Why some animals dropped in index after May 2010
Some animals dropped in index when evaluations of Growth & Meat changed ...
SIL altered the way it describes genetic merit for Growth & Meat (carcass merit) in May 2010. Since then, some users have expressed concern when they saw index values drop dramatically for sheep previously highly rated.
Essentially there were four things causing change.
- Shifting effects of carcass size out of the Meat index to the Growth index
- Changing Meat index eBVs from tissue weight to tissue yield (kg per kg carcass)
- Separating Lamb Growth from the penalty for Adult Size (DP sheep only)
- Removing the penalty on Fatness (DP sheep only)
Essentially all size effects are now in the Growth indexes (Lamb Growth & Adult size) while the Meat Yield index tells something about carcass value, above and beyond carcass size, in terms of lean yield and lean distribution (DP & TS sheep) as well as fatness in TS sheep. For most sheep this led to decreases in Meat index and increases in Growth index.
Typically, most animals highly rated for Growth and Meat showed drops in Meat index (from old Meat index to new Meat Yield index) while Growth increased (from old Growth index to new Lamb Growth index). This was because size effects are removed from the new Meat eBVs and to compensate more value is put on the carcass weight (CW) eBV in the Lamb Growth index. On average the net effect was not great but with higher indexes the changes in each component index were usually large. So where Meat dropped, Growth rose.
The second reason was that animals with high LEAN eBV (a weight), but low LEAN for their size (carcass weight eBV), now have low, usually negative, lean yield eBVs. Such eBVs produce negative Meat Yield indexes for DP sheep and depress TS Meat Yield indexes.
Thirdly, for DP sheep, the old Growth index bundled Lamb Growth eBVs with the Adult size eBV but the former were rewarded and the latter penalised in the index. It was not easy to see why an animal had a good index - high lamb growth or low adult size. Lamb Growth (DPG) is now separated from Adult Size (DPA) to clearly discriminate between these effects.
Finally, for DP sheep, removal of the penalty on fatness means animals with low fat lose the benefit they got from that in the old Meat index. SIL removed this penalty for DP sheep (but not for TS sheep) due to breeder concern that the old DP Meat index favoured genotypes that were too lean as ewes.
For more detail on this, SIL has produced a Fact Sheet describing the index changes you can expect to see as a result of the changes SIL made in May 2010. Accompanying this are two tables of examples (DP & TS) for the old & new indexes and all their eBVs to illustrate how these changes impact on indexes.
Click here for the Growth & Meat changes Fact Sheet
Click here for the Growth & Meat changes - Tables of examples