Altering the carcase has weakened its impact on lean meat yield

Increasing lean meat yield % (LMY%) and rapid growth are important profit drivers in the sheep supply chain. To optimise these traits, Australian prime lamb producers use the Carcase Plus Index (CP) for selecting sires. The index originally combined breeding values for weight, eye muscle depth and decreased fat depth, with weightings of 60:20:20 (old CP). Due to perceived gains in leanness in Terminal sires, and concern over reducing intramuscular fat levels in lamb meat, this index was altered to the current weightings of 65:30:5 (new (CP). Given that selection for reduced fat depth results in an increase in LMY%, we hypothesised that the new CP will return less LMY% and therefore reduced carcase value compared to the old CP Lamb carcases (n=1,800) from the Sheep CRC Information Nucleus Flock were collected from 6 research stations over 5 years. Carcases were scanned in 3 sections, fore, saddle and hind, using Computed Tomography (CT) to determine fat, lean and bone weights. Data was analysed using the allometric equation y=axb, fitted in its log-linearised form log y = log a + b.log x. The impact on carcase value was determined for new and old CP indexes in a 23 kg carcase. Within the 76 unit range of Terminal sire CP index values, the old CP Index delivered 2.2, 1.2 and 0.5 percentage units more lean in the fore, saddle and hind sections (P