The impact of lamb genotype on carcass composition and the relationship with intramuscular fat

The financial value of a carcass is influenced by lean meat yield percentage (LMY%), which represents the proportion of the carcass that is lean meat (muscle). Australian lamb producers can select for this trait indirectly via three existing Australian Sheep Breeding Values (ASBVs), namely post-weaning weight (PWWT), c-site fat depth (PFAT) and eye muscle depth (PEMD). Previous research assessing the effect of these breeding values on carcass composition has focussed on individual point measurements of tissue depth or the weights of a small number of indicator muscles. In contrast, this PhD has used computed tomography (CT) scanning to assess the impact of ASBVs on composition at a whole carcass level. Chapters 3 and 4 of this thesis focus on the change in body composition (fat, lean and bone), in the fore, saddle (middle) and hind sections of the carcass as measured by CT. An allometric approach was adopted in the analysis of the data from 1,665 lamb carcasses, which allowed a robust interpretation of the impact of the carcass breeding values and production factors on carcass composition. It was discovered that increasing sire PEMD and reducing sire PFAT increased LMY% (7.7% and 9.5% units) across the range of these breeding values. Furthermore, lean was redistributed to the saddle region (3.8% units for PEMD and 3.7% units for PFAT). Additionally both breeding values reduced carcass fatness across the sire range by 24.7% and 16.6% for PFAT and PEMD respectively. Increasing sire PWWT had minimal impact on LMY% and carcass fat. Carcass bone was most influenced by reducing sire PFAT, with the other breeding values having little (PWWT) or no (PEMD) effect. Chapter 5 uses further analysis of data collected in Chapters 3 and 4 to report on the financial implications of using ASBVs to select for improved LMY%. The use of CT to report the financial improvement to carcass lean value achieved through genetic selection has not previously been comprehensively reported. Lambs were compared at a standard carcass weight (23kg) and age (269 days), which allowed the influence of the breeding values on both LMY% and HCWT to be accounted for in the calculation of carcass lean value. Reducing sire PFAT had the greatest impact on carcass lean value followed by PEMD and PWWT. The eating quality and nutritive value of lamb are essential traits for consumer satisfaction. Selection for leanness has been shown to reduce intramuscular fat (IMF) in the loin (longissimus lumborum) muscle and so have a detrimental impact on eating quality. For this reason, selection to improve LMY% must be balanced against the consumer focused traits. The measurement of IMF% in relation to genetic selection is undertaken in the loin muscle, with little known about the other muscles of the carcass. Chapters 6 and 7 detail the correlation of IMF% between 5 muscles located in different carcass regions (fore, saddle and hind). These correlations were generally found to be strong, particularly for that of the loin IMF% with the fore and hind section muscles. From an industry perspective this is a significant outcome, as it implies that genetic selection for IMF% in the loin will also cause correlated changes in the other muscles. The relationship between LMY% and IMF% in each of these muscles was also assessed, and found to be consistently negative. Lastly, CT was assessed for its effectiveness to predict IMF%. Although a negative correlation was found between average pixel density and IMF%, the ability of CT scanning to predict IMF% demonstrated relatively poor precision. In summary this work has utilised CT to quantify the impact of ASBVs on carcass composition in lamb. It details the financial impact of these effects due to changes in lean % and its distribution throughout the carcass. Additionally it explores the relationship between IMF% in different regions of the carcass and the impact of selection for LMY%. Finally it assesses the ability of CT to predict IMF% which would be of benefit to industry in the event that CT measurement becomes mainstream within commercial abattoirs.