Changes to the Scottish Blackface Breeding Evaluation produced for Signet Recorded Flocks

Samuel Boon, September 2016

Based on work completed by EGENES/SRUC and funded by AHDB Beef and Lamb & Quality Meat Scotland

 

Summary

AHDB and QMS recently funded two enhancements to the Scottish Blackface Breeding Evaluation and these are incorporated in your latest results.

• Improvements have been made to the way EBVs are calculated, particularly traits like the Litter Size Born EBV – and breeders now have access to two new traits.
• Breeders will notice changes in the scaling of a number of EBVs and the ranking of certain animals.
• The work on “Genetic Groups” provides new sheep entering the analysis with a much fairer starting point and should encourage new flocks to get involved with performance recording.

Due to the number of changes that are incorporated into this new evaluation, breeders are strongly advised to use the new breeding values when assessing sheep and not to make direct comparisons to their rating in previous evaluations.

A technical note is provided here for those breeders wishing to know more about these developments.

 

Enhancement 1. New genetic parameters

Estimated Breeding Values are calculated using knowledge of the heritability of different traits (the degree to which variation between animals is influenced by their genes) and the relationships (correlations) between these traits. Periodically these “genetic parameters” are reviewed to ensure that they are appropriate for the sheep/breed being analysed.

In 2016 AHDB funded SRUC/EGENES to review the genetic parameters used in the evaluation of Scottish Blackface sheep.

 

How have heritability values changed?

The table below shows the new heritability values that are being used in the Scottish Blackface evaluation.

Estimates of heritability (h²) for Scottish Blackface

 

Trait	Old Heritability Value	New Heritability Value
8 Week Weight	0.12	0.15
Maternal Ability	0.16	0.12
Scan Weight	0.29	0.13
Muscle Depth	0.33	0.22
Fat Depth	0.18	0.26
Mature Size	0.43	0.34
Birth Weight (New Trait)		0.19
Litter Size (Born) (New Trait)		0.07
Litter Size (Reared)	0.05	0.07
CT Lean Weight	0.28	0.15
CT Fat Weight	0.29	0.17
FEC S	0.15	0.13
FEC N	0.20	0.11

 

How do I interpret heritability values?

The heritability value indicates the proportion of the variation between animals that can be explained by their genes (or you could think of it as the degree to which an animal’s appearance is influenced by its genes).

If a growth trait has a heritability of 0.4 then 40% of the liveweight variation between animals would be due genetic differences between them and 60% would be due to environmental influences.

Health and reproduction traits tend to have a low heritability – but this doesn’t mean they are not worth assessing; even a small change in these traits can be highly profitable.

 

What are the new genetic correlations?

The new genetic correlations are shown in the table below.

How do I interpret genetic correlations?

The size of the correlation indicates how closely related traits are to one another. A correlation of 0 indicates “no relationship”, 0.90 would be a very strong, positive relationship between traits and -0.90 would indicate a strong negative relationship.

In this case one of the strongest genetic relationships exists between 8 Week Weight and Scan Weight (0.81). This is unsurprising as the genes that control growth rate to 8 weeks of age are also likely to influence growth rate in later life. There is also a strong relationship between Litter Size Born and Litter Size Reared – sheep genetically programmed to produce more lambs tend to rear more lambs (unsurprisingly).

 What do these changes mean?

• The new EBVs are more accurate and more closely aligned to the performance recorded Scottish Blackface population
• Changes in EBVs were mostly in an expected direction, based on work in other breeds
• One significant change was the genetic relationship between fat depth and litter size reared. This relationship was thought to be slightly negative, but the data shows it to be slightly positive – when combined with the change in heritability of fat depth, the changes in Litter size EBV are quite noticeable and this impacts on the overall index of recorded sheep.

Enhancement 2. Genetic Groups

Why do we need a genetic groups model in Scottish Blackface sheep?

In previous Scottish Blackface genetic evaluations, animals with an unknown sire or dam (i.e. previously unrecorded sheep) brought into performance recorded flocks were initially given EBV and Index values representative of the average of all of the animals in the analysis with an unknown sire and dam.

Bearing in mind that genetic progress is being made in the performance recorded population, these starting points tend to lag further and further behind the current breed average, meaning that the initial indexes given to new animals with unknown parents is low.

Given that the non-recorded breed may have also made genetic progress it may be deemed inappropriate to group new males born in 2014 for example, as having similar genetic merit as ewes born in the 1980’s and 90’s.

A modification to this approach is the application of a genetic groups system, in which animals with unknown sire and dam are grouped according to their sex and year of birth. This allows modern animals to enter the analysis with initial genetic predictions closer to the genetic merit of the current population, hence moderating the impact of bringing unknown animals into a recorded flock on their performance figures (EBVs and indexes) and giving them a fairer starting point.

Impact of a Genetic Groups Model for Scottish Blackface Sheep

The new Scottish Blackface genetic group’s model now has three groups fitted, animals born pre-1995, 1995-1999 and 2000 onwards.

This work showed previous analyses were underestimating the genetic merit of unrecorded sheep that are performance recorded with Signet and the new system gives these animal with an unknown sire and dam a fairer starting point.

Whilst this provides a good and robust solution for new flocks, breeders are still reminded that the best way to test a new ewe or ram coming into the scheme is to get it to drop plenty of progeny and to fully record them.

 Benefits of this work

Both of these projects have contributed to an enhancement of the genetic analysis of Scottish Blackface sheep. Breeders and commercial ram buyers now have

• A better estimate of the genetic merit of recorded Scottish Blackface sheep
• A clearer understanding of the genetic merit of the non-recorded Scottish Blackface sheep that have entered the analysis in recent years
• A positive, promotional message to encourage recording within the breed – particularly amongst new, previously unrecorded flocks

Information produced by Signet Breeding Services. Email: [email protected]