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Ultrasound Evaluation to Improve Carcass Merit

J. Willard Lemaster
4-H Animal Science Extension Specialist
University of Maryland - College Park
Maryland 4-H Center

Introduction
Since the 1950's, ultrasound technology for biological application has been available for use. The equipment used then is referred to as A-mode and was only capable of measuring fat and muscle depth. Ultrasound consists of very high frequency sound waves. Frequencies between one and 7.5 megahertz (MHz) are generally used for animal use with 3.5-5.0 MHz for live animal evaluation and 5.0-7.5 MHz most common for reproductive uses.

Pulses are produced in a transducer by the vibrations of piezoelectric crystals. These pulses are transmitted through tissue until they reach a tissue interface, such as between fat and lean tissue. At the interface, a portion of the sound wave continues to penetrate the tissue while part of the wave is reflected back to the transducer. The transducer acts as a receiver and the reflected waves produce mechanical energy as they strike and deform the piezoelectric crystals.

The B-mode ultrasound, ultrasound that is used currently, consists of a linear array of several transducers that are fired in succession to send sound waves into the tissue. These sound waves interact to form patterns of energy within the tissue. This technique is used to focus the linear array transducers in order to optimize the depth resolution upon reception of the ultrasound signals. The display for B-mode is a two-dimensional display of dots or pixels. The brightness of each dot is proportional to the strength of the returning echo. Real-time ultrasound is a version of B-mode, but the display of dots on the screen is updated almost instantly. Ultrasound images appear in various colors and shades on the display unit. Bone and fat will appear white in color, while muscle, tissue and the corpus luteum will appear a dense grey, and fluid (follicles and cysts) will appear black in color.

Methodology
Body composition measurements are taken with an Aloka 500 real-time ultrasound machine equipped with a 3.5 MHz transducer designed for animal use. In the normal scenario for estimating carcass traits via ultrasound, a "Certified Technician" travels to a designated location with portable ultrasound equipment. Upon restraint of an animal, the technician would apply a "couplant" (usually vegetable oil) to the back of the animal at a designated location. The couplant prevents the interference of air between the transducer and the animal. This allows for maximum conduction of sound waves. Real-time ultrasound will allow for an image to be produced immediately. This image can be captured to a computer's hard drive allowing for the images to be interpreted at a later time.

Figure 1

Ultrasound measurements for backfat thickness (BF) and longissimus dorsi area (Ribeye area, REA) are taken between the 12th and 13th ribs on each animal (Figure 1). Some additional measurements that are taken from cattle are for intramuscular fat (Marbling/Percent Intramuscular Fat, PIMF) is also taken across the area between the 12th and 13th ribs parallel to the spine. Lastly, the rumpfat measurement is taken on an imaginary line between the hip bone and the pin bone with the superficial gluteus medius muscle used as a landmark.

Ribeye Area
Ribeye area is measured in square inches and is positively and highly correlated with percent retail product (%RP). This trait is a moderately high heritable (>.4) trait. This means that the trait will be passed on to progeny. Ultrasound measurements of REA are accurate within one square inch of the actual REA measurement. Correlations between ultrasound REA and carcass REA vary between .62 and .94. Much of this variation may be due to the accuracy of the technician.

Backfat
Backfat thickness is measured in inches, and is a good indicator of %RP. However, unlike REA, it is negatively and highly correlated with %RP. This means, as BF increases, %RP decreases. This trait is similar to REA in heritability (>.4). Ultrasound measurements of BF are accurate to within 0.07 inches of the actual carcass BF. Ultrasound BF is highly correlated (.96) with carcass BF. Some believe ultrasound BF may be more accurate than carcass BF, because of the fact that no BF has been removed during the ultrasound process. Unlike in the packing plant, varying amounts of BF may be removed when the hide is removed from the carcass.

How Does a Producer Use Ultrasound Information?
The most important thing to remember is comparisons should be made between sire groups and/or within contemporary groups. Differences between individuals should not be considered accurate unless these animals are within a contemporary group, and their differences in carcass traits are much greater than the standard deviations mentioned above for each of the traits.

One should closely monitor BF; this is an indicator of development of the animal and/or the maturity status of the animal. One must remember these are just a couple of traits to evaluate, and one should not get carried away with single-trait selection. With single-trait selection, one may be creating more problems than what they may be attempting to correct with single-trait selection. Before any selection on future genetics, one must have a defined set of goals established for their own operation.

Conclusions
The ability of ultrasound to predict carcass traits has made great strides in the last ten years, but there is still need for improvement in the area of PIMF for cattle to be made. Furthermore, the accuracy of a certified technician is very important to achieve the highest accuracy in data collection. Highly skilled technicians have demonstrated accuracy levels that compare favorably with measurements taken on the carcass. Accuracy is also dependant upon the ultrasound equipment and software being used to collect and process the images. Ultrasound technology enables the producer to make accurate and rapid decisions toward increasing carcass quality and subsequent profit.

As ultrasound EPDs are being generated by various breeds, this is providing an invaluable tool for seedstock producers to produce breeding stock with genetics for high-quality, high-value carcasses for the future. In conclusion, ultrasound is another tool for a commercial producers, along with seedstock producers, to utilize to make more accurate and informed decisions about future genetics without having to wait years for carcass data to be generated. Furthermore, the accuracy of the ultrasound data is as only as good as the accuracy of the technician, equipment and producers that are reporting the data.