Download Genetic Merit

1 of 3
This article appeared in Heartland Sheep & Mainland Sheep, the spring sheep specials
produced by Country-Wide, in 2008
Genetic Merit – what is it?
Georgie Walker & Mark Young, SIL
‘What you see is not always what you get’ very much applies to sheep breeding. When you
visually assess the size of a ram, you are looking at his ‘phenotype’, not his genetic merit.
Phenotype of an animal is the sum of genotype (genes or DNA) and environment effects
(feeding and management).
For example, a small, lean looking (phenotype) ram at weaning may have very good growth
genes (genotype) but may have been fed less well due to being born late, as a triplet with a
hogget mother (environment). So he did not express his genetic potential for growth.
Alternatively he may have been fed well (environment) but not have good growth genes
(genotype). How do we know which situation applies?
We need to use all information available to us to make a “best-bet” estimate of genetic merit
as only genetics is passed on to offspring.
This concept of actual performance (phenotype) being the sum of environmental effects and
genetic effects is fundamental to modern animal breeding. It can be represented as:
Performance
(Phenotype)
Weaning
weight
=
=
Genetics
(Genotype)
Breeding Value
(genes
influencing
weaning
weight)
+
+
Environment
o Date of birth
o Birth litter size
o Rearing litter size
o Age of dam
o Flock
o Mob feeding effect
Adjustments due to
environmental effects
Environmental effects are a very important part of the Performance equation. Another very
important source of information SIL uses to get “best” estimates of genetic merit is how other
family members performed. Two ewes may have similar lambing history, but one has sisters
with an average litter size of 1.1 while the other has sisters with an average litter size of 1.6.
Which one is the better bet? Other things being equal, it is clear that the ewe from the family
with the higher litter size is a better bet to help us increase lambing percentage.
SIL ram breeders record performance and environment effect information that allows SIL to
produce accurate estimates of genetic merit. Effectively these are predictions of how animals
14:23 29-Sep-08
Genetic Merit 05 WEB.doc
sil
2
rate relative to each other, when known environmental effects have been corrected for, and
the performance of relatives accounted for.
SIL uses the term “breeding value” (BV) for its estimate of genetic merit. BVs are measured
in the same units as the trait. For many traits a higher value is better but for some traits a
lower value is better (e.g. wool fibre diameter and faecal worm egg count). A ram passes on
half his BV to his offspring.
It is difficult to assess overall merit across a range of traits particularly when they are very
different traits or are measured in different units. SIL addresses this by grouping traits
together, where they affect a similar production characteristic, and converting merit to a
monetary value. These are called sub-indexes. SIL has sub-indexes for: Growth, Meat
(carcass merit), Reproduction, Survival (lamb), Wool and Health traits.
These sub-index trait groups can be added together to create an Overall Index of genetic
merit. With this system, two animals may be of similar overall merit, but one may be superior
for lamb growth while the other is superior for lamb survival and fleece weight.
SIL’s genetic merit triangle (see diagram) shows the process of how genetic merit is
calculated and represented:
Overall
Index
Sub indexes added up
– a farm profit focus
Sub-Indexes
(Goal trait groups)
BVs consolidated
- a trait profit focus
Breeding Values
Field measurements
Pedigree information
Measurements adjusted for
known environmental influences
and performance of relatives
combined to create BVs
What traits are in an overall index depends on the farm production system. SIL’s three main
Indexes systems are for Dual Purpose (ewe breed), Terminal Sire (meat breed) and Wool
sheep.
SIL performs genetic evaluations for many flocks and flock groups. Some sheep are in more
than one of these evaluations. BVs and indexes can be different from one evaluation to
another since there may be different information available on which to rate these animals
against their contemporaries.
14:23 29-Sep-08
Genetic Merit 05 WEB.doc
sil
3
SIL regularly performs one large-scale genetic evaluation that is across-flock and acrossbreed. Called SIL-ACE, it uses information from almost 3 million animals in over 300 flocks.
Results listing the most highly rated sires are published on the SIL website (www.sil.co.nz).
Having a genetic plan that identifies what traits are important to your farm will mean you can
make most effective use of SIL figures. Work out what traits are most important for you to
improve and make use of your ram breeder’s expertise by asking them to present animals to
you that have good figures for these traits.
TAKE HOME MESSAGES
o
o
o
o
o
What you see is not always what you get! Animals highly rated for growth may not
be the biggest of those presented for sale.
Don’t select animals on their own birth rank, or their dam’s lambing history, to
improve lambing percentage. Use the BV for number of lambs born (NLB)
Use SIL indexes and BVs to get best estimates of genetic merit – this is what the ram
will pass on!
Ask your breeder to explain SIL information to help you choose rams best suited to
your needs.
SIL-ACE is a national analysis that compares rams from many different flocks and
breeds. Index figures for a ram from SIL-ACE can be compared to those of rams
from the same analysis but they cannot be compared to index figures from an
analysis done for just one single flock or for another across-flock group
Georgie Walker & Mark Young work for Sheep Improvement Ltd (SIL), the national system
for genetic evaluation of sheep that is part of Meat & Wool New Zealand.
More information about the SIL system for rating sheep on genetic merit is available on the
SIL website (www.sil.co.nz). SIL can also answer queries you may have about the SIL system
for rating rams on genetic merit – you can telephone 0800-SILHELP (0800-745-435) or send
an email to [email protected]
14:23 29-Sep-08
Genetic Merit 05 WEB.doc
sil