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.