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J. Anim. Sci. 2005. 83:316-323
© 2005 American Society of Animal Science

ANIMAL GENETICS

Genetic and phenotypic relationships between individual subcutaneous backfat layers and percentage of longissimus intramuscular fat in Duroc swine1

D. W. Newcom2, T. J. Baas3, C. R. Schwab and K. J. Stalder

Dept. of Animal Science, Iowa State University, Ames 50011


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results and Discussion
 Implications
 Literature Cited
 
Progeny (n = 589) of randomly mated Duroc pigs were used to determine the genetic and phenotypic relationships between individual s.c. backfat layers and i.m. fat percent (IMF) of the longissimus. Five days before slaughter, cross-sectional ultrasound images were collected at the 10th rib by a National Swine Improvement Federation-certified ultrasound technician using an ultrasound machine (Aloka 500 SSD) fitted with a 12-cm linear array transducer. Off-midline backfat (SBF) and loin muscle area (SLMA) were measured. Individual s.c. backfat layers were measured at the same location: outer (OBF), middle (MBF), and inner (IBF). Off-midline backfat (CBF) and loin muscle area (CLMA) were measured on the carcass 24 h postmortem. A slice from the 10th rib of the loin muscle was obtained for determination of IMF. Heritability estimates and genetic correlations were calculated fitting all possible two-trait animal models in MATVEC (Wang et al., 2003Go). The heritabilities for OBF, MBF, IBF, CBF, SBF, and IMF were 0.63, 0.45, 0.53, 0.48, 0.44, and 0.69, respectively. The genetic correlations of OBF, MBF, and IBF with IMF were 0.36, 0.16, and 0.28, respectively, and the genetic correlations of CBF and SBF with IMF were 0.25 and 0.27, respectively. Genetic correlations between OBF and MBF, OBF and IBF, and MBF and IBF were 0.43, 0.45, and 0.67, respectively. Results demonstrate that individual backfat layers are highly heritable, of similar magnitude to total backfat, and have similar genetic correlations with IMF. Individual backfat layers could become candidate traits for implementation into a multiple-trait genetic evaluation to improve IMF, while minimizing the detrimental effect on total backfat depth.

Key Words: Backfat Layers • Intramuscular Fat Percent • Pigs


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results and Discussion
 Implications
 Literature Cited
 
Fresh pork quality continues to be a major concern for producers, packers, processors, wholesale and retail suppliers, exporters, and consumers. Several indicators of meat quality (pH, tenderness, i.m. fat percent [IMF], lean color) have been investigated to determine overall consumer acceptance of fresh pork (NPPC, 1995Go). Measuring these traits in the live animal is difficult, but real-time ultrasound technology has been used to estimate IMF for genetic improvement purposes (Ragland, 1998Go; Newcom et al., 2002aGo).

Loin IMF has been reported to be related to s.c. backfat depth, with the two traits having a moderate, positive genetic correlation (Sellier, 1998Go). Genetic selection for decreased backfat thickness has resulted in a decrease in IMF content (Sonesson et al., 1998Go), and ultimately a decrease in palatability of fresh pork (Barton-Gade, 1990Go). Positive correlations between depth of the innermost backfat layer and marbling scores in pigs have been reported (Moody and Zobrisky, 1966Go). Recent investigation into the three individual s.c. backfat layers has led to speculation that the innermost layer develops last, at a physiological time period similar to when IMF is deposited (Eggert, 1998Go). Quantification of the relationship between individual fat layers and IMF in the live pig may allow for enhanced genetic selection opportunities to improve IMF in the loin. It may be possible to improve meat quality, while maintaining current backfat levels or further decreasing total backfat depth.

The first objective of this study was to determine the phenotypic and genetic relationships between the three individual s.c. backfat layers and IMF in swine. The second objective was to determine the relationships between individual subcutaneous backfat layers and indicators of lean composition in pigs. A third objective was to determine the effect of live weight on carcass and ultrasound measures.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results and Discussion
 Implications
 Literature Cited
 
Barrows and gilts (n = 499 and 90, respectively) from randomly mated purebred Duroc pigs were used in this study. Distribution of records by sire, dam, litter, gender, and contemporary group is shown in Table 1Go. The average inbreeding coefficient was 1.36%. Pigs were housed in one of three types of finishing facilities. The first was a curtain-sided finishing building with totally slotted floors, and pigs were allowed 0.74 m2 of floor space. The second was a curtain-sided building with partially slotted floors, and pigs were allowed 0.76 m2 of floor space. The third was a totally enclosed monoslope building with solid concrete floors and a flush gutter system, in which pigs were allowed 0.74 m2 of floor space. Pigs were allowed ad libitum access to feed and water in all three buildings. All pigs were fed a traditional corn-soybean meal diet that met or exceeded NRC (1998)Go requirements. Pigs within a contemporary group were housed in the same facility.


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  		Table 1. Distribution of records by sire, dam, litter, contemporary group, and gender in Duroc pigs
 
Pigs were weighed off-test weekly at a group average of approximately 115 kg. Adjusted days to 113.5 kg (DAYS) were calculated using recommendations in the Guidelines for Uniform Swine Improvement Programs (NSIF, 1997Go). Pigs were ultrasonically evaluated 5 d before slaughter with an Aloka 500 SSD ultrasound machine fitted with a 3.5-MHz, 12-cm linear array transducer (Corometrics Medical Systems, Inc., Wallingford, CT). One cross-sectional ultrasound image from the 10th- to the 11th-rib interface was collected. Total 10th-rib off-midline backfat (SBF) and loin muscle area (SLMA) were measured using the cross-sectional image. Individual backfat layers at the 10th rib (outer [OBF], middle [MBF], and inner [IBF]) were evaluated at the same location where SBF was measured. The sum of the three individual layers was calculated, and a tolerance of ± 10% of SBF was permitted. If the sum were different from the total by more than 10%, SBF and each individual layer were measured again.

After slaughter and a 24-h chill, 10th-rib off-midline backfat (CBF) and loin muscle area (CLMA), along with last-rib mid-line backfat depth (LRBF), were evaluated on each carcass according to Pork Composition and Quality Assessment Procedures (NPPC, 2000Go). Ratios of OBF, MBF, and IBF to SBF, and the ratios of MBF:OBF, IBF:OBF, and IBF:MBF were calculated as percentages. A 3.2-mm slice from the 10th-rib loin surface was used to determine IMF by the method of Bligh and Dyer (1959)Go. Descriptive statistics for traits measured are shown in Table 2Go.


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  		Table 2. Descriptive statistics for growth, carcass, and ultrasound traits in Duroc pigs
 
Phenotypic correlations were calculated using Proc Corr in SAS (SAS Inst., Inc., Cary, NC). A mixed linear model was fitted using Proc Mixed in SAS to estimate least squares means by gender, differences of least squares means between genders, and regression coefficients for off-test weight. The model included fixed effects for gender and contemporary group, random effects for sire and dam, and off-test weight as a linear covariate. The quadratic effect of live weight was tested and was not significant (P = 0.15).

Genetic parameters were calculated fitting all possible two-trait animal models in MATVEC (Wang et al., 2003Go) to obtain heritability and genetic correlation estimates utilizing the full relationship matrix. Standard errors of the genetic parameters were estimated using the Delta Method of Lynch and Walsh (1998)Go. The analysis model included fixed effects for gender and contemporary group and a random effect for animal (genetic). Off-test weight was included in the model as a linear covariate. Additive genetic and residual variances, heritabilities, and standard errors for each trait were averaged across two-trait analyses.


   Results and Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
Implications
 Literature Cited
 
Gender Effects
Least squares means by gender for growth, carcass, and ultrasound measurements are shown in Table 3Go. Barrows grew faster (P < 0.001), had more CBF and SBF (P < 0.001) and LRBF (P < 0.01), and less CLMA and SLMA (P < 0.001) than gilts. These results agree with previous research (NPPC, 1995Go; Moeller et al., 1998Go; Newcom et al., 2002bGo). Barrows also had more backfat depth in each individual layer (P < 0.001) than gilts, which does not agree with the findings of Lonergan et al. (1992)Go, who reported barrows were fatter than gilts at the 10th rib, but the difference was entirely due to the middle fat layer. They found no differences in outer or inner layer fat thickness between barrows and gilts. These contradictory results may be explained by the different genetic lines (commercial vs. purebred pigs) used in the studies and/or a difference in mean backfat level (<17 vs. 21 mm) between the studies. Barrows had a greater percentage of IMF (P < 0.001) than gilts, which closely follows previous reports (Goodwin, 1994Go; Hamilton et al., 2000Go; Newcom et al., 2004Go). Barrows had greater percentages of MBF:SBF (P < 0.001), IBF:SBF (P < 0.05), MBF:OBF (P < 0.001), and IBF:OBF (P < 0.01) than gilts; however, gilts had a greater percentage of OBF:SBF (P < 0.001) than barrows.


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  		Table 3. Least squares means (±SE) by gender and coefficients for regression of growth, carcass, and ultrasound traits on off-test weight in Duroc pigs
 
Regression Analysis
Coefficients for the regression of carcass and ultrasound traits on off-test weight from the PROC MIXED analysis are shown in Table 3Go. The regression coefficients for CBF and SBF were 0.236 and 0.201 mm/kg, respectively, which are similar to the findings of Lo et al. (1992)Go and Newcom et al. (2002b)Go calculated from carcass measures. These values are slightly smaller than those reported by Moeller et al. (1998)Go from ultrasound measures. The regression coefficients for CLMA and SLMA were 0.180 and 0.216 cm2/kg, respectively, which are similar to values reported by Newcom et al. (2002b)Go, but larger than reported by Lo et al. (1992)Go and smaller than reported by Moeller et al. (1998)Go. Moeller (1990)Go found similar regression coefficients for 10th-rib backfat and slightly larger regression coefficients for loin muscle area using serial measures and intra-pig linear regression. The linear regression coefficient for LRBF was 0.176 mm/kg, which is similar to the value of 0.179 mm/kg reported by Moeller (1990)Go, but smaller than the value of 0.217 mm/kg reported by Moeller et al. (1998)Go. The differences in regression coefficients could be due to differences in mean off-test weight between studies or the methods used to calculate regression coefficients. The current study used pigs with a mean off-test weight approximately 5 to 10 kg heavier than those used by Lo et al. (1992)Go, Moeller (1990)Go, and Moeller et al. (1998)Go, and calculated regression coefficients were based on a single measurement with off-test weight as a covariate, unlike the intra-pig linear regression from serial ultrasonic scan measurements used by Moeller (1990)Go and Moeller et al. (1998)Go.

For individual backfat layers, coefficients for the regression of OBF, MBF, and IBF on off-test weight were 0.059, 0.079, and 0.065 mm/kg, respectively. These values are larger than those reported by Eggert (1998)Go, but that study indicated significant quadratic and cubic regression coefficients, which were not found in the present study. The regression coefficients for the ratios of individual layers to total backfat were –0.201, 0.108, and 0.131 %/kg for OBF:SBF, MBF:SBF, and IBF:SBF, respectively. This indicates that as BW increased in a linear fashion, the percentage of the total backfat made up by the outer layer decreased, showing that at the mean off-test weight of the present study, the rate of growth of the outer layer of backfat decreased as weight increased. The regression coefficients for MBF:OBF and IBF:OBF were 0.421 and 0.395%/kg, respectively, indicating the depth of the middle and inner backfat layers were increasing at a faster rate than the outer layer in the weight range observed in the present study. Moody and Zobriski (1966), Mersmann and Leymaster (1984)Go, and Fortin (1986)Go reported similar results, with the middle backfat layer growing at a rate faster than the outer layer. The regression coefficient for IBF:MBF did not differ (P = 0.16) from zero, indicating the MBF and IBF are growing at approximately the same rate with respect to each other. This finding disagrees with the results of Fortin (1986)Go, who reported the MBF was deposited at a faster rate than the IBF.

The coefficient for the regression of IMF on off-test weight was 0.014%/kg. This increase in IMF with increased live weight is consistent with previous research (Lawrie et al., 1963Go; Allen et al., 1967Go; Schuler et al., 1970Go). Lo et al. (1992)Go reported a regression coefficient of 0.007%/kg in a population of Duroc and Landrace pigs, which is smaller than that in the present study, but this could be due to the fact that the pigs used by Lo et al. (1992)Go were slaughtered at a lighter mean weight. Candek-Potokar et al. (1998)Go reported IMF increased 29% in pigs fed ad libitum from 100 to 130 kg of BW. Cisneros et al. (1996)Go reported IMF increased 0.027%/kg increase in BW from 100 to 160 kg, a regression coefficient that is larger than the value in the current study, which could, in fact, be the result of their weight range being different than the range of weights in this study (Table 2Go).

Several researchers have reported little or no effect of slaughter weight on IMF (Berry et al., 1970Go; Martin et al., 1980Go, 1981Go). Beattie et al. (1999)Go reported a regression coefficient for IMF of 0.004%/kg of live weight, which was not different from zero, when evaluating pigs with a lower mean IMF (<1.0%) than in the present study and measured within a growing period from 70 to 100 kg. Garcia-Macias et al. (1996)Go and Latorre et al. (2004)Go found that IMF was not affected by slaughter weight when evaluating pigs from 90 to 120 kg and from 116 to 133 kg, respectively; this is different than the results from the current study and could be due to the lower mean IMF (<2.8%) compared with the present study. Mayoral et al. (1999)Go reported that fat content within the LM increased with increased weight; however, they also reported that IMF as a percentage of muscle weight remained constant. These findings are different from the current study, where percentage of IMF increased as live weight increased, and this increase in live weight led to an increase in the weight (size) of the LM (0.180 cm2/kg CLMA and 0.216 cm2/kg SLMA).

Variance Component and Genetic Parameter Analysis
Heritability estimates and genetic and phenotypic correlations for OBF, MBF, IBF, CBF, SBF, LRBF, and IMF are shown in Table 4Go. Heritability estimates for OBF, MBF, and IBF were 0.63, 0.45, and 0.53, respectively. No previously published heritability estimates were found for individual backfat layers, but the values are within the range of heritability estimates for total carcass backfat summarized by Clutter and Brascamp (1998)Go. Genetic correlations between OBF and MBF, OBF and IBF, and MBF and IBF were 0.43, 0.45, and 0.67, respectively, whereas phenotypic correlation coefficients were slightly greater for all three trait combinations (Table 4Go). In Duroc-sired pigs, Eggert (1998)Go reported that phenotypic correlations between OBF and MBF, OBF and IBF, and MBF and IBF were 0.43, 0.58, and 0.81, respectively. The phenotypic correlation between OBF and MBF reported by Eggert (1998)Go was smaller, whereas the correlations between OBF and IBF and between MBF and IBF were greater than in the present study. Eggert (1998)Go also found the correlation between OBF and MBF to be different from zero at the P < 0.10 level, which contrasts with the very highly significant level (P < 0.001) found in the present study.


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  		Table 4. Heritability estimates (±SE) and genetic and phenotypic correlations (±SE) for the three individual subcutaneous backfat layers, 10th-rib backfat measured on the carcass and ultrasonically, last rib backfat, and intramuscular fat percent in Duroc pigsa
 
The heritability for longissimus IMF found in this study was 0.69 (Table 4Go). This is greater than the average of 0.50 published by Sellier (1998)Go, but within the range of 19 estimates (0.26 to 0.86) from that review. The genetic correlations of OBF, MBF, and IBF with IMF were 0.36, 0.16, and 0.28, respectively (Table 4Go). These estimates are considered moderate and in the appropriate direction, as fatter pigs tend to have greater amounts of IMF (Wariss et al., 1990Go; Knapp et al., 1997Go; Huff-Lonergan et al., 2002Go). Corresponding phenotypic correlations were 0.26, 0.30, and 0.34, respectively. Eggert (1998)Go reported phenotypic correlations of OBF, MBF, and IBF with IMF for Duroc-sired pigs to be substantially greater than those observed in the present study (0.66, 0.76, and 0.71, respectively). This difference could be due to differences in project design: purebred vs. crossbred pigs, measuring individual layers with ultrasound vs. measurement on the carcass, differences in number of observations (589 vs. 16), or differences in individual fat layer depths and IMF (mean differences between Eggert (1998Go) and the present study were –1.28, 6.37, and 2.74 mm, and –0.48% for OBF, MBF, IBF, and IMF, respectively).

The genetic correlations of CBF, SBF, and LRBF with IMF were 0.25, 0.27, and 0.42, respectively (Table 4Go). These values are similar to reports from previous researchers (Sellier, 1998Go), and similar to the genetic correlations of individual backfat layers with IMF. Phenotypic correlations of CBF, SBF, and LRBF with IMF were 0.30, 0.32, and 0.13, respectively. The values for the 10th rib estimates are similar to the genetic correlations, but are lower for the last rib measurement. Eggert (1998)Go reported a phenotypic correlation of 0.83 between backfat at the 10th rib and IMF, substantially greater than in the present study. Huff-Lonergan et al. (2002)Go reported the correlation to be 0.45 from a Berkshire x Yorkshire F2 population. Cameron (1990)Go reported that genetic and phenotypic correlations between backfat measured at the P2 site (mid-line, last rib) and IMF were 0.05 and 0.22, respectively, which are different than what was found in the present study. These differences could be due to variation in the populations studied, measurement methods, and mean values of traits of interest. The results from the present study support previous literature reports that fatter pigs tend to have more IMF.

Genetic correlations of OBF, MBF, and IBF with CBF were 0.71, 0.85, 0.84, respectively, which are similar to the genetic correlations with SBF, as well as the corresponding phenotypic correlations (Table 4Go). Correlations between the three individual backfat layers and LRBF were lower than those reported for 10th-rib backfat. Eggert (1998)Go reported phenotypic correlations between CBF and OBF, MBF, and IBF of 0.67, 0.95, and 0.92, respectively. The value for the correlation between CBF and OBF is similar to that in the current study, but the correlations of CBF with MBF and IBF are greater than in the present study.

Heritability estimates for CBF, SBF, and LRBF were 0.48, 0.44, and 0.17, respectively (Table 4Go). The estimates for CBF are similar to previously reported averages (Stewart and Schinckel, 1989Go; Clutter and Brascamp, 1998Go). Newcom et al. (2002b)Go reported the heritability of 10th-rib carcass backfat was 0.40. The values from the present study are smaller than those reported by Berger et al. (1994)Go and Moeller (1994)Go, who found estimates greater than 0.70. Moeller (1994)Go reported the heritabilities of SBF and LRBF to be 0.87 and 0.55, respectively, which are substantially greater than those in the current study. The genetic correlation between CBF and SBF was 0.98, and was similar to the value (0.99) reported by Moeller (1994)Go. The genetic correlations of CBF and SBF with LRBF were 0.73 and 0.80, respectively; these values are similar to those reported by Moeller (1994)Go.

Heritability estimates and genetic and phenotypic correlations between ratios of the three individual s.c. backfat layers and IMF are shown in Table 5Go. Heritability estimates of the ratios OBF:SBF, MBF:SBF, IBF:SBF, MBF:OBF, and IBF:OBF were of similar magnitude as estimates for CBF and SBF, but smaller than for OBF or IBF alone. The heritability estimate for IBF:MBF (0.36) was slightly lower than for the other ratios. The genetic correlations of the ratios of individual layers to SBF and to other layers with IMF were small and close to zero, with the exception of IBF:SBF and IMF. This correlation could indicate that an increased rate of IBF deposition may be genetically related to increased IMF within the loin. Phenotypic correlations between the ratios of layers and IMF differed (P < 0.001) from zero, except for IBF:MBF and IMF. Eggert (1998)Go reported phenotypic correlations of ratios of individual layers to total backfat and to other layers with IMF were not significantly different from zero, which is in contrast to the findings of the present study. Nonetheless, that study supports the findings of the current study with respect to the negative correlation between OBF:SBF and IMF, indicating that as the proportion of SBF made up by OBF increases, longissimus IMF tends to decrease.


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  		Table 5. Heritability estimates (±SE) and genetic and phenotypic correlations (±SE) between ratios of three individual subcutaneous backfat layers and intramuscular fat percent in Duroc pigs
 
Heritability estimates and genetic and phenotypic correlations for IMF, CLMA, SLMA, and DAYS with individual backfat layers are shown in Table 6Go. Genetic correlations between IMF and CLMA, SLMA, and DAYS were –0.33, –0.15, and –0.05, respectively. Corresponding phenotypic correlations were similar to the genetic correlations. Eggert (1998)Go reported a phenotypic correlation between loin muscle area and IMF of –0.56, which is greater than the relationship observed in the current study. Huff-Lonergan et al. (2002)Go reported a phenotypic correlation between loin muscle area and IMF of –0.27, which is similar to the findings from the present study. Loin muscle area is an indicator of lean weight and lean percent in a pig carcass; thus, the correlations between IMF and lean weight or lean percent may be of a magnitude similar to those between IMF and loin muscle area. Sellier (1998)Go reported a mean genetic correlation between IMF and carcass leanness of –0.34, with a range of –0.55 to –0.07. Cameron (1990)Go found the genetic and phenotypic correlations between lean weight and IMF were –0.41 and –0.25, respectively, which indicates that the relationships between IMF and lean weight or lean percent are in the same negative direction and are of similar or lesser magnitude compared to the relationship between IMF and loin muscle area found in the present study.


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  		Table 6. Heritability estimates (±SE) and genetic and phenotypic correlations (±SE) for the three individual subcutaneous backfat layers, 10th-rib loin muscle area measured on the carcass and ultrasonically, adjusted days to 113.5 kg, and intramuscular fat percent in Duroc pigsa
 
Heritability estimates for CLMA, SLMA, and DAYS were 0.70, 0.73, and 0.69, respectively. Moeller (1994)Go reported heritability estimates of 0.79, 0.71, and 0.91 for CLMA, SLMA, and days to 105 kg, respectively. That study estimated the heritability for ADG as 0.66, similar to the heritability estimate for DAYS in the present study. Clutter and Brascamp (1998)Go reported a mean heritability of 0.31 for ADG, with a range of 0.03 to 0.49. These differences indicate that measurement methodology (ADG or days to a common weight) in the evaluation of a trait (growth) can affect heritability estimates.

The genetic correlation between CLMA and SLMA was 0.99, higher than the estimate reported by Moeller (1994)Go. This difference could be due to a greater mean loin muscle area or heavier off-test weight in the current study, or due to differences in genetic and phenotypic variation of loin muscle area between studies. Genetic correlations between loin muscle area and DAYS were small (–0.29 and –0.22, respectively, for CLMA and SLMA), and the phenotypic correlations also were small (–0.10 and –0.27, respectively, for CLMA and SLMA). These values are similar to those reported by Moeller (1994)Go. Results show growth and loin muscle area are lowly correlated, so selection for either trait is possible without a severe detrimental effect on the other.

Heritability estimates and genetic and phenotypic correlations for the three individual s.c. backfat layers, CLMA, SLMA, and DAYS are shown in Table 6Go. The genetic correlations of OBF, MBF, and IBF with CLMA are –0.46, –0.24, and –0.33, respectively. Corresponding correlations with SLMA are –0.43, –0.05, and –0.13, respectively. Similar values, whether measured ultrasonically or on the carcass, suggest that layers are related in a similar manner to both measures of loin muscle area. Additionally, the relationships between individual backfat layers and loin muscle area are similar to the relationship between total backfat depth and loin muscle area. Genetic correlations of OBF, MBF, and IBF with DAYS are –0.37, –0.32, and –0.24, respectively. These relationships are similar to the relationship between SBF and DAYS (–0.40), but of greater magnitude and in the opposite direction from the relationship between CBF and DAYS (0.05). Phenotypic correlations between individual backfat layers and loin muscle area are of slightly smaller magnitude than the genetic correlations. The phenotypic correlations between individual backfat layers and DAYS are of similar magnitude as the genetic correlations.

Our results show that the relationships between individual backfat layers and IMF were similar to the relationships between CBF and SBF, and IMF. In addition, the relationships between individual backfat layers and measures of carcass muscle composition were similar to the relationships between CBF, SBF, and indicators of carcass lean composition. More research is needed to investigate the effect of including total backfat, or an individual backfat layer, on the genetic improvement of IMF.


   Implications
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
 Implications
Literature Cited
 
From the results of this study, estimates of individual backfat layers or ratios of individual layers to total fat depth have the potential to be included in a multiple-trait analysis when attempting to improve intramuscular fat percent in the loin muscle. Due to the heritability of the innermost backfat layer and its genetic relationships with the outer layer and intramuscular fat percent, emphasis could be placed on the innermost layer of backfat and intramuscular fat percent simultaneously to make genetic improvements in both total backfat thickness and percentage of intramuscular fat. Emphasis could also be placed on decreasing the proportion of total backfat made up by the outermost layer to improve intramuscular fat percent. However, in a commercial setting, producers and technicians would most likely not spend the time to measure an individual backfat layer. Increasing the proportion of total backfat made up by the innermost layer and decreasing the proportion of total backfat made up by the outermost layer should lead to the greatest improvement in loin intramuscular fat percent.


   Footnotes
 
1 This journal paper of the Iowa Agric. and Home Econ. Exp. Stn., Ames, IA, Project No. 3456, was supported by Hatch Act and State of Iowa funds. Back

2 Current address: Genetic Improvement Services of NC, Inc., P.O. Box 9, Newton Grove, NC 28366. Back

3 Correspondence: 109 Kildee Hall (phone: 515-294-6728; fax: 515-294-5698; e-mail: tjbaas{at}iastate.edu).

Received for publication June 14, 2004. Accepted for publication November 2, 2004.


   Literature Cited
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
 Implications
 Literature Cited
 


Allen, E., R. W. Bray, and R. G. Cassens. 1967. Changes in fatty acid composition of porcine muscle lipid associated with sex and weight. J. Food Sci. 32:26–29.

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Berger, P. J., L. L. Christian, C. F. Louis, and J. R. Mickelson. 1994. Estimation of genetic parameters for growth, muscle quality, and nutritional content of meat products for centrally tested purebred market hogs. Pages 51–63 in Research Investment Report 1994. Natl. Pork Prod. Council, Des Moines, IA.

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Cisneros, F., M. Ellis, F. K. McKeith, J. McCaw, and R. L. Fernando. 1996. Influence of slaughter weight on growth and carcass characteristics, commercial cutting and curing yields, and meat quality of barrows and gilts from two genotypes. J. Anim. Sci. 74:925–933.[Abstract]

Clutter, A. C., and E. W. Brascamp. 1998. Genetics of performance traits. Pages 427–462 in Genetics of the Pig. M. F. Rothschild and A. Ruvinsky, ed. CAB Int., New York, NY.

Eggert, J. M. 1998. Growth and characterization of individual backfat layers and their relationship with longissimus intramuscular fat. Ph.D. Thesis. Purdue Univ., West Lafayette, IN.

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