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* Department of Animal Science, Iowa State University, Ames 50011 and
and
National Pork Board, Des Moines, IA 50306
2 Correspondence:
109 Kildee Hall (phone: 515-294-6728; fax: 515-294-5698; E-mail:
tjbaas{at}iastate.edu).
| Abstract |
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Key Words: Carcass Composition Genetic Parameters Pigs
| Introduction |
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Genetic parameter estimates for backfat and muscle content have been studied extensively and are readily available in the literature (Stewart and Schinckel, 1989; Ducos, 1994; Clutter and Brascamp, 1998). Carcass composition traits are highly heritable and respond very favorably to selection. If payment programs evolve, and packers change from pricing based on carcass composition to a weight of primal and(or) boneless subprimal pricing strategy, producers need to emphasize these traits in their breeding programs.
In order for any selection program to be successful, variance component and genetic parameter estimates for the traits in question must be known. This study was designed to estimate variance components and genetic parameters for pork carcass components, including primal and boneless subprimal cut weights in pigs.
| Materials and Methods |
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Data from two national progeny testing and genetic evaluation programs were evaluated in this study. The first dataset was from 285 homozygous halothane normal purebred Yorkshire (144) and Duroc (141) pigs from two replications of the National Pork Boards Genetics of Lean Efficiency project (R. N. Goodwin, personal communication). Barrows and gilts from both breeds were delivered to the Minnesota Swine Testing Station, New Ulm, MN, at 10 to 20 d of age. A standard health and vaccination protocol was implemented, and pigs were randomly assigned to pens within breed and sex. All pigs were randomly assigned to one of two target slaughter weights (113.4 and 131.5 kg) upon entry. Pigs were reared in a modified open-front finisher with a partially slotted floor and given 1.5 m2 of pen space per pig. At a pen average of 31.7 kg, pigs were placed on test and given ad libitum access to feed and water.
The second dataset utilized was from 171 homozygous halothane normal purebred Yorkshire (31), Duroc (19), and Other breeds, including Berkshire (52), Chester White (21), Hampshire (9), Landrace (29), Poland China (6), and Spot (4) from two seasons of the National Barrow Show Sire Progeny Test (Goodwin, 2000). Barrows and gilts from all three breed groups were delivered at approximately 8 weeks of age. Pigs were penned by sire group in an open front building with a bedded, solid concrete floor and given 1.4 m2 of pen space per pig. At a pen average of approximately 31.7 kg, animals were placed on test and given ad libitum access to feed and water until they reached a target end weight of 108.9 kg. Data from both projects were combined for analysis.
Animals were weighed off-test weekly upon reaching their target end weight. Pigs were delivered to Hormel Foods/Quality Pork Processors in Austin, Minnesota and harvested after an overnight rest period.
Carcasses entered the cooler forty-five minutes post-mortem. After a two-hour blast chill (-20°C), carcasses were selected for dissection based upon centrality of the dorsal split and lack of trim. At 24 hours post-mortem, the left side of each selected carcass was transported to Geneva Meats, Geneva, Minnesota, for dissection. A total of 456 carcasses were selected from 1,350 pigs slaughtered.
The right side of each carcass was used for measurement of 10th rib off-midline backfat (BF10) and loin muscle area (LMA). Carcasses were ribbed by a cut perpendicular to the vertebral column between the 10th and 11th ribs from the anterior end. Tenth rib off-midline backfat and loin muscle area were measured according to Pork Composition and Quality Assessment Procedures (NPPC, 2000).
The dissection procedure described in Berg et al. (1999) was performed on all selected carcasses 48 hours post-mortem. Primals and boneless subprimals were cut according to Institutional Meat Purchase Specifications (NAMP, 1997). Boneless subprimal cuts and individual muscles were trimmed to 0.0 cm external fat. Primal weights included the 401 Ham (HAM), 410 Loin (LOIN), 405 Picnic shoulder (PIC), 406 Boston Butt (BB), and 409 Belly (BELLY). Each primal was separated into its respective boneless subprimals and muscles. The inside (INS), outside (OUT), and knuckle (KNU) muscles of the ham were summed to yield a boneless ham (BHAM). The longissimus dorsi (LD) and psoas major (TEND) were summed to yield a boneless loin product (BLOIN). The boneless picnic (BPIC) and boneless Boston Butt (BBUTT) were summed to yield a boneless shoulder (BSHLDR). The boneless subprimal components BHAM, BLOIN and BSHLDR were summed to yield total primal boneless lean (LEAN). Summary statistics from the two projects are shown in Table 1
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Groups of traits were designated for analysis. The first group included HAM, LOIN, BB, PIC, BELLY, BF10, and LMA. A second group included BHAM, BLOIN, BSHLDR, and LEAN. Primals were also grouped with their respective boneless subprimals, individual muscles, and LEAN.
All traits were first analyzed with a single trait sire model using PROC MIXED of SAS (SAS Inst. Inc., Cary, NC) to test for significant effects in the model and estimate between sire (
s) and residual variance (
e). The initial model was
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where Yijklm = observation for a trait, ai = fixed effect of ith breed; bj = fixed effect of jth sex; ck = fixed effect of kth group (where group is defined as a slaughter group within project); abij = fixed effect of the interaction of ith breed with jth sex; wijklm = covariate of off-test weight for ith breed with jth sex; ßij = linear regression coefficient of the dependent variable on off-test weight, wij; sil = random effect of sire l within breed i; and eijklm = random residual error. Distribution of records by breed, gender, and project is shown in Table 2
. A significance level of P = 0.25 was established as a maximum level for an effect to remain in the model. The interaction of breed with sex was not significant (P > 0.25) and was removed from the final model.
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s and
e from the single trait sire model as initial variance component estimates. Multiple trait analyses by group were performed using averaged variance components from the two-trait analyses as initial variance estimates. The version of DFREML utilized for the multiple trait analyses allowed for the estimation of SE of variance components and genetic parameters by using the average information matrix. All SE for variance component and genetic parameter estimates reported are from the within-group multiple trait analyses. | Results and Discussion |
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Least squares means by gender and breed for primal and boneless subprimal cut weights, individual muscle weights, LEAN, BF10, and LMA are given in Table 3
. Gilts had heavier weights (P < 0.01) than barrows for all boneless subprimals, individual muscles, and LEAN, and for all primal cuts, except BELLY. Gilts also had less (P < 0.01) BF10 and more (P < 0.01) LMA than barrows. Barrows had a heavier (P < 0.01) BELLY when compared to gilts. These gender differences for BF10 and LMA are similar to those found in previous studies (NPPC, 1995; Moeller, 1994).
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Regression Coefficients
Regression coefficients for off-test weight by breed for primal and boneless subprimal cut weights, LEAN, BF10, and LMA are shown in Table 4
. The Other breeds group was included in the analysis, but individual breed results are not reported due to the variation in sample numbers per breed within the group. Regression coefficients by breed group for LOIN, BB, PIC, and BF10 were not different (P > 0.05). Regression coefficients for LMA (P < 0.05), LEAN (P < 0.05), BELLY (P < 0.01), HAM (P < 0.05), BLOIN (P < 0.05), and BHAM (P < 0.01) were not different when comparing the Yorkshire and Duroc breeds. The interaction of off-test weight with sex did not affect weight of primal and boneless subprimal cuts, LEAN, BF10, and LMA (P > 0.05). Lo et al. (1992) reported a regression coefficient of 0.2304 ± 0.0245 for BF10 on off-test weight, similar to this study. They found a smaller regression coefficient for LMA (0.0971 ± 0.173) than in this study.
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Heritability estimates and genetic and residual correlations for BF10 and LMA are shown in Table 5
. Heritability estimates for BF10 and LMA were 0.40 and 0.62, respectively, and the genetic correlation between BF10 and LMA was -0.45. These results are similar to previous literature estimates from Clutter and Brascamp (1998) (average h2 = 0.49 for backfat) and Stewart and Schinckel (1989) (average h2 = 0.52 and 0.47 for BF10 and LMA, respectively, and average rg = -0.35 between BF10 and LMA). Two studies have used National Barrow Show data to estimate genetic parameters for BF10 and LMA. Heritability estimates for BF10 and LMA were 0.72 and 0.76, respectively, as reported by Berger et al. (1994) and 0.79 and 0.71, respectively, by Moeller (1994). Berger et al. (1994) estimated the genetic correlation between BF10 and LMA to be -0.57, while Moeller (1994) estimated the genetic correlation to be -0.68. Results from the National Genetic Evaluation Terminal Line Program (NPPC, 1995), which followed a protocol similar to the Genetics of Lean Efficiency but evaluated crossbred pigs, showed heritability estimates for BF10 and LMA to be 0.46 and 0.48, respectively. The genetic correlation between BF10 and LMA in that study was estimated to be -0.61.
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Heritability estimates and genetic and residual correlations for primal cut weights are given in Table 5
. Heritability estimates for HAM, LOIN, and BELLY were 0.57, 0.51, and 0.51, respectively. Heritability estimates for the two primal cuts from the shoulder, BB and PIC, were 0.09 and 0.21, respectively. Johansson et al. (1987) estimated genetic parameters for primal cut percentage from European progeny testing programs. They reported heritability estimates for ham percentage (0.28 to 0.43), back percentage, (0.40 to 0.44), streak percentage (0.29 to 0.61), shoulder percentage (0.35 to 0.55), percent meat plus bone in the ham (0.51 to 0.87), and for percent meat plus bone in the back (0.25 to 0.73) for the Landrace, Yorkshire, and Hampshire breeds.
The two largest primal cuts, HAM and LOIN, were highly correlated with each other (0.62). The primal weights for HAM, LOIN, and PIC were negatively associated with BELLY (-0.57, -0.42, and -0.82, respectively) and BF10 (-0.62, -0.51, and -0.60, respectively). The primal weights for HAM, LOIN, and PIC were also positively associated with LMA (0.53, 0.78, and 0.70, respectively). The genetic correlation between BELLY and LMA was -0.23. These results show primal cuts with higher lean to fat ratios (HAM, LOIN, BB, and PIC) are positively associated with lean predictors (LMA) and negatively associated with predictors of fat content (BF10). The primal cut with a lower lean to fat ratio (BELLY) showed opposite relationships with LMA and BF10.
Selection for decreased BF10 has been successful in the Duroc, Yorkshire, Hampshire, and Landrace breeds (NSR, 2001). The results in this study suggest that decreasing BF10 and increasing LMA increases HAM and LOIN but decreases BELLY. This could be detrimental as BELLY is a high value primal cut.
Boneless Subprimals
Heritability estimates and genetic and residual correlations for boneless subprimal cut weights are shown in Table 6
. Heritability estimates for BHAM, BLOIN, and BLEAN were 0.76, 0.72 and 0.69, respectively. These estimates are higher than those for the primal cuts. Variation in fat deposition within each primal could lead to this higher estimate of heritability when compared to the individual primal.
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Carcass Component Groups
Heritability estimates and genetic and residual correlations for ham components from a six-trait analysis that included HAM, BHAM, LEAN, INS, OUT, and KNU are given in Table 7
. Heritability estimates for HAM, BHAM, and LEAN were similar to estimates from the primal and boneless subprimal analyses. Heritability estimates for INS, OUT, and KNU were 0.80, 0.80 and 0.61, respectively. The estimates for INS and OUT were higher than those for HAM and similar to those for BHAM.
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Heritability estimates and genetic and residual correlations for loin components from a five-trait analysis that included LOIN, BLOIN, LEAN, LD, and TEND are shown in Table 8
. The heritability estimates for LOIN and LEAN were similar to estimates from the primal and boneless subprimal analyses. The heritability estimate for BLOIN was lower than the estimate from the boneless subprimal analysis. Heritability estimates for LD and TEND were 0.64 and 0.21, respectively.
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Heritability estimates and genetic and residual correlations for weights of shoulder components from a six-trait analysis that included BB, PIC, BSHLDR, LEAN, BBUTT, and BPIC are given in Table 9
. Heritability estimates for BB, PIC, BSHLDR, and LEAN were similar to estimates from the primal and boneless subprimal analyses. Heritability estimates for BBUTT and BPIC were 0.17 and 0.38, respectively. These estimates were slightly higher than those for the intact primal, similar to what was seen with the other subprimals.
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| Implications |
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| Footnotes |
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Received for publication December 19, 2001. Accepted for publication July 22, 2002.
| Literature Cited |
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