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Ultrasound Applications for the Beef Industry

Livestock Update, April 2003

Scott P. Greiner, Ph.D., Extension Animal Scientist, Beef, VA Tech

Ultrasound technology has significantly impacted the beef industry in recent years. Adoption of ultrasound technology has been a result of the industry's shift from commodity to consumer-driven, with a corresponding focus on carcass merit and end product value. There are two primary applications for ultrasound: 1) as a selection tool, and 2) as a management tool. Use as a selection tool involves collection of ultrasound carcass data on yearling bulls and heifers, and the subsequent use of this data to generate carcass EPDs. The use of ultrasound as a management tool involves practices such as sorting feedlot cattle into carcass outcome groups, and projecting optimum slaughter endpoints. The common denominator between these two applications is that ultrasound can be used to predict carcass traits such as fat cover, ribeye area, and intramuscular fat (marbling) on the live animal. However, using ultrasound technology to generate carcass EPDs vs. sort feedlot cattle are distinctly different applications.

Ultrasound Use in Genetic Evaluation
Historically, breed associations have relied on structured sire carcass progeny testing programs to amass data for carcass EPD calculation. However, this process is time consuming, expensive, and results in only a modest amount of data on a few sires. Ultrasound scanning of yearling seedstock for carcass merit (and use of this data to generate EPDs) has been extensively researched and determined to be an effective means to alleviate some of the shortcomings of carcass progeny testing. This use of yearling bull and heifer ultrasound data to generate carcass EPDs which will be applicable to slaughter progeny is contingent on three primary points: 1) ultrasound measures carcass traits in the live animal in a consistently accurate manner, 2) ultrasound traits are heritable, and 3) the genetic relationships between ultrasound carcass traits in seedstock animals and the same measures in finished cattle are favorable.

Ultrasound Accuracy: Generally, the term accuracy has been used to refer to the relationship between an ultrasound measurement taken on the live animal and a subsequent measurement taken shortly thereafter on the animal's carcass. For the purposes of genetic evaluation, ultrasound needs to accurately asses body composition at the time the measurements are taken (i.e. ultrasound measures replace carcass measures).

Many studies have been conducted over the years to define this relationship. A summary of early ultrasound work reported average correlations between ultrasound and carcass traits of .86 and .73 for fat thickness and ribeye area, respectively (Duello, 1993; M.S. Thesis, Iowa State University). Recent research reviews have confirmed that ultrasound can be used to accurately and repeatably measure carcass traits in the live animal in a precise manner (Williams, 2002. J. Anim. Sci. 80:E183).

The majority of the variation in ultrasound accuracy has been attributed to differences in technician proficiency. Consequently, the industry has developed certification standards to measure technician accuracy, and breed associations utilize technicians that have been accredited for compilation of ultrasound data for use in genetic evaluations. Technician statistics from the Fall 2002 Annual Proficiency Testing and Certification Program confirm ultrasound is an accurate predictor of carcass traits. Average correlations for the 43 participating technicians were ..89, .86, and .70 for fat thickness, ribeye area, and intramuscular fat, respectively. Furthermore, standard errors of prediction indicate that 67% of the time the average technician was within .06 in. fat thickness., .76 sq. in. ribeye area, and .87 % intramuscular fat of the actual measure. These field results further confirm the structured research findings that when performed by a well-trained, certified technician ultrasound predicts carcass merit with acceptable precision. Furthermore, the fact that live animal equations developed from ultrasound measurements have been shown to be similar in accuracy to equations developed from carcass measurements for predicting beef carcass composition is further testimony to the usefulness of the technology (Greiner et al., 2003. J. Anim. Sci. 81:466).

The relationship between marbling score and intramuscular fat and the role of each in selection programs for carcass quality are frequently misunderstood and even debated within the industry. Marbling score is a component of the USDA beef quality grading system, and refers to visible fat found between muscle fiber bundles within the ribeye muscle. Marbling score is assessed visually by a USDA grader during the process of assigning a USDA Quality Grade, and is therefore a subjective measure. In addition to the quantity, the distribution and texture of visible fat flecks within the ribeye are considered during assessment of marbling score. USDA Quality Grade (Select/Choice/Prime) is primarily a function of marbling score for most of the U.S. fed cattle population.

Intramuscular fat also quantifies the fat found between muscle fiber bundles within the ribeye muscle. Intramuscular fat is determined by chemical extraction of lipid from a thin facing of the exposed ribeye muscle. Therefore, percent intramuscular fat is an objective measurement that quantifies the total fat content within the ribeye muscle. Research studies have found a strong relationship between marbling score and percent intramuscular fat, with correlation values ranging from .70 to .90 (Wilson et al., 1998. ISU Beef Res. Report R1529). Since both marbling score and intramuscular fat quantify lipid with in ribeye muscle, why is this relationship not perfect (correlation 1.0)? With one measure being subjective and the other objective, a perfect relationship would not be expected. Additionally, marbling score considers amount, distribution, and texture of fat whereas intramuscular fat simply quantifies the amount (percent) of fat within the ribeye. The following table outlines the relationship between intramuscular fat percentage, marbling score, and USDA quality grade. Note that within a quality grade, a range for both marbling score and percent intramuscular fat exist.

Relationship Between Chemical % Intramuscular Fat and USDA Quality Grade
% Intramuscular Fat USDA QG Degree of Marbling (Marbling Score)
< 2.30 Standard Traces (3.0-3.9)
Slight (4.0-4.9)
Small (5.0-5.9)
Modest (6.0-6.9)
Moderate (7.0-7.9)
Sl. Abundant (8.0-8.9)
Mod. Abundant (9.0-9.9)
(from Wilson et al., 1998. ISU Beef Res. Report R1529)

Both marbling score and intramuscular fat records are useful for genetic evaluation programs. Marbling score data acquired through structured sire evaluation progeny testing programs is utilized to generate Marbling EPDs. Software has been developed that predicts percent intramuscular fat using ultrasound. Ultrasound predicted intramuscular fat models have demonstrated the ability to explain 80-85% of the actual variation in chemically determined intramuscular fat, and a strong relationship between ultrasound predicted intramuscular fat and actual carcass marbling score has been demonstrated (Perkins et al., 1997. J. Anim. Sci. 75:178; Duckett and Kline, 1997. J. Anim. Sci. 75:113).

Heritability: Heritability is the proportion of the measurable difference observed between animals for a given trait that is due to genetics (and can be passed to the next generation). A summary of 10 research studies shows average heritability estimates of .32. .28, and .41 for ultrasound ribeye area, fat thickness, and intramuscular fat %, respectively (Bertrand et al., 2001. J. Anim. Sci. 79:E190). As a basis for comparison, the 2003 Spring Sire Evaluation of the American Angus Association used a heritability of .20 for weaning weight.

Genetic Relationships with Finished Cattle: The genetic relationship between ultrasound measures in seedstock animals and the same measures in finished cattle has been the center of ultrasound research in recent years. Put another way, does selection for increased intramuscular fat using ultrasound in yearling bulls and heifers also result in slaughter steers and heifers with more marbling (when both are expressed on an age constant basis)? The answer is yes, despite the fact that yearling bulls have much lower levels of intramuscular fat than finished cattle. The research that has been conducted in this area indicates a favorable genetic relationship between ultrasound measures of fat thickness, ribeye area, and intramuscular fat in the live animal and corresponding carcass traits measured in finished steers and heifers. Genetic correlations that are high (.70 or greater) can be cited for each trait (Bertrand et al., 2001. J. Anim. Sci. 79:E190).

The research literature validates the usefulness of ultrasound in genetic evaluation programs. The power of ultrasound resides in the ability to quickly amass large amounts of data, through the scanning of yearling bulls and heifers, for use in carcass trait genetic evaluations in a relatively cheap fashion. Moser (1997. Ph.D. Thesis, University of Georgia) estimated the cost of structured sire carcass testing programs at $97 per progeny. This compares to a cost of less than $20 per bull or heifer to collect ultrasound measurements. The Spring 2003 Angus Sire Evaluation Report indicates that 222,164 yearling bull and heifer records from 10,018 sires and 135,748 dams were used to calculate ultrasound body composition EPDs. The Spring 2003 Angus carcass evaluation includes 69,273 steer and heifer carcass records from 3,990 sires. The fact that the progeny carcass records have been assembled over the last 15-20 years (since the inception of the sire carcass testing program), and ultrasound records only since 1999 emphasizes a primary advantage to ultrasound- generating a large database in a relatively short period of time. As a result, selection is enhanced as EPDs are available on more animals in less time. As further evidence, rankings of proven sires compare favorably for carcass-derived EPDs and ultrasound-derived EPDs (rank correlations >.80 for all traits; Doyle Wilson, Iowa State University, personal communication).

Ultrasound Use as a Management Tool
The second primary application of ultrasound technology to the industry, as a management tool, also has great promise. Decision-support systems that are effective in sorting cattle prior to harvest or projecting optimum harvest endpoints also utilize ultrasound technology. Ultrasound measurements of fat cover, muscle, and intramuscular fat are used in several systems along with a variety of other parameters that may influence carcass outcome or profitability including frame size, gain performance, weight, and feed/cost of gain parameters (Williams, 2002. J. Anim. Sci. 80:E183). With these systems, ultrasound is frequently used to assess body composition at various times during the feeding phase- at delivery, reimplant time, or 30 days prior to harvest as examples. These ultrasound measures are then combined with other pertinent information, and using computer-based models outputs are generated to assist in management decisions.

As mentioned earlier, it is important to distinguish between feedlot application of ultrasound and that used for genetic improvement. Feedlot models take ultrasound measurements, and combined with other information, project phenotype of the animal sometime in the future. This is distinctly different than application to yearling bulls and heifers, when ultrasound data is utilized to conduct genetic evaluations. For genetic evaluation purposes the interest is in accurately characterizing carcass traits in the live-animal at a particular point in time. This point in time is generally within a given age range, for which guidelines and recommendations for seedstock bulls and heifers have been developed to enhance the probability that the seedstock cattle will express differences in ultrasound measurements to be useful in EPD calculation. For this reason, ultrasound data taken at times outside the recommended window are not utilized in genetic evaluations. Put another way, we should not expect intramuscular fat measurements taken outside the recommended window to be necessarily indicative of a bull's genetics for marbling. The genetic relationships between seedstock ultrasound measures and carcass traits in slaughter cattle have been shown to be favorable when ultrasound measures are taken at the appropriate time. EPD calculation with ultrasound traits utilizes the same concepts applied to other performance measures such as growth, which includes proper contemporary group identification. Contemporary groups are animal that have been treated alike and given the same opportunity to perform, and deviation of an individual from the average of the contemporary group is the useful component to genetic evaluation. These aspects, combined with the acceptable levels of accuracy, heritabilities, and favorable genetic relationships with carcass traits in finished cattle allow ultrasound measures to work for cattlemen.

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