CHANGES IN FAT DEPTHS AND MUSCLE DIMENSIONS IN GROWING LAMBS AS MEASURED BY REAL-TIME ULTRASOUND

Carcass characteristics of 3-4-month-old lambs from 2 experiments were measured with a realtime ultrasound scanner 4 times over 4 months. In experiment 1, all lambs grazed 3 pasture regimes consecutively over 4 months, and in experiment 2, 3 nutritional treatments were compared. Lambs contemporary to these, from a third experiment, were scanned bef ore slaughter, and the equivalent carcass measurements obtained. Signi ficant (P<0.001) changes in GR tissue depth, subcutaneous fat depth at the C site, and M. longissimus thoracis et lumborum (LD) dimensions w ere detected as liveweight increased with time in experiments 1 and 2. Depth of LD was found to increase more than width during the growth p hase studied. High repeatabilities were obtained for GR, with partial correlations of 0.72 and 0.86, using liveweight as the adjustment fact or: For fat depth, repeatabilities were moderate at 0.57 and 0.73, res pectively. An inconsistent result was found for LD depth and width, wi th correlations of 0.68 and 0.29 for depth and 0.36 and 0.51 for width . In experiment 3, there were small differences between GR, fat depth, and the area of the LD muscle as measured in vivo and on the carcass, but the accuracy with which fat depth could be estimated from ultraso nic measurements for individual lambs was low compared with the GR mea surement. Depth of LD measured in vivo was significantly (P<0.001) gre ater than on the carcass, but the converse was true for LD width, indi cating a change in shape. Partial correlations between in vivo and car cass measures were 0.60 for GR, 0.17 for fat depth, 0.36 for LD depth, and -0.15 for LD width. For LD area, there was a range of 0.31-0.42 d epending on the method used to determine area on the carcass. It was c onsidered that real-time ultrasound offered potential when used to mon itor changes in GR. Measurement of fat depth in lean lambs was subject to significant error and is not recommended. Of the muscle dimensions , LD depth offers potential, but repeatability may be unsatisfactory i n lightweight lambs.