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* University of Louisiana, Monroe 71209;
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Louisiana State University Agricultural Center, Baton Rouge 70803;
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Texas A&M Research and Extension Center, Corpus Christi 78406; and
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Central Community College, Hastings, NE 68902
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Key Words: Brahman steer • carcass trait • genetic parameter
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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), DeRouen et al. (1992), and Wheeler et al. (2001) reported that meat from Brahman cattle is less tender than meat from nonBrahman cattle. Also, Luckett et al. (1975), Johnson et al. (1990), and O’Conner et al. (1997) reported that shear force of meat increased as the percentage of Bos indicus inheritance increased in crossbreds. Additionally, Damon et al. (1960), DeRouen et al. (1992), and Wheeler et al. (2001) reported that carcasses of Brahman steers had lower marbling scores than nonBrahman steers, resulting in lower carcass quality grades.
With the emphasis being placed on carcass and tenderness traits in the beef cattle industry today, obtaining information on heritabilities and genetic correlations of these traits is necessary in order to design a scheme to improve purebred Brahman cattle. Most reviews of genetic parameters for carcass traits have been from Bos taurus cattle (Marshall, 1994; Koots et al., 1994; Utrera and Van Vleck, 2006). Crews and Franke (1998), Elzo et al. (1998), and Riley et al. (2002, 2003) published genetic parameters for carcass traits of Brahman and Brahman crossbred cattle. Johnston et al. (2003) reported slightly greater heritability estimates for shear force of tropically adapted breeds in Australia than for temperate breeds.
To learn more about genetic variation in the Brahman breed, the objective of this study was to gain additional information about heritabilities and genetic correlations for postweaning growth, carcass, and tenderness traits in purebred Brahman steers and to predict EPD of sires represented in the sample of steers.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Paternal half-sib Brahman male calves (n = 467), sired by 68 bulls, were purchased from 17 breeders of purebred Brahman in Louisiana over 5 yr (1996 to 2000). Calves were spring-born, weaned in the fall, and transported from the breeders’ farms to the LSU Agricultural Center Central Station in Baton Rouge. Owners of these Brahman herds indicated their sires were selected on the basis of some combination of BW at 12 or 24 mo of age, muscle thickness, breed characteristics, parental or individual show ring performance, or pedigree. None of the producers considered carcass merit in sire selection. Most bull calves purchased from the producers were average or above average for weaning weight in that herd-year group.
Management
Calves were castrated, dehorned if necessary, given appropriate vaccinations, and dewormed within 2 wk after arrival at the LSU Agricultural Center Central Station. Each calf was assigned an identification number for data collection and for association with its ancestral pedigree. After processing, calves were placed on common Bermudagrass (Cynadon dactylon) pastures that had undergone regrowth, with 2.3 kg of a high-roughage, corn-based diet (12% CP, DM basis per head·d–1. Bermudagrass hay was available at all times. Calves were placed on ryegrass (Lolium multiflorum) pasture on approximately 1 December each year, stocked at approximately 730 kg of calf BW per ha, and grazed for an annual average of 120 d. Steers were implanted with Synovex (Fort Dodge Animal Health, Overland Park, KS) before placement on ryegrass the first year and with Ralgro (Schering-Plough Animal Health Corporation, Kenilworth, NJ) the remaining 4 yr. Upon completion of the grazing period steers were shipped to a feedlot in south Texas for finishing.
Feedlot and Slaughter
Steers were weighed upon arrival at the feedlot, implanted with Ralgro, and placed in 1 pen for feeding. Steers were slaughtered in 2 or 3 groups each year. When a group of steers in the pen reached an estimated average of 7 to 10 mm of backfat thickness and a BW of 500 to 570 kg, that group was slaughtered. Time of slaughter for each group was visually estimated by trained feedlot personnel. Steers were fed an average of 148 d in the feedlot over the 5 yr. Steers were processed at Sam Kane Beef Processors in Corpus Christi, TX, at an average age of 545 d. Carcasses were electrically stimulated with high voltage during the slaughter process. After a 24-h chill, carcasses were ribbed and carcass data were recorded. A 15-g LM sample was collected at this time for measurement of calpastatin enzyme activity. Calpastatin activity was determined at Central Community College in Hastings, NE, following the procedures of Whipple et al. (1990a) and Shackelford et al. (1994). Calpastatin activity was not measured the last year of the study.
Cooking and Shear Force Measurements
A boneless wholesale rib (#112; NAMP, 1997) was obtained from the right side of each carcass at the time the carcasses were processed. The ribs were transported by refrigerated truck to the Louisiana State University Animal Sciences Department Meats Laboratory. Two 2.54-cm thick steaks were cut from the small end of each rib section, trimmed of outside fat and connective tissue, vacuum packaged, and each was randomly assigned to aging for 7- or 14-d. After the steaks were aged for the appropriate time at 4°C, they were frozen at –20°C. When steaks from all steers had been aged and frozen, they were thawed for 24 h at 4°C and then broiled to an internal temperature of 70°C (medium doneness) on a Farberware Open-Hearth Broiler (Model FSR200, Faberware Co., Bronx, NY). Internal temperatures were monitored with a 30-ga., type-T constantan-copper thermocouple connected to a strip-chart recorder (Honeywell Inc., Fort Washington, PA). Steaks were turned once at about 35°C and removed at 70°C. After the steaks were held for 24 h at 4°C, six 1.27-cm cores were removed from each steak parallel to the orientation of the muscle fibers. Cores were sheared with a Model 4501 Instron Universal Testing Machine (Instron Corp., Canton, MA) equipped with a Warner-Bratzler V-blade shearing attachment with a cross-head speed of 100 mm/min. The average of the 6 shear force measurements on each steak was used as the shear force observation for each steer. This procedure follows the guidelines established by the American Meat Science Association (AMSA, 1995).
Statistical Analysis
The multiple trait derivative free REML (MTDFREML) programs of Boldman et al. (1995) were used to estimate heritability, genetic correlations between all traits, and sire EPD. The MIXED procedure (SAS Inst. Inc., Cary, NC) was used to obtain beginning additive genetic and residual variances for MTDFREML. The PROC MIXED procedure included sire as a random effect, and contemporary group (year x slaughter group) and the linear covariate of slaughter age as fixed effects. Sires were confounded with herd; however, herd was not a significant source of variation in the early analyses. Artificial insemination was not used across the private herds. Connectedness across herds was assumed to occur through the numerator relationship matrix, A, used in the animal model (Kennedy and Trus, 1993). The fact that all herds were located in Louisiana and that breeders often exchanged germplasm supported this assumption.
Four-generation pedigrees were available for all steers. A total of 2,155 animals contributed information to the A matrix. Heritabilities and sire breeding values were estimated with single-trait analyses in MTDFREML, whereas 2-trait analyses in MTDFREML were used to estimate phenotypic and genetic correlations. To obtain estimates of SE for the 2-trait analyses, the data were edited to include only steers that had observations for those 2 traits.
Analyses were run initially at a convergence of 10–6 and then rerun to a convergence of the simplex of 10–9. Cold restarts were made to insure the global maximum was reached. This was assumed when the –2(log likelihood) did not change to the second decimal place. Variances and covariances obtained from the analyses were assumed to be based on a constant age-at-slaughter endpoint. Slaughter age was used to adjust for the range of birth dates of the calves. Contemporary groups were determined by the year and group in which each steer was slaughtered. Thirteen contemporary groups were involved in the study, with an average of 33 steers per group.
Response variables based on objective observations included feedlot ADG, BW at slaughter, HCW, backfat thickness, LM area, quality grade, yield grade, calpastatin enzyme activity, shear force after 7- and 14-d of aging, and hump height. Skeletal and lean maturity, percent KPH, and marbling score data were visually estimated at ribbing of the carcasses. Yield grade was calculated from an equation from the meats manual of Boggs and Merkel (1979). Subjective measurements were determined by the same trained meats specialist all 5 yr of the study. Ribeye area was determined using a direct-grid reading.
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RESULTS AND DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Descriptive statistics for growth, carcass, and tenderness traits are given in Table 1. A total of 430 steers contributed data to most responses. Calpastatin activity was limited to 355 observations. Average BW at slaughter and backfat thickness were within the range of criteria that was given to the feedlot personnel for the slaughter groups. Slaughter and carcass weights were slightly greater and backfat thickness was slightly less than Brahman steers and heifers fed and slaughtered in Florida (Riley et al., 2002).
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Heritability estimates for traits discussed in this study are presented in Table 2. High heritability estimates (>0.40) were found for BW at slaughter, HCW, LM area, quality grade, yield grade, and calpastatin activity. Moderate estimates of heritability (0.20 to 0.39) were obtained for feedlot ADG, backfat thickness, marbling score, 7- and 14-d shear force, and hump height, whereas low estimates of heritability (<0.20) were found for skeletal maturity, lean maturity, and percent KPH.
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for purebred and composite populations of cattle. The estimates of heritability for BW at slaughter (0.59 ± 0.16) and HCW (0.57 ± 0.15) were the greatest in the study and were similar to mean estimates of 0.52 ± 0.14 and 0.45 ± 0.12 reported by Koots et al. (1994) and estimates of 0.47 and 0.55 reported by Riley et al. (2002). Utrera and Van Vleck (2006) reported a mean heritability of 0.42 for carcass weight adjusted to a constant age. Lower estimates ranging from 0.15 ± 0.11 to 0.28 ± 0.08 were also reported for these traits (Gregory et al., 1995; Wulf et al., 1996).
Our estimate of heritability for LM area was 0.50 ± 0.16. This estimate is lower but not different from estimates of 0.62, 0.65 ± 0.13, and 0.69 ± 0.14 reported by Van Vleck et al. (1992) and Wheeler et al. (1996, 2001). Marshall (1994) and Koots et al. (1994) reported mean estimates of 0.37 and 0.43, respectively, for LM area. Utrera and Van Vleck (2006) reported a mean heritability estimate of 0.41 for LM area from 36 estimates.
The heritability estimate for backfat thickness was 0.36 ± 0.17. This estimate is larger than the 0.25 ± 0.08 reported by Gregory et al. (1995) and smaller than those reported by Wheeler et al. (1996, 2001) and Marshall (1994; 0.44 to 0.84 ± 0.14) and the estimate of 0.63 reported by Riley et al. (2002). Utrera and Van Vleck (2006) reported a mean heritability estimate of 0.41 for adjusted backfat thickness on 36 estimates.
For marbling score, the heritability estimate of 0.37 ± 0.16 found in this study was lower than the range of estimates (0.43 to 0.93 ± 0.02) reported for marbling score by several authors (Van Vleck et al., 1992; Shackelford et al., 1994; Wheeler et al., 2001). Wulf et al. (1996) reported a lower estimate at 0.16 ± 0.11. Riley et al. (2002) reported a heritability estimate for marbling score on Brahman steers and heifers of 0.44. Marshall (1994) reported a mean estimate of 0.35 on 9 estimates, whereas Utrera and Van Vleck (2006) reported a mean heritability estimate of 0.45 on 29 estimates. Estimates of heritability for carcass quality grade were limited. Riley et al. (2002) reported a heritability estimate for carcass quality grade of 0.47 that is similar to our estimate of 0.42 ± 0.16.
The estimate of heritability for yield grade was 0.46 ± 0.17, which is lower than estimates reported by Wheeler et al. (1996, 2001; 0.76 ± 0.13, 0.85 ± 0.14) and Wulf et al. (1996; 0.76 ± 0.26). Estimates of heritability for skeletal and lean maturity were not different from zero (0.10 ± 0.10 and 0.00 ± 0.08, respectively). No literature estimates were found for these comparisons. All steers were classified as A-maturity; therefore genetic variation for maturity was relatively small.
The heritability estimate for percent KPH was not different from zero (0.00 ± 0.08). Elzo et al. (1998) reported low estimates of heritability for percent KPH ranging from 0.01 to 0.14 in several groups of cattle. Moderate levels of heritability for percent KPH (0.32 ± 0.12; 0.28 ± 0.12) were reported by Wheeler et al. (1996, 2001). A relatively high mean estimate of heritability for percent KPH of 0.48 on 8 estimates was reported by Utrera and Van Vleck (2006). Heritability estimates for percent KPH from dissected carcasses may be larger than when KPH is subjectively measured by carcass observation.
The heritability estimate for hump height in this study was 0.38 ± 0.16. Riley et al. (2002) reported an estimate of 0.54 for purebred Brahman cattle. Sherbeck et al. (1996) found that shear force increased as hump height in Brahman crossbred cattle increased.
Heritability for calpastatin activity was 0.45 ± 0.17 and slightly lower than estimates reported by Wulf et al. (1996), Marshall (1994), and Shackelford et al. (1994) of 0.52 ± 0.21, 0.70, and 0.65 ± 0.19, respectively. However, our estimate was greater than those reported by O’Conner et al. (1997) and Riley et al. (2003), which were 0.15 ± 0.15 and 0.07, respectively.
Estimates of heritability for shear force after different aging times have been variable, ranging from 0.06 to 0.37 (Wulf et al., 1996; Wheeler et al., 1996; O’Conner et al., 1997; Riley et al., 2003). Crews and Franke (1998) found that heritability for shear force decreased (0.24 to 0.02) as Brahman breeding decreased. This is contrary to the report of Elzo et al. (1998) who reported greater heritability estimates as percent Brahman decreased (0.17 to 0.58). The estimates of heritability in this study for shear force at 7- and 14-d of aging were 0.29 ± 0.14 and 0.20 ± 0.11, respectively. These values are consistent with most of the reports in the scientific literature. However, Shackelford et al. (1994) reported an estimate of 0.53 ± 0.15 across groups of cattle.
Estimates of heritability for carcass and shear force traits were reported from temperate and tropical breeds of cattle in Australia. Johnston et al. (2003) reported the heritability of shear force was slightly larger for tropical breeds (0.30; Brahman, Belmont Red, and Santa Gertrudis) than for temperate breeds (0.09; Angus, Hereford, Murray Gray, and Shorthorn). However, Reverter et al. (2003) in a companion paper reported that heritability estimates for carcass traits were similar for tropical and temperate breeds.
Ferguson et al. (2000) reported that electrical stimulation of carcasses prior to chilling resulted in lower shear force and in lower calpastatin enzyme activity. We were not able to determine the effect of electrical stimulation on shear force and calpastatin activity in these cattle because all were electrically stimulated. It is not clear if electrical stimulation influenced the magnitude of genetic parameters estimated for these traits.
Phenotypic and Genetic Correlations
Phenotypic and genotypic correlations are given in Table 3. Most phenotypic correlations were relatively small with values less than 0.30. Lean maturity and percent KPH were not included in the 2-trait analyses because their estimated additive genetic variance was zero. Feedlot ADG was positively correlated with BW at slaughter (0.67) and HCW (0.61). Body weight at slaughter was also positively correlated (0.90) with HCW. The LM area was positively correlated with BW at slaughter (0.31) and HCW (0.40). Yield grade was positively correlated with backfat thickness (0.79) and negatively correlated with LM area (–0.49). Fatter carcasses were associated with greater yield grades and smaller LM areas.
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, 2001; 0.86 ± 0.04 and 0.89 ± 0.03). Yield grade was highly correlated genetically with LM area (–0.66 ± 0.16). Wheeler et al. (1996, 2001) reported large genetic correlations between yield grade and LM area (–0.79 ± 0.21; –0.72 ± 0.19). Marbling score was highly correlated (0.99 ± 0.02) genetically with quality grade because marbling score is the main indicator of carcass quality grade.
Calpastatin enzyme activity was negatively correlated genetically with HCW (–0.69 ± 0.24) but positively correlated with shear force after steaks were aged for 7 d (0.74 ± 0.28) and 14 d (0.72 ± 0.26). Whipple et al. (1990b) identified calpastatin activity as one possible cause for the difference in tenderness between Bos indicus and Bos taurus crosses. Whipple et al. (1990a) and Shackelford et al. (1991) found that calpastatin activity at 24 h postmortem was strongly associated with beef tenderness (r = 0.66 and 0.39, respectively). Other researchers (Marshall, 1994; Shackelford et al., 1994; Wulf et al., 1996) reported high positive correlations of 0.58 and 0.50 ± 0.22 between calpastatin enzyme activity and shear force.
Shear force at 7 and 14 d of aging was negatively correlated genetically with yield grade (–0.90 ± 0.30 and –0.66 ± 0.30, respectively). Wheeler et al. (2001) also found a negative genetic correlation between shear force and yield grade at 7 d (–0.50 ± 0.26) and 14 d (–0.41 ± 0.28) of aging. Seven-day shear force was also negatively correlated with backfat thickness at –0.82 ± 0.27. Wheeler et al. (2001) also reported a negative genetic correlation (–0.41 ± 0.25). Shear force has been known to be greater in carcasses with less backfat thickness due to cold shortening (Koohmaraie et al., 1996; King et al., 2003). Bidner et al. (1997) showed that removing the backfat from carcasses resulted in faster cooling of the carcass, a greater pH, and a greater shear force. We do not know if cold shortening is responsible for the high genetic correlation between backfat thickness and shear force in these data. The genetic correlations between 7-d shear force and yield grade and 7-d shear force and backfat thickness indicate that selection for improved tenderness may be associated with increased backfat thickness and low yielding carcasses. However, EPD of approximately 30% of the sires with negative EPD for 7-d shear force were associated with negative EPD for yield grade and for backfat thickness. This indicates that it is possible to find sires in this sample that would improve 7-d shear force and not cause fatter or lower yielding carcasses. The phenotypic correlation between backfat thickness and shear force was near zero.
The genetic correlation for backfat thickness and marbling score was 0.04 ± 0.33. Our finding was similar to low values ranging from –0.19 to 0.26 ± 0.24 (Wilson et al., 1993; Hirooka et al., 1996; Wheeler et al., 1996; Pariacote et al., 1998; Shanks et al., 2001; Crews et al., 2004). When analyzing data from purebred Brahman cattle, Elzo et al. (1998) and Riley et al. (2002) reported estimates of 0.03 and 0.56, respectively. Moderate values of 0.42 ± 0.14 and 0.44 ± 0.18 were reported by Wheeler et al. (2001) and Gregory et al. (1995), respectively.
Marbling score was not associated genetically with shear force at 7 d (0.08 ± 0.34) or 14 d (–0.02 ± 0.36) of aging. Wheeler et al. (1994) reported that the small positive association of marbling score with palatability was similar in Bos taurus and Bos indicus cattle and that marbling explained at best only 5% of the variation in palatability traits. Dikeman et al. (2005) said that selection for marbling would result in little improvement in meat tenderness. In a summary of several papers, Marshall (1994) reported a range of estimates from –0.25 to –0.53. Results by Elzo et al. (1998) gave a value of –0.06 for purebred Brahman cattle, whereas Sherbeck et al. (1996) showed similar findings with small relationships between marbling score and tenderness on steers of varying percentages of Brahman inheritance. Moderate values were reported by Wheeler et al. (2001) when aging steaks for 7 d (–0.27 ± 0.27) and 14 d (–0.30 ± 0.30).
Expected Progeny Differences
Expected progeny differences were calculated for each of the 68 sires. Descriptive statistics of the sire EPD are given in Table 4. The mean EPD for each trait was near zero. Ranges of sire EPD for the traits in this study are similar to ranges for carcass trait EPD of the 230 bulls in the 2006 American Brahman Breeders Association Sire Summary (American Brahman Breeders Association, Houston, TX). Interestingly, ranges of sire EPD for carcass weight and LM area in this study were generally similar to those in the 2006 Angus sire summary (http://www.angus.org/sireeval/averages.html), but Angus bulls had greater range for marbling score and for backfat thickness. Sire EPD for shear force after a 14-d aging period in 103 Simmental, 23 Shorthorn, and 69 Hereford bulls involved in the National Cattlemen’s Beef Association Carcass Merit Project were similar across breeds and ranged from –0.48 to 0.36 kg (Dikeman et al., 2005). Ranges for Simbrah sire EPD for carcass traits were generally similar to those in this study (http://www.simmental.org/userimages/2007%20spring%20sire%20summary.pdf).
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In conclusion, the magnitude of genetic parameters estimated for carcass and tenderness traits in a sample of Brahman steers indicates that selection within the Brahman breed may improve carcass quality and consumer-related traits. However, selection of some sires for improvement in tenderness may result in fatter and less yielding carcasses. Further research is needed to determine if these correlations are as high in Brahman cattle as found in this study. The ranges of Brahman sire EPD for carcass quality and tenderness traits were not radically different from EPD ranges of sires in other breeds.
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Footnotes |
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2 Acknowledgement: The authors thank Manual A. Persica III and John T. Carothers, Research Associates in the Department of Animal Sciences, Louisiana State University Ag Center, Baton Rouge for assistance in preparing, aging, and cooking steaks and in obtaining shear force data for this project.
3 Corresponding author: smith{at}ulm.edu
Received for publication September 23, 2006. Accepted for publication February 26, 2007.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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