ABSTRACT: Accuracy and repeatability of live-an- imal ultrasound measures, and the relationships of these measures with subprimal yields and carcass value, were investigated using data from 172 wethers. Wethers were F 1 progeny from the mating of 4 termi- nal sire breeds to Rambouillet ewes and were finished in a feedlot to a mean BW of 62.9 kg (SD = 9.5 kg). Before transport to slaughter, LM area, LM depth, and backfat thickness were measured from transverse ultrasound images taken between the 12th and 13th ribs. After slaughter, these measures were taken on each carcass. Carcasses were fabricated into subpri- mal cuts, and weights were recorded. Ultrasound ac- curacy and repeatability were assessed using bias, SE of prediction, SE of repeatability, and simple correla- tions. Relationships among ultrasound and carcass measures, and between these measures and carcass yield and value, were evaluated using residual corre- lations and linear prediction models. Ultrasound bias approached 0 for LM area, and backfat thickness was overestimated by only 0.69 mm. The SE of prediction and r were 1.55 cm 2 and 0.75 for LM area, and 1.4 mm and 0.81 for backfat thickness, respectively. The SE of repeatability was 1.31 cm 2 and 0.75 mm for LM area and backfat thickness, respectively. At a standardized BW and backfat thickness, wethers with larger LM area and LM depth yielded larger and more valuable carcasses, and these relationships were detectable with ultrasound. For each SD increase in carcass LM area, dressing percentage increased 1.57 percentage points, gross carcass value increased US$5.12, and boxed car- cass value increased US$6.84 (P < 0.001). For each SD increase in ultrasound LM area, dressing percentage increased 0.95 percentage points, gross carcass value increased US$3.15, and boxed carcass value increased US$3.86 (P < 0.001). When LM area effects were ad- justed for carcass weight, the response in boxed car- cass value attributed to disproportionate increases in high-value subprimal cut weights was small. Associa- tions of dressing percentage and carcass value with ul- trasound and carcass LM depth were significant (P < 0.01) but smaller than corresponding associations with LM area. These data indicate biological and economical incentives for increasing LM area in wethers, and live- animal ultrasound can provide reliable estimates of carcass measures. These results are applicable to ter- minal sire breeders and producers who market sheep using carcass-merit pricing systems. Key words: backfat thickness, carcass yield, longissimus muscle area, sheep, ultrasound ©2008 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2008. 86:3203–3214 doi:10.2527/jas.2007-0836 INTRODUCTION Lamb meat production has become the emphasis of the US sheep industry (Jones, 2004). Throughout the marketing chain, lamb buyers (i.e., producers, pack- ers, retailers, and consumers) demand improvements to production efficiency, product yield, carcass compo- sition, and meat quality (TAMRC, 1991; Ward et al., 1995; LeValley et al., 2008). For the industry to remain competitive, it must produce carcasses that meet, or exceed, these buyer demands (Beermann et al., 1995). Ultrasound can afford breeders, producers, and re- searchers the ability to estimate carcass compositional B-mode, real-time ultrasound for estimating carcass measures in live sheep: Accuracy of ultrasound measures and their relationships with carcass yield and value 1,2 T. D. Leeds,* M. R. Mousel,* D. R. Notter,† H. N. Zerby,‡ C. A. Moffet,* and G. S. Lewis* 3 *USDA, ARS, US Sheep Experiment Station, Dubois, ID 83423; †Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg 24061; and ‡Department of Animal Sciences, The Ohio State University, Columbus 43210 1 The use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the USDA or the ARS of any product or service to the exclusion of others that may be suitable. 2 The authors acknowledge T. Kellom, M. Williams, T. Northcutt, and the USSES operations staff for animal procedures and data col- lection; and D. O’Diam, A. Radunz, J. Gevin, K. Brueggemeier, and A. Naber for slaughter and fabrication procedures and data collec- tion. 3 Corresponding author: gregory.lewis@ars.usda.gov Received December 28, 2007. Accepted June 6, 2008. 3203 Published December 5, 2014