Changes to the Poll Dorset and Dorset Horn Breeding Evaluation
(Part 2 – Index Weightings) - January 2014
EBLEX funded SRUC/EGENES to look at the breeding index weightings used within
the Dorset Breeding Evaluation.
Changes were required because:
 Breeders wished to see an ATAN penalty applied to the evaluation to penalise
sheep with low, negative Fat Depth EBVs
 Both indexes needed to be “rebalanced” following changes to the genetic
parameters.
 The original Terminal Sire index had been based on CT traits and needed to
be re-worked based on ultrasound traits to enable the ATAN function to be
applied correctly.
The relative emphases placed on the traits in the Dorset Maternal Index are shown
in Table 1. These are very similar to the weightings agreed 18 months ago which
placed a greater emphasis on prolificacy.
Table 1. Maternal Index
% emphasis
(new)
Eight Week Weight EBV
44
Maternal Ability EBV
43
Litter Size Born EBV
10
Fat Depth EBV *
3
 Small emphasis on fat depth required to enable ATAN to be modelled.
The relative emphases placed on the traits in the Dorset Terminal Sire Index are
shown in Table 2 alongside a version of the previous weightings. Reducing the
emphasis on leaner breeding lines will lead to the breeding of slightly fatter sheep,
they will have significantly better muscling across the loin – and despite the reduction
in emphasis on growth (relative to muscling) the predicted rates of genetic gain in the
growth traits are actually very similar to those achieved using the previous index.
Table 2. Terminal Sire Index
Eight Week Weight EBV
Scan Weight EBV
Muscle Depth EBV
Fat Depth EBV
% emphasis
(old index re-worked)
2
62
19
17
% emphasis
(new)
4
45
45
6
Summary
The new Maternal Index is similar to the previous version. The new Terminal Sire
Index places less emphasis on leaner breeding lines and more emphasis on those
with superior muscling. Genetic gains associated with growth traits are very similar.