LAMB CARCASS CHARACTERISTICS .3. DESCRIBING CHANGES IN CARCASSES OF GROWING LAMBS USING REAL-TIME ULTRASOUND AND THE USE OF THESE MEASUREMENTS FOR ESTIMATING THE YIELD OF SALEABLE MEAT

In experiment 1, 76 cryptorchid (n=37) and ewe (n=39) lambs were scann ed for GR (depth of muscle and fat tissue from the surface of the carc ass to the lateral surface of the 12th rib, 110 mm from the midline) a nd M. longissimus thoracis et lumborum (LD) depth (USB) on 4 occasions over 3 months at intervals of 28 days with a real-time ultrasound sca nner. Carcass GR measurements were obtained after the final live measu re at 5.5 months of age. Significant (P<0.001) changes in GR and USB w ere detected as liveweight increased. All changes could be best explai ned by linear models (P<0.001) and there was a significant (P<0.001) s ex effect evident for GR indicating that this is a useful measurement for monitoring changes in carcass fatness in live lambs. The correlati on (r=0.31) between ultrasonic and carcass GR was significant (P<0.05) , however, mean ultrasonic and carcass measurements of GR were signifi cantly different (P<0.05) for ewe, but not cryptorchid lambs. A sex ef fect was found with the relationship between in vivo GR, liveweight an d carcass GR indicating that there was no significant relationship bet ween GR measured ultrasonically and on the carcass when liveweight and sex were accounted for. In experiment 2, 86 ewe and 84 cryptorchid la mbs were slaughtered in sex groups, the ewes at 6 months of age and th e cryptorchids at 8 months of age. Before slaughter subcutaneous fat d epth (USFat C) over the LD and LD depth (USB) were measured using a re al-time scanner. Of the carcasses, 84 (42 ewes, 42 cryptorchids) were prepared into 'trim' lamb (TL) cuts. There was a significant (P<0.001) correlation between USFAT C and the carcass equivalent (Fat C) for ew es and cryptorchids respectively (r=0.63; r=0.47). This was also the c ase for USB and carcass LD depth (B) with significant (P<0.001) correl ations of r=0.53 and r=0.56 for ewes and cryptorchids respectively. Li veweight per se was a poor predictor of Fat C and B irrespective of me asurement group and was of minimal value when used in combination with USFat C or USB measurements. The amount of variation in Fat C and B t hat was explained by the corresponding ultrasonic measurement and live weight ranged from R(2)=0.29-0.39. Irrespective of the measurement gro up considered or the yield type, measurement of USFat C significantly (P<0.001) improved the accuracy with which percentage yield could be e stimated over and above liveweight and increased the amount of variati on (R(2)) in percentage yield that was explained. However, it is recom mended that if ultrasonic measurements are to be used for predictive p urposes, GR and LD depth be adopted.