|NOTTER, DAVID - Virginia Polytechnic Institution & State University|
|ZERBY, HENRY - The Ohio State University|
|SURBER, LISA - Montana State University|
|Leeds, Timothy - Tim|
|MOELLER, STEVEN - The Ohio State University|
|Taylor, Joshua - Bret|
Submitted to: Sheep and Goat Research Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2014
Publication Date: 12/1/2014
Publication URL: d1cqrq366w3ike.cloudfront.net/http/DOCUMENT/SheepUSA/SGRJ_V29_29-35_Notter_12-14.pdf
Citation: Notter, D.R., Mousel, M.R., Zerby, H.N., Surber, L.M., Leeds, T.D., Moeller, S.J., Lewis, G.S., Taylor, J.B. 2014. Impact of changes in weight, fat depth, and loin muscle depth on carcass yield and value and implications for selection and pricing of rams from terminal-sire sheep breeds. Sheep and Goat Research Journal. 29:36-44.
Interpretive Summary: The development of breeding objectives and associated selection indexes is critical for integrated programs of genetic improvement. In particular, linkages between breeding objectives in seedstock flocks and profitability in commercial production are essential to ensure the relevance of the seedstock sector. With this in mind, we set out to: 1) describe the relationship between ultrasonic measurements of loin muscle area and depth in lambs; 2) develop equations to predict yield and value of carcasses of crossbred lambs sired by terminal-sire sheep breeds from body or chilled carcass weights, ultrasonic measurements of fat depth, and predicted ultrasonic measurements of loin muscle depth; and 3) link these prediction equations to estimated breeding values (i.e., EBV) for postweaning weights and ultrasonic fat and loin muscle depths to develop indexes for selection and pricing of rams that are to be used as terminal sires. We confirmed that postweaning body weight was the primary determinant of terminal-sire value across a wide range of production systems and marketing scenarios. Effects of increasing loin muscle depth and decreasing fatness were also positive, but smaller than effects of increasing postweaning growth. Furthermore, we determined that the LAMBPLAN Carcass Plus index was an effective terminal sire index for U.S. producers (LAMBPLAN is an Australia-based sheep genetics and breeding value data-management resource).
Technical Abstract: Breeding objectives and selection indexes are necessary to support comprehensive genetic improvement programs. This study used off-test body weights (OTBW) or chilled carcass weights (CCW), ultrasonic measurements of fat depth (USFD, mm), and predicted ultrasound loin muscle depths (USLMD, mm) from 456 wether lambs to predict carcass value and link predictions to estimated breeding values (EBV) of terminal sires. Carcasses were processed by closely trimming high-value cuts (rack, loin, leg, and sirloin), and carcass value (TrCVal) was determined for each carcass. Increasing OTBW had positive effects on carcass value but did not affect dressing percentage (DP). Increasing USFD increased CCW and DP but decreased TrCVal. Increasing USLMD had positive effects on CCW, DP, and TCVal. The EBV for postweaning weight (PWWT), USFD, and USLMD of average and elite Suffolk rams were compared to develop breeding objectives for lambs harvested at a constant time on feed, harvest weight, or harvest fatness, and for a scenario with larger-than-current price premiums for leanness and muscling. At constant harvest weights, the breeding objective was I2 = 1.2 EBVPWWT – EBVUSFD + 0.8 EBVUSLMD, but changed to I4 = 0.3 EBVPWWT – EBVUSFD + 0.4 EBVUSLMD if carcass price was strongly influenced by leanness and muscling. Genetic correlations among indexes exceeded 0.85. Index I4 was strongly correlated with the Australian LAMBPLAN Carcass Plus index, indicating that selection on Carcass Plus would be effective under U.S. conditions. All indexes were dominated by PWWT EBV. Effects of increasing muscling were substantial, but changing USFD EBV had only modest effects.