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Forty-six single male lambs ofChurra Tensina mountain breed with a live-weight ranging from 19.9 . to 24.4 kg ('Temasco' carcass commercial type) were scanned by ultrasound (ALOKAmodel SSD- 900, with a 7.5 MHz probe) to determine M. longissimus dorsi depth and subcutaneous fat thickness between the lOth-lith, 12th-13th dorsal vertebrae and 1st-2nd, 3rd-4th lumbar vertebrae. Lambs were slaughtered and carcasses were cooled at 4 °C for 24 h and halved. The left side was divided according to a standardised jointing procedure, based on six anatomical regions: shoulder, long leg, anterior ribs, ribs, flank and neck. Each joint was then disseCted into ,muscle, bone plus remainder and subcutaneous, inter-muscular, kidney and pelvic fat. Thein vivo ultrasound measureme~ts plus slaughter weight were fitted to predict carcass tissue composition by Stepwise regression analysis. All the developed models were highly significant (P

The objectives of this study were to study the relationship between in vivo ultrasound measurements and cold carcass measurements at 4 anatomical points of the backbone, and to establish regression equations to estimate carcass composition within the cold carcass weight range for Ternasco lambs (8 to 12.5 kg) by using ultrasonic measurements taken at a single location. Measurements of subcutaneous fat and skin thickness and of muscle depth and width were taken over the 10th to 11th and 12th to 13th thoracic vertebrae and the 1st to 2nd and 3rd to 4th lumbar vertebrae. These measurements were taken at 2 and 4 cm from the nearest end of the LM to the backbone and at 1/3 of the LM width with the probe perpendicular to and parallel to the backbone. The left sides of the carcasses were dissected into muscle, fat, and bone. Body weight (22.6 kg) and cold carcass weight (10.8 kg) were representative of Ternasco light lambs. Muscle depth measured at 2 cm, 4 cm, and 1/3 of LM width remained regular, with slight ups and downs along the spine. All the pairs of in vivo ultrasound and cold carcass measurements were significantly different (P < 0.05) and had small correlations. All the ultrasound measurements of muscle depth at any location or at any distance to the backbone were less than their equivalent cold carcass measurements, with differences ranging from 0.8 to 5.9 mm. Differences between ultrasound fat thickness + interface (US_FDGI) and cold carcass fat thickness were less than differences between ultrasound fat thickness and cold carcass fat thickness, ranging from −0.9 to −1.0 mm for the former and from −2.1 to −0.5 mm for the latter. The small differences in absolute values between US_FDGI and cold carcass fat thickness suggest that US_FDGI is the best measure of the real fatness level of the lambs. The best prediction equations for muscle, bone, and fat were developed with in vivo ultrasound data measured at the 1st to 2nd lumbar vertebrae perpendicularly to the backbone, but they had limited predictive value. To predict the muscle content of carcass, BW and muscle depth were included, and they explained 59% of variation. Fiftyone percent of total fat was predicted by BW and fat thickness, whereas only 17% of the variation in bone was predicted by 2 fat-related variables. The BW of lambs was an important predictor to improve regression equations but ultrasound measurements were the most important variables when a narrow range of BW was used. Research was funded by project INIA-RTA03-031.