Dual energy X-ray absorptiometry (DXA) can be used to predict live animal and whole carcass composition of sheep

Abstract

An Hologic QDR4500A dual energy X-ray absorptiometer (DXA) was used to measure live body composition in 15 second-cross wether lambs ((Border Leicester × Merino) × Poll Dorset) aged 6–8 months (17–50 kg) and 13 mature ewes (composite terminal sire line based largely on Poll Dorset, White Suffolk and Texel breeds) (44–71 kg). After slaughter, DXA scans were also performed on the whole carcasses. Values determined by DXA including total tissue mass (TTM), lean tissue mass (LTM), fat tissue mass (FTM) and bone mineral content (BMC) for the half carcass were evaluated by comparison with chemically determined composition. In the case of BMC the relationship was with chemically determined ash content. Live weight and chemically determined lean, fat and ash were strongly related to DXA-derived values for whole body TTM, LTM, FTM and BMC, respectively (R² = 0.999, 0.988, 0.980 and 0.977, respectively). Carcass weight and chemically determined lean, fat and ash were strongly related to DXA-derived values for carcass TTM, LTM, FTM and BMC, respectively (R² = 0.998, 0.984, 0.986 and 0.906, respectively). However, because DXA estimates were different from chemically determined values in this sample of carcasses, they needed to be adjusted with the use of appropriate regression equations to correct the in-built algorithms used to predict human body composition. These data demonstrate the efficacy of DXA as a non-destructive method for determining the composition of the whole body and carcasses of sheep.

Introduction

In Europe and Australasia there have been increasing changes in production systems and marketing of sheep meat, with consumers demanding larger and leaner lamb cuts (McLaughlin, 1992, Thatcher and Gaunt, 1992, Stanford et al., 1998, Pethick et al., 2010). Currently, the Australian lamb industry uses a number of traits such as weight, GR (‘Grade Rule’) fat depth (measured over the 12th rib, 110 mm from the midline) and m. longissimus lumborum depth as the basis of genetic improvement (Hopkins et al., 2004). While these measures may be used as indicators of lean meat yield, their usefulness is likely to diminish if selection pressure is placed on these single measures, as has been the case for the pig (Suster et al., 2003, D'Souza et al., 2004). In many parts of the world mature sheep are used for purposes other than meat production such as wool and milk. Body composition can vary considerably during the reproductive and seasonal cycles and to adequately manage animals in these systems there is a need to measure body composition in the live animal. Although liveweight is correlated with body composition (Searle, 1970, Ponnampalam et al., 2007), changes in live weight over time are difficult to interpret in the absence of more detailed information on body composition (Stanford et al., 1998). Also, increasingly sheep are being used as model animals to study obesity (Henry et al., 2001, Anukulkitch et al., 2007) and in order to do this, accurate measures of body composition are required.

A method that has been used to determine fat, lean tissue and bone mineral composition in live pigs is dual energy X-ray absorptiometry (DXA) (Mitchell et al., 1998, Lukaski et al., 1999, Marcoux et al., 2002, Suster et al., 2003). There have been a few studies that have reported close relationships between DXA-predicted and chemically determined (Clarke et al., 1999, Dunshea et al., 2007) or dissected (Mercier et al., 2006) body composition in the half carcass of lambs. However, there has been only one that has related these measures in the live animal (Pearce et al., 2009) and the animals were not slaughtered until 11 days after scanning, sufficient time for changes in body composition and gut fill to occur. Therefore, the major aim of this study was to determine whether DXA could be used to predict chemical composition of the whole body and carcass of the sheep in vivo.