Abstract:
In experiment 1,76 cryptorchid (n = 37) and ewe (n = 39) lambs were scanned for GR (depth of muscle and fat tissue from the surface of the carcass to the lateral surface of the 12th rib, 110 mm from the midline) and M. longissimus thoracis et lumborum (LD) depth (USB) on 4 occasions over 3 months at intervals of 28 days with a real-time ultrasound scanner. Carcass GR measurements were obtained after the final live measure at 5.5 months of age. Significant (P<0.001) changes in GR and USB were detected as liveweight increased. All changes could be best explained by linear models (P<0.001) and there was a significant (P<0.001) sex effect evident for GR indicating that this is a useful measurement for monitoring changes in carcass fatness in live lambs. The correlation (r = 0.31) between ultrasonic and carcass GR was significant (P<0.05), however, mean ultrasonic and carcass measurements of GR were significantly different (P<0.05) for ewe, but not cryptorchid lambs. A sex effect was found with the relationship between in vivo GR, liveweight and carcass GR indicating that there was no significant relationship between GR measured ultrasonically and on the carcass when liveweight and sex were accounted for. In experiment 2, 86 ewe and 84 cryptorchid lambs were slaughtered in sex groups, the ewes at 6 months of age and the cryptorchids at 8 months of age. Before slaughter subcutaneous fat depth (USFat C) over the LD and LD depth (USB) were measured using a real-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 ewes and cryptorchids respectively (r = 0.63; r = 0.47). This was also the case for USB and carcass LD depth (B) with significant (P<0.001) correlations of r = 0.53 and r = 0.56 for ewes and cryptorchids respectively. Liveweight per se was a poor predictor of Fat C and B irrespective of measurement 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 that was explained by the corresponding ultrasonic measurement and liveweight ranged from R2 = 0.29-0.39. Irrespective of the measurement group considered or the yield type, measurement of USFat C significantly (P<0.001) improved the accuracy with which percentage yield could be estimated over and above liveweight and increased the amount of variation (R2) in percentage yield that was explained. However, it is recommended that if ultrasonic measurements are to be used for predictive purposes, GR and LD depth be adopted.