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,3
* University of Nebraska, Lincoln, and ,
,
USDA, ARS, Roman L. Hruska U.S. Meat Animal Research Center,
and
Clay Center 68933-0166 and
and
Lincoln, NE 68583-0908; and
and
Department of Animal Science and Industry, Kansas State University, Manhattan 66506-0201
3 Correspondence:
A218 Animal Sciences, University of Nebraska, Lincoln 68583-0908 (phone: 402/472-6010; fax: 402/472-6362; E-mail:
lvanvleck{at}unlnotes.unl.edu).
| Abstract |
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Key Words: Crossbreeding Genetic Correlation Heritability
| Introduction |
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Selection for aggregate genetic improvement is most effective when relationships among the traits selected are known (Mohuiddin, 1993). An estimate of maternal genetic ability for weaning weight has been included in genetic evaluation programs for some time, but relatively little is known about its relationship to carcass characteristics (Green, 1996; Koots et al., 1994), despite important contributions to this area (MacNeil et al., 1984; Crews and Kemp, 1999; Shanks et al., 2000). Correlations between total genetic effects for weaning weight and some economically important carcass traits have, in some cases, been favorable (Koch, 1978; Koots et al., 1994) and may represent opportunities for increased productivity. If genetic antagonisms exist, however, they may hinder effective selection and reduce profitability. The objective of this study, therefore, was to estimate correlations among direct and maternal genetic effects for weaning weight and direct genetic effects for carcass traits in beef cattle.
| Materials and Methods |
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The management protocol for F1 heifers was to produce first calves at 2 yr of age. Postweaning heifer management has been described by Laster et al. (1976; 1979), Gregory et al. (1979b), and Thallman et al. (1999). Heifers born in Cycle I were bred by AI to Hereford, Angus, Holstein, South Devon, and Brahman sires. Cycle II heifers were mated by AI to Hereford, Angus, Brahman, and Santa Gertrudis sires. In the final two cycles, all heifers were bred by natural service to Red Poll bulls, and females older than 2 yr were subsequently bred by natural service to Simmental sires in multibull breeding pastures.
Weaning weights (WWT, n = 23,681) included both F1 males and females, as well as calves of F1 females. Weaning weights were adjusted to 205 d by adding to birth weight the product of 205 and average daily gain to actual day of weaning (BIF, 1996).
After weaning, steers were allocated to replicated pens and fed in groups according to sire breed. A postweaning adjustment period of 25 to 40 d was followed by an average of 262 total days on feed. Each year, steers were serially slaughtered in commercial packing plants in three or four groups over a period of 56 to 84 d. Koch et al. (1976; 1979; 1982) and Wheeler et al. (1996) provide details of postweaning management and slaughter protocol.
At slaughter, hot carcass weight (HCWT) was recorded for each steer. After a 24-h chill, ribeye area (REA); kidney, pelvic, and heart fat percentage (KPH), adjusted fat thickness (AFAT), and marbling score (MARB) were determined. Marbling score was scored on a 100-point scale within each of seven categories (AMSA, 2001). Fat thickness, measured at the 12th rib, was adjusted visually for distortion from hide pull and atypical fat cover distribution over the chuck, round, and other parts of the carcass. For Cycles I through III (1970 through 1976), the right side of each carcass was processed at Kansas State University, Manhattan, KS. For Cycle IV, processing was performed at the U.S. Meat Animal Research Center, Clay Center, NE. Carcass halves were processed into boneless, closely trimmed retail cuts, fat trim, and bone. Weights from retail product, fat trim, and bone were converted into percentages of actual carcass weights (RET, FAT, and BON, respectively). Longissimus dorsi steaks were aged for 7 d and frozen for future evaluation. After thawing 24 h at 2 to 4°C, the steaks were prepared for Warner-Bratzler shear force tests. Across all four cycles, steaks were cut 2.54 cm thick, and within location, a consistent protocol to determine shear force values was used. For evaluations at Kansas State University, steaks were cooked to an internal temperature of 65°C, cooled at room temperature for 30 min, and eight 1.27-cm cores were removed. For evaluations at the U.S. Meat Animal Research Center, steaks were cooked to an internal temperature of 70°C, stored at 5°C for 24 h, and six 1.27-cm cores were removed. Warner-Bratzler shear force (WBSF) was measured by shearing each core with an Instron 1132/Microcon II United Testing Instrument (Instron, Canton, MA) equipped with a Warner-Bratzler shear device (Wheeler et al., 1996).
Table 1
shows the summary statistics for weaning weight and carcass traits after elimination of outliers greater than four standard deviations from the mean.
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Fixed factors for models for all traits were determined through preliminary analyses using procedure GLM of SAS (SAS Inst. Inc., Cary, NC). Fixed factors (main effects and interactions) and covariates were tested and removed from the model if found nonsignificant (P > 0.01), with nonsignificant effects rejected sequentially, and those effects with greatest p-values rejected first. Fixed factors for weaning weight were age of dam, year of birth, sex, and breed composition, along with a covariate for calendar birth day. Fixed factors for carcass traits were age of dam, year of birth, and line, with age at weaning and age at slaughter as linear covariates.
Animal models were used for final analyses of all data. The model for weaning weight also contained a random maternal genetic effect, as well as the uncorrelated random effect of maternal permanent environment. Models for carcass traits did not contain a maternal genetic component but included a maternal effect as an uncorrelated random effect to account jointly for maternal genetic and permanent environmental effects of the dam. The multiple-trait derivative-free restricted maximum likelihood (MTDFREML) suite of programs (Boldman et al., 1995) was used for univariate and bivariate analyses for all traits. Iterations for each trait were stopped when the variance of twice the negative logarithm of the likelihood function (-2 log L) was less than 1 x 10-9 for single trait analyses or 1 x 10-6 for pairwise analyses. Initial starts used variance component estimates based on the literature or previous research with this dataset, and each subsequent restart used starting values from the previous run. Convergence was declared when differences in -2 log L were small (generally less than 0.01), and ratios of variance components as a function of phenotypic variance and correlations changed by less than 0.01. Associated standard errors for estimates in single-trait analyses were estimated using an average information algorithm derived by Johnson and Thompson (1995) as implemented by Dodenhoff et al. (1998).
| Results and Discussion |
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Negative estimates of direct-maternal genetic covariance have been reported for many beef cattle populations and are included in the national genetic evaluations of many breed associations (e.g., Lee and Pollak, 1997).
Estimates of heritability for carcass traits, proportion of phenotypic variance due to maternal effects, and direct and maternal genetic correlations with weaning weight are shown in Table 3
. Estimates of heritability for carcass traits were moderate to large. Generally, estimates were consistent with previous literature (Koots et al., 1994). Variances due to maternal effects as proportions of phenotypic variance were not significantly different from zero for most traits and were small for HCWT, RET, and FAT.
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Several authors have estimated correlations between growth and carcass traits, but without fitting maternal effects to the models for growth traits. Gregory et al. (1995) found a moderate positive genetic correlation between weaning weight and carcass weight (0.42 ± 0.18) from data involving both purebred and composite animals.
Estimates of genetic correlations between direct and maternal weaning weight and percent lean yield were not significantly different from zero in the study by Crews and Kemp (1999) (0.15 ± 0.20 and 0.03 ± 0.19, respectively), while estimates between direct and maternal WWT and RET in our study were negative (-0.12 and -0.33, respectively). However, percent lean yield as used by Crews and Kemp (1999) was calculated from other characteristics, such as hot carcass weight and yield grade, and was not directly measured. Gregory et al. (1995) reported a negative estimate of genetic correlation between RET and WWT (-0.28 ± 0.21) and a positive genetic correlation between BON and WWT (0.37 ± 0.30). Here, estimates of genetic correlations were negative and small with WWT (-0.13 and -0.09, respectively).
Estimates of genetic correlations for direct and maternal effects of WWT with AFAT were moderate and positive (0.26 and 0.25, respectively). Gregory et al. (1995) reported a positive genetic correlation between WWT and AFAT (0.23 ± 0.23). Crews and Kemp (1999) estimated genetic correlations between direct and maternal components of WWT and REA to be positive (0.34 ± 0.17 and 0.18 ± 0.18, respectively). Our estimates were of similar magnitude (0.29 and 0.29, respectively). Gregory et al. (1995) also reported a positive genetic correlation between direct genetic effects for WWT and REA (0.45 ± 0.21).
Estimates of genetic correlation between KPH and the direct and maternal genetic effects for WWT were small to moderate and positive. Genetic correlation between direct effect of WWT and MARB was estimated to be negative (-0.12), while the estimate of genetic correlation involving the maternal effect for WWT and direct effect for MARB was positive (0.28). Gregory et al. (1995) reported a positive genetic correlation between WWT and MARB (0.27 ± 0.18).
Near-zero estimates of genetic correlations were found between direct and maternal effects of WWT and WBSF. Gregory et al. (1995) reported similar nonsignificant genetic correlations between WWT and WBSF or tenderness score.
Selection for increased direct genetic value for WWT would be expected to increase HCWT, FAT, AFAT, REA, and KPH, to decrease RET, BON, and MARB, and to have little effect on WBSF. Emphasis on maternal milk or maternal effects on WWT may lead to positive correlated responses in HCWT, FAT, AFAT, REA, MARB, and KPH, with little effect on WBSF. All genetic correlations with carcass traits that were positive for direct genetic effects for WWT were also positive for maternal genetic effects, except for MARB, RET, and BON, which all had small positive estimates of genetic correlations with direct effects for WWT. The estimates of genetic correlations between maternal effects of WWT and carcass traits were moderate to large for carcass traits involving fatness or increased retail weight as a function of increased body weight.
Previous work has shown genetic correlations between weaning weight and some carcass traits to be moderate to large, and results from this study indicate that when the genetic effect for weaning weight is partitioned into direct and maternal components, genetic maternal ability for weaning weight may be correlated with some of the carcass traits studied.
| Implications |
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| Footnotes |
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2 Current address: 3450 Litton Reaves Hall, Virginia Tech, Blacksburg 24061-0306 (phone: 540/231-4740; fax: 540/231-3010; E-mail: rsplan{at}vt.edu). ![]()
Received for publication September 4, 2001. Accepted for publication July 24, 2002.
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