Volume 45 Number 4 Winter 1998

Sonic Swine
Ultrasonic Equipment Helping Produce Leaner Pork

Daryl Kuhlers, Nada K. Nadarajah, Steve Jungst, Elisabeth Huff-Lonergan,
Steve M. Lonergan, and Brian L. Anderson

Consumers are demanding leaner red meat with less fat, and the meat industry is striving to meet that demand. A few meat packing plants already are paying a premium price to market hog producers on the basis of percent lean in individual carcasses. In general, a relationship exists between total percentage of lean meat in pork carcasses with reduced backfat thickness on the tenth rib and larger loin eye muscle area. Hence, selective breeding for a combination of low fat and more muscled pigs should play a major role in future pork production and market success. An AAES breeding program has helped define ways to correlate ultrasound measurements of live animals and carcasses to sound breeding decisions.

Real-time ultrasound technology allows producers to measure backfat thickness and muscle depths in live pigs. Ultrasonic estimation of fat and muscle dimensions of live animals is feasible and is a relatively cheap method to make useful predictions of percent lean yield in carcass. This information also can be used by animal breeders in selecting breeding stock to improve carcass quality in future generations of pigs.

Correlating ultrasound measurements of backfat and loin muscle depths on live pigs to selection of superior breeding animals for improvement of carcass yield and quality has been the focus of an on-going AAES research project. The study involving almost five generations of selective breeding has resulted in a population of pigs superior for lean meat production.

The study was initiated at the Auburn University Swine Breeding Unit in 1993 to select a line of Landrace pigs for larger loin muscle area using ultrasound techniques to measure the backfat thickness and loin eye muscle area of live pigs. The breeding experiment data were analyzed using the latest genetic evaluation methods to select superior breeding animals as parents on the basis of their genetic values (breeding value) estimates to produce the next generation of pigs. So far, four generations of selection have been completed and the fifth generation is underway. In order to evaluate the selection response and to compare the differences with an unselected base population of Landrace pigs, a control line was also maintained under similar feeding and management conditions throughout this study.

An Aloka 500V real-time ultrasound machine was used to measure fat thickness and loin eye muscle area on live pigs at about 168 days of age. Based on those measurements and subsequent analyses of the breeding data, a line of pigs was selected solely for increased real-time ultrasound loin eye area for the past four generations along with an unselected control line. Scanned data for loin eye muscle area and backfat thickness at the tenth rib on 1,142 pigs, sired by 78 boars and out of 187 sows over four generations, were available for the current analyses. Additional data from the fifth generation will be incorporated into the final analyses of this investigation.

Ultrasound equipment—Aloka500V (with small screen) —is used to measure the loin muscle and fat depths in live animals. The computer stores the images for later evaluation.

Ultrasound measurements for loin muscle area in the select line pigs were found to be larger in each subsequent generation. The difference between the selected and the control lines for ultrasound loin eye muscle area on live animals in the fourth generation was 1.5 square inches. The correlated response in the select line for the ultrasound measure of backfat thickness was a reduction of 0.1 inch over the control line pigs in the fourth generation.

In each generation, measurements were recorded for carcass length, tenth rib backfat thickness, loin eye muscle area, and percentage trimmed lean cuts from one barrow (a castrated boar) per litter. Additional measures of carcass quality were recorded on loin muscle, including meat color (1 = pale, 5 = dark) and marbling (1 = practically devoid, 5 = moderately abundant). In generations 3 and 4, firmness (1 = very soft, 5 = very firm) scores on pork chops cut from the tenth rib also were obtained. A total of 155 carcasses was evaluated.

Figure 1. Changes in loin eye area (top), backfat thickness (middle), and lean cuts percentage (bottom) because of genetic selection. 

Differences in carcass loin eye muscle area between the selected and control lines were 0.4, 0.8, 1.2, and 1.3 square inches in generations 1, 2, 3, and 4, respectively (Figure 1, top graph). For carcass length, the selected lines were 0.5, 0.2, 1.2, and 1.4 inches shorter than the control line in generations 1 through 4, respectively. For backfat thickness at tenth rib, the selected line showed a reduced fat thickness of 0.07, 0.21, 0.14, and 0.15 inches, respectively, over generations 1 to 4 compared to the control line (Figure 1, middle graph). On average, the select line pigs showed superior muscling as measured by the percentage of trimmed lean cuts of 0.9, 2.2, 2.6, and 2.0% in generations 1, 2, 3, and 4, respectively, over the control line pigs (Figure 1, bottom graph).

The differences between select and control lines for the subjective meat quality traits of color were -0.27, -0.11, -0.59, and -0.54, and for marbling, the line differences were 0.27, 0.34, -0.48, and -0.55, in generations 1, 2, 3, and 4, respectively. For firmness, the differences between select and control lines were -0.91 and -0.95 in generations 3 and 4, respectively.

Pork chops from a control line pig (left) and select line pig (right). Notice the large size and pale color of the chop from the select line pig.
 A select line loin (center) compared to two loins from the control line for size and color. 

These results suggest that selection for loin eye area using real-time ultrasound will result in desirable changes in carcass loin muscle area, percent lean muscle cuts, and backfat thickness at tenth rib. However, undesirable changes in carcass length, color, marbling, and firmness may also occur.

Kuhlers is Professor and Nadarajah is Research Associate of Animal and Dairy Sciences; Jungst is former Research Associate of Animal and Dairy Sciences, currently with PIC (USA), Inc; Huff-Lonergan and Lonergan are former Assistant Professors of Animal and Dairy Sciences, now at Iowa State University;
and Anderson is Manager of the AAES Swine Breeding Unit.

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