Relationship between real-time ultrasound and carcass measures and composition in heavy sheep

Abstract

Fat depth over the M. longissimus thoracis et lumborum (LL) at the 12th rib (USFat C) and the depth of the LL (USEMD) were measured before slaughter using a real-time ultrasound machine in 147 mixed sex, 22-month-old sheep of five genotypes. Equivalent measures were obtained on the carcasses (Fat C and EMD) and each carcass side was scanned by dual energy X-ray absorptiometry to provide an estimate of composition (percentage lean and fat). There was a significant (P < 0.001) correlation between USFat C and Fat C (fat depth over the LL at the 12th rib measured on the carcass) at r = 0.67. This was also the case for USEMD and EMD (muscle depth of the LL at the 12th rib measured on the carcass) with a significant (P < 0.001) correlation of r = 0.55. Liveweight per se was a poor predictor of Fat C and was of minimal value when used in combination with ultrasonic fat depth measurements. The prediction of Fat C was significantly underestimated by USFat C and this increased as the animals became fatter. The relationship between carcass and ultrasonic measurements of EMD was poor, but better when liveweight was used in combination with USEMD. The prediction of EMD was significantly overestimated by USEMD and this increased as the animals became heavier. Combining USFat C measurement with liveweight significantly (P < 0.001) improved the accuracy (R2) and precision (r.s.d.) with which either the percentage of fat or lean could be estimated. Measurement of USEMD was of no value for the estimation of the percentage of fat or lean. There was no significant (P > 0.05) sex effect on any of the relationships. Ultrasonic measurement of subcutaneous fat depth and muscle depth in heavy fat animals is subject to undefined error, but still provides a means to predict in vivo fat levels and muscle depth. However, the bias associated with the predictions suggest caution should be exercised when measuring heavy fat sheep and the need for further work to confirm the findings of the present study.