Using Ultrasound Estimates of Loin Muscle Measurements to Obtain Weight-adjusted Ribeye Area in Targhee Rams
Lisa Surber¹, Rodney Kott¹, and David Notter²
¹Montana State University, Bozeman, Bozeman, MT 59717
²Virginia Polytechnic Institute and State University, Blacksburg, VA 24061
Background
In recent years, ultrasound measurement of ribeye area (REA) has allowed objective measurements of important carcass traits to be used in livestock selection programs. The sheep industry is behind the swine and beef industry in use of these carcass measurements in selection programs. US lambs are thought to have larger REA than imported lambs and this trait distinguishes US lambs from imported product. Some observational data suggest that REA in US lambs may have decreased in recent years.
Due to its relative low cost and portability, ultrasound technology has been incorporated in national genetic programs for lamb carcass quality improvement in many parts of the world. Many sheep breed organizations are initiating carcass value traits and expected progeny differences (EPD). However, there are no standardized procedures to accurately ensure repeatability among technicians, and, due to differences in production and management, no established weight and/or age at which to measure REA.
Purpose Statement
The objective of this research is to develop a recommended method to adjust ram ultrasound scanning estimates. The rationale behind these adjustments is for comparison purposes at production sales, and incorporation into EPD analyses in the National Sheep Improvement Program.
Summary of Findings
Ultrasound technology was used to assess carcass characteristics in sheep. Forty-two Targhee rams were used to examine growth in weight and REA (Figure 1). MSU Targhee rams were measured for REA at 12th/13th rib transverse using ultrasound approximately every 4 weeks from November through May under a typical ram development program for western range rams. Results (Figure 1). This data indicated that a 190-lb ram should have a 3-inch REA, and that REA changed by ±0.015 inches for every 10-lb change above or below 190 lb. Body weight differences accounted for 67 % of variation in REA. However, because typical growth curves are not linear, linear adjustment is not appropriate, and provides no information on the optimal age or weight for measurement in genetic evaluation programs. In order to have a measurement that is comparable in animals of different sizes and useful to the sheep industry, REA should be measured at a time when animals are at a uniform physiological stage.
At the same degree of maturity, measures of cross-sectional area such as REA are expected to be proportional to the 2/3 power of body weight. This expectation comes from a geometric argument that the volume (weight) of an individual may be approximated from the product of its height, length, and width whereas a cross-sectional area is expected to be proportional to only the product of height and width. We propose that the optimal time to measure REA in growing rams is when REA is most nearly proportional to the 2/3 power of body weight (Figure 2).
A proportionality constant of greater than 0.67 suggests that compensatory growth in REA is occurring, and a constant of less than 0.67 indicates maturity has progressed to the point that growth in REA has slowed. Data in Figure 2 suggests that the ideal scanning weight for these Targhee rams would be between 180 and 200 lbs with limits of 170 to 210 lbs. This weight range corresponds to the most linear period of growth in Figure 1. Adjusted REA would be obtained as:
190 lb Adj. REA = (Target wt / Actual wt)0.67 X (Actual REA).
Conclusions
Measurements of REA should be adjusted for body weight for use in genetic evaluation. Otherwise, selection pressure will primarily be for weight at scanning. A similar analysis using intensively managed Suffolk ram lambs at Virginia Tech suggests that Suffolk lambs should be scanned at about 150 lbs. We have always been concerned that linear conversion preferentially benefits lighter rams and penalizes heavier rams. Because linear adjustment involves fitting a straight line to a non-linear growth curve, the weight-adjusted REA over- and underestimates REA at the ends of the curve. Use of a power function for adjustment reduces this bias. Our data thus suggest that scanning Targhee yearling rams in a typical Western range development program at 190 lbs (approximately 12 months of age) is comparable to scanning intensively managed Suffolk ram lambs at 150 lbs (5 to 7 months of age).
Applications
These data and procedures can and have been applied to the sheep industry in three areas. At the Miles City Ram Sale, ultrasound data is now as important as wool micron measurements or performance testing to producers purchasing rams. Over 90% of the rams sold at that sale now report ultrasound REA. Our work has helped develop a selection strategy that incorporates measurement of carcass merit into selection programs. Our ultrasound procedures will be used to develop a standardized protocol for ultrasound scanning of sheep. Lastly, our data will assist in development of a carcass merit EPD in the Targhee breed.