most aligned with the breeding objective of the individual breeding program for which they are being selected.

Angus Selection Indexes

Ten selection indexes are published for animals within the TransTasman Angus Cattle Evaluation. Each selection index caters for a defined breeding system, target market and production system.

Breeding Systems

The majority of selection indexes are maternal selection indexes and are designed for use in a self replacing breeding program where a portion of female progeny are retained as replacements. The exception is the Angus Terminal Sire Index which is designed for use in situations where Angus bulls are being used as a terminal sire over mature breeding females and all progeny, both male and female, are slaughtered.

Target Markets

The Angus Breeding and Angus Breeding Low Feed Cost selection indexes are general purpose selection indexes that are suitable for use in the majority of commercial beef operations.

The Domestic, Domestic Low Feed Cost, Heavy Grain, Heavy Grain Low Feed Cost, Heavy Grass, Heavy Grass Low Feed Cost and AngusPRO selection indexes are specific to beef operations targeting a defined market endpoint with their steer progeny.

Production Systems

The Angus Breeding, Domestic, Heavy Grain, Heavy Grass, AngusPRO and Angus Terminal selection indexes are designed for use in a production system where pasture is fully utilised for the majority of the year.

Table 1 - Angus Selection Indexes

Breeding System

Production system

Pasture is fully utilised for the majority of the year.

An increase in the maintenance requirements of the mature female breeding herd results in additional costs being incurred.

Mature Weight Objective

Angus Breeding Index ($A)

Domestic Index ($D)

Heavy Grain Index ($GN)

Heavy Grass Index ($GS)

AngusPRO ($PRO)

The Angus Breeding Low Feed Cost, Domestic Low Feed Cost, Heavy Grain Low Feed Cost and Heavy Grass Low Feed Cost selection indexes are suitable for use in production systems where pasture is not fully utilised for the majority of the year, or the cost of supplying additional feed is low, and hence, where an increase in the maintenance requirements of the mature female breeding herd results in minimal additional costs being incurred.

The low feed cost versions of the selection indexes do not aim to limit any increase in mature cow weight, which is an objective in the other versions of these selection indexes. The breeding system, target market and production system catered for by each individual selection index is summarised in the table below.

Pasture is not fully utilised for the majority of the year, or the cost of supplying additional feed is low.

Not applicable

Angus Breeding Low Feed Cost Index ($A-L)

Domestic Low Feed Cost Index ($D-L)

Heavy Grain Low Feed Cost Index ($GN-L)

Heavy Grass Low Feed Cost Index ($GS-L)

Angus Terminal Sire Index ($T)

Important things to consider when reviewing selection index values

When reviewing the selection indexes for an animal, there are a number of important considerations to make.

1. Animals with extremely favourable EBVs for traits of high economic importance will still rank highly, irrespective of their EBVs for other traits

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 EBV 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.

2. Trait contributions do not indicate the response to selection that will occur.

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.

Angus Terminal Sire Index ($T)

Choosing the selection index of relevance to your breeding program

Are you selecting sires for use in a self-replacing (i.e. heifers are retained for breeding) or terminal breeding program (i.e. all heifers and steers are slaughtered)?

In your production system, is pasture fully utilised for the majority of the year?

In your production system, do you consider the cost associated with supplying additional feed as animal feed requirements increase, via additional pasture production and/or supplementary feeding, is low?

Are you comfortable if the weight of your mature breeding herd increases as a result of your selection decisions?

YES. I AM COMFORTABLE IF THE WEIGHT OF MY MATURE COWS INCREASES

Are you targeting a single market with you steer progeny?

Which market?

GRASS FINISHED HEAVY STEER

Heavy Grass Low Feed Cost Index ($GS-L)

Heavy Grain Low Feed Cost Index ($GN-L)

Angus Breeding Low Feed Cost Index ($A-L)

Are you targeting a single market with you steer progeny? Angus Breeding Index ($A)

DOMESTIC LONG FED GRAIN STEER

Domestic Low Feed Cost Index ($D-L)

GRASS FINISHED HEAVY STEER

Heavy Grass Index ($GS)

TERMINAL LONG FED GRAIN STEER YES NO. I DO NOT WANT THE WEIGHT OF MY MATURE COWS TO INCREASE

Which market? NZ ANGUSPURE

Heavy Grain Index ($GN)

DOMESTIC

Domestic Index ($D)

AngusPRO Index ($PRO)