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* Department of Animal Science, University of Arkansas, Fayetteville 72701 and
and
Southwest Missouri State University, Springfield 65804
2 Correspondence:
459 Shealy Dr., Room 231A, Univ. of Florida (phone: 352-392-2390; fax: 352-392-7652; E-mail:
stelzleni{at}animal.ufl.edu).
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Key Words: Genetic Correlation • Genetic Parameters • Heritability • Ultrasound
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Estimates of genetic parameters for carcass traits estimated by live-animal ultrasound are plentiful (Turner et al., 1990; Robinson et al., 1993; Shepard et al., 1996). Two authors have reported heritabilities and genetic correlations for live-animal ultrasound measurements of muscle and fat depots in yearling Brangus cattle (Johnson et al., 1993; Moser et al., 1998). However, limited data are available using a single ultrasound technician operating standardized equipment and software. Before ultrasound can be effectively utilized, each breed association must validate ultrasound heritabilities and correlations. Therefore, the objective of this research was to determine estimates of heritabilities and genetic correlations for the longissimus muscle area, fat thickness, intramuscular fat and yearling weight of yearling bulls and heifers in the Brangus breed utilizing ultrasound technology.
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Materials and Methods |
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The RTU equipment utilized for data collection was an Aloka 500V system equipped with a 3.5-MHz, 17-cm transducer (Aloka USA, Inc., Wallingford, CT) and a superflab to ensure proper fit of the transducer to the curvature of the animal’s natural body shape. The superflab (also known as a wave guide or stand-off pad) is a piece of poly-rubber that conforms to the animal’s back and side to allow good contact with both the medial and lateral ends of the ribeye. The superflab is used in conjunction with collecting the image of LMA and FT. A superflab is not used for the collection of the longitudinal image for estimating IMF. Herring et al. (1998) found the Aloka 500V to be superior in accuracy and repeatability to the Aloka 210DX, for measurements of the LMA. Image visualization and data capture was accomplished using Critical Vision (CVIS) software (Critical Vision, Inc., Atlanta, GA). Herring et al. (1998) noted that CVIS software was one of the most accurate for measuring IMF. Ultrasound and carcass measurements are sufficiently correlated to use in research and genetic improvement programs (Perkins et al., 1992; Smith et al., 1992; Shepard et al., 1996).
For scanning, transducer placement was first determined by palpating the left side of the animal between the 12th and 13th ribs. Once the scanning area was determined, corn oil (Mazola, CPC Foodservice, Englewood Cliffs, NJ) was applied, and the area was curried free of dirt and debris and oiled again before transducer placement. Animal hair was not clipped. The ultrasound probe was placed toward the midline, between and parallel to the 12th to 13th rib bones and moved laterally until the longissimus muscle came into full view on the screen (Perkins et al., 1992). All 12th-rib FT, longissimus muscle, and percent IMF images were captured for later viewing. The technician traced the outline of the muscle image, and then counted pixels to determine LMA. Fat thickness was estimated at the 3/4 position from the chine bone end of the longissimus muscle (USDA beef carcass grade standards) using the cross-sectional ribeye image. A single longitudinal image of the longissimus muscle was taken (included the 11th, 12th, and 13th ribs) for calculation of IMF. The CVIS software predicts the percentage of IMF (ether-extractable equivalent) from the longitudinal LMA image.
Data were edited to ensure uniformity of the equipment, software, and guidelines of the IBBA. Only purebred Brangus bulls and heifers intended to be used in the future as seedstock or replacement animals were retained in the data set. Two-generation pedigrees were obtained from the IBBA database. Live-animal ultrasound measurements that predated 1995 were excluded, eliminating those images collected using the split screen technology of the Aloka 210DX.
Yearling weights and ultrasound data were only considered from those animals that were weighed between 320 and 410 d of age (IBBA guidelines). Animals were assigned to contemporary groups based on sex, breeding season, and environment. Contemporary groups with progeny from only one sire were eliminated from the data set. The progenies of 309 sires and 170 paternal grandsires were represented in 23 contemporary groups. In the 23 contemporary groups, the sires had a range of 3 to 49, with a median value of 20. The progenies included in the contemporary groups had a range of 6 to190 and a median value of 45. Descriptive statistics for the edited data set used for analysis are shown in Table 1
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All possible two-trait analyses were performed to obtain genetic and phenotypic correlations. The pedigree contained 4,134 records. The MTDFREML program used does not provide information on standard errors of estimated genetic parameters. Cold restarts were used to attempt to avoid local maximums (Boldman et al., 1993). Convergence was assumed when the variance of the simplex reached 1 x 10-8 for each analysis, and the first four decimals of -2 log likelihood did not change.
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Results and Discussion |
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. The moderate to high heritability estimated for LMA indicates substantial additive genetic effects on the area of the LMA. The estimate for heritability found in this study falls between the estimates reported by Johnson et al. (1993) of 0.40, and Moser et al. (1998) of 0.29 for ultrasonically obtained values of the longissimus muscle in Brangus cattle. Shepard et al. (1996) reported a low heritability of 0.11 in ultrasonically measured yearling Angus cattle, and Arnold et al. (1991) reported heritabilities of 0.25 for weight constant, and 0.28 for age constant estimates of ultrasonically obtained values for the longissimus muscle in Hereford cattle. Previously reported heritabilities for carcass measurements of the LMA are 0.17 averaged across purebreds (Gregory et al., 1995); 0.28 in Hereford bulls (Lamb et al., 1990); and 0.32 in Angus cattle (Wilson et al., 1993). Johnson et al. (1993) suggested that heritabilities for ultrasound measures could be different from those of actual carcass measurements due to the error associated with obtaining and interpreting ultrasonic images. Heritabilities could also be different because live yearling RTU measurements are not the same biological trait as the carcass analog. Moser et al. (1998) stated that heritability increased to 0.44 when only LMA measurements from 1994 and later, using the Aloka 500V, were analyzed. Fat thickness heritability for beef cattle has ranged from 0.04 to 0.52 for ultrasound measures (Benyshek, 1981; Lamb et al., 1990; Turner et al., 1990). The heritability found in this study was also in the middle range of those pertaining to ultrasound measures across beef breeds, and higher than heritabilities previously reported for the Brangus breed. Johnson et al. (1993) reported a heritability of 0.14, and Moser et al. (1998) found a heritability of 0.11 for ultrasonically measured 12th-rib fat thickness. The difference in heritability estimates in this study might be attributed to the smaller sample size and larger additive genetic variance in this study. Although estimates of heritability of ultrasonically measured FT vary, the moderate estimate in this study suggests that selection could reduce external FT.
The estimation of genetic parameters for IMF predicted by ultrasonic measurement was of primary interest in this study. Information on the heritability of IMF (marbling) by ultrasound technology was not found in the literature. Literature was found on carcass marbling scores, and the correlation of ultrasonic images of IMF and ether- extractable fat. The estimate of heritability for IMF in this study was low, and suggests that IMF is substantially influenced by factors other than direct additive gene effects. The estimate of heritability for IMF reported in this study does differ from the heritability estimates of marbling scores previously published. Estimates of heritability for marbling score ranged from 0.26 to 0.47 (Benyshek, 1981; Arnold et al., 1991; Wilson et al., 1993). Herring et al. (1998) reported a correlation of 0.61 between the predicted and actual percentage of ether-extractable fat for the CVIS system. Limited research has been done to determine the best ultrasound machine to predict IMF (Herring et al., 1998), and to determine the repeatability needed to estimate percent IMF for ultrasound machines (Hassen et al., 1999). Hassen et al. (1999) reported that taking repeated measurements per animal could improve the precision of predicting percent IMF. Therefore, increasing the number of images per animal to at least four should improve the precision of ultrasound IMF values.
Genetic and Phenotypic Correlations
Little research has been completed comparing direct correlations of ultrasound measurements; instead, ultrasound measurements have often been correlated to carcass measurements. Genetic and phenotypic correlations are presented in Table 2. Genetic correlations for ultrasound traits in this study were low to moderate. The low, almost zero, genetic correlation between LMA and FT (-0.09) suggests that there are no pleiotropic effects between these two traits. Also, there is a low genetic correlation between LMA and IMF. The low correlations for these three traits indicate that a change in the LMA would not induce a change in either the FT or marbling traits of the animal. Moser et al. (1998) found a genetic correlation of 0.13 for ultrasonically obtained images of LMA and FT. However, Arnold et al. (1991) reported a stronger genetic correlation of 0.48 between ultrasonically measured LMA and FT, and a -0.37 genetic correlation between the carcass measurement of the LMA and 12th-rib fat thickness. Information was not found in the literature for comparisons between ultrasonically obtained images of LMA and IMF with what was found in this study.
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The moderate genetic correlation between LMA and YW found in this study is in agreement with the genetic correlation reported by Moser et al. (1998) and Johnson et al. (1993), who reported genetic correlations of 0.49 and 0.39, respectively, for ultrasonically measured LMA. The moderately positive genetic correlation between FT and YW found in this study does not agree with either the strength of association found by Moser et al. (1998) of 0.11, or by direction of association found by Johnson et al. (1993) of -0.53. The moderate correlations found in these traits suggest that there are pleiotropic properties involved that should be taken into account when selecting for these traits.
Phenotypic correlations for all traits in this study were small to moderate. Correlations between IMF and LMA, and IMF and YW were both near zero. The phenotypic relationship between LMA and FT and IMF and FT were both low (less than 0.2). This suggests that in the environment in which these animals were reared, there is no to little relationship between the performance of these traits. The phenotypic correlations for YW and LMA and YW and FT were moderate (0.44 and 0.32, respectively), showing a mild association between the performances of these traits.
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Implications |
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Footnotes |
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Received for publication September 21, 2001. Accepted for publication July 26, 2002.
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Literature Cited |
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This article has been cited by other articles:
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  X. J. Yang, E. Albrecht, K. Ender, R. Q. Zhao, and J. Wegner Computer image analysis of intramuscular adipocytes and marbling in the longissimus muscle of cattle J Anim Sci, December 1, 2006; 84(12): 3251 - 3258. [Abstract] [Full Text] [PDF]
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