Estimating beef carcass composition using the speed of ultrasound

The reciprocal of the speed of 2·25 MHz (nominal) ultrasound (RV) was measured at selected sites through the soft tissues of 72 beef carcasses from Friesian. Hereford and Hereford × Friesian bulls and steers at or within 1 h of stunning. The measurement sites were: through M. adductor horizontal to the caudal edge of the symphysis pubis (S2), through M. scalenus just cranial to the first rib (S3) and through M. longissimus thoracis between the tenth and eleventh ribs, approximately 6 cm from the midline (S4). The thickness of extractable lipid (d(f)) at each site was defined as the product βd, where β was the volume fraction of extractable lipid and d was the tissue thickness. The thickness of lipid-free tissue (d(ff)) was defined as (d-d(f)) and β was calculated from the ultrasonic measurements using an empirical linear relation between β and RV. When cold, the intact sides were scored independently for fatness and conformation by two experienced judges according to the EAAP system and their mean scores determined. Sides were split at the level of the last rib and the depth of subcutaneous fat measured at 25%, 50% and 75% of the width of M. longissimus thoracis from the midline. The three ultrasonic characteristics (RV, d(f) and d(ff)) at each site, the mean fat depth and the mean fat and conformation scores were examined for correlation with % fat and % lean in the sides determined by complete physical dissection. The best predictors of % fat were mean d(f) at S3 and S4, mean d(f) at S2, S3 and S4 and mean EAAP fat score which were all of comparable precision (rsd, respectively, 2·07, 2·06 and 2·08). The best predictors of % muscle were mean d(f) at S3 and S4, mean d(f) at S2, S3 and S4 and mean RV at S2, S3 and S4 (rsd, respectively, 1·79, 1·79 and 1·90). The corresponding residual standard deviations for mean EAAP fat score and mean fat depth were 2·18 and 2·20.