Enhancement of the terminal carcass production index to incorporate birth weight and lambing ease in Australian sheep

Sires ranking highly on Australian terminal sheep indexes tend to have higher birth weight (BWT), which potentially translates to increased incidence of difficult births in commercial flocks. The Terminal Carcass Production industry index was therefore modified to include a penalty targeting a genetic change of -0.005 kg in BWT over 10 years. An alternative scenario was also considered, where an economic value for lambing ease (LE" %) was added to the index. The largest impact of the BWT penalty approach was a reduction in the response for growth traits with little improvement in LE but with minimal impact on carcass yield and eating quality. The economic value for LE was calculated as $1.15/1% improvement in lambing ease/ ewe/year. Compared to the first approach, including LE in the index produced is preferred with similar outcomes for birth weight but with less impact on response for the growth traits and also resulted in a positive response in LE.