Changes in fat depths and muscle dimensions in growing lambs as measured by real-time ultrasound

Abstract

Carcass characteristics of 3-4-month-old lambs from 2 experiments were measured with a real-time 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 before slaughter, and the equivalent carcass measurements obtained. Significant (P<0.001) changes in GR tissue depth, subcutaneous fat depth at the C site, and M. longissimus thoracis et lumborum (LD) dimensions were detected as liveweight increased with time in experiments 1 and 2. Depth of LD was found to increase more than width during the growth phase studied. High repeatabilities were obtained for GR, with partial correlations of 0.72 and 0.86, using liveweight as the adjustment factor. For fat depth, repeatabilities were moderate at 0.57 and 0.73, respectively. An inconsistent result was found for LD depth and width, with correlations of 0.68 and 0.29 for depth and 0.36 and 0.5 1 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 ultrasonic measurements for individual lambs was low compared with the GR measurement. Depth of LD measured in vivo was significantly (P<0.001) greater than on the carcass, but the converse was true for LD width, indicating a change in shape. Partial correlations between in vivo and carcass 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 depending on the method used to determine area on the carcass. It was considered that real-time ultrasound offered potential when used to monitor 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 in lightweight lambs.