J. Anim. Sci. 2003. 81:1943-1949
© 2003 American Society of Animal Science
Heritability of body length and measures of body density and their relationship to backfat thickness and loin muscle area in swine
Z. B. Johnson1 and
R. A. Nugent, III2
Department of Animal Science, University of Arkansas, Fayetteville 72701
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Abstract
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The objective of this study was to estimate heritability for body length (LEN) at the end of performance testing and to estimate genetic correlations with backfat (BF) thickness and loin muscle area (LMA) in Landrace, Yorkshire, Duroc, and Hampshire breeds of swine. Also examined were two measures of body density involving body length and weight and their relationships to backfat and loin muscle area. Data consisted of performance test records collected in a commercial swine operation from 1992 to 1999. Boars from 60% of the litters were culled at weaning based on a maternal breeding value of the dam. Remaining boars and all females were grown to 100 d of age (15,594, 55,497, 12,267, and 9,782 Landrace, Yorkshire, Duroc, and Hampshire pigs, respectively). At this time, all pigs were weighed (WT100) and selected for performance testing based on a combination of maternal and performance indexes, which differed by breed. All pigs were weighed at the end of the 77 d performance test (WT177) when BF, LMA, and LEN were measured. Two measures of body density involving length were calculated: Body mass index (BMI) = WT177/LEN2 and body density (DENSITY) = WT177/LEN. For each breed, genetic parameters were estimated using an animal model with random litter effects and multiple-trait REML procedures. A series of three-trait models including WT100 and combinations of two other traits in each analysis was conducted. Fixed effects included contemporary group and age as a covariate. Average estimates of heritability were 0.16 to 0.32 for LEN (unadjusted for WT177), 0.12 to 0.26 for LEN (adjusted for WT177), 0.23 to 0.33 for DENSITY, and 0.16 to 0.25 for BMI. Genetic correlations between LEN and LMA were low. Genetic correlations between LEN (unadjusted for WT177) and BF were 0.10 to 0.41. Adjusting LEN for WT177 gave correlations of 0.11 for Landrace and Hampshire and negative correlations (-0.06 and -0.19, respectively) for Yorkshire and Duroc. Genetic correlations between LMA and DENSITY and between LMA and BMI were comparable and ranged from 0.44 to 0.54. Genetic correlations between BF and DENSITY were slightly higher (0.53 to 0.68) than those between BF and BMI (0.37 to 0.67). In these data, not much relationship between BF and body length at a constant weight and age was found. There was a negative relationship between LMA and LEN at a constant weight and age, implying that longer pigs had smaller LMA.
Key Words: Backfat • Body Density • Body Length • Longissimus dorsi • Pigs
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Introduction
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Bereskin and Steele (1988)
reported an estimate of heritability for carcass length of 0.71 and a correlation of carcass length with average backfat of -0.71. It was hypothesized that, in the live animal, longer pigs would be leaner pigs. Also, it was thought that the body length measurement could provide additional information to supplement prediction of backfat thickness and loin muscle area. Body length is likely to be highly correlated with weight at the same age; therefore, any analysis to obtain correlations of body length with measures of body fat should probably include some adjustment for BW. One way to do this is to include weight as a covariate. Another option would be to make use of the body mass index (BMI), which is an easy-to-measure parameter that is widely used to estimate body fat percentage in humans. Snijder et al. (1999) reported that several studies have found a high correlation between BMI and body fat percentage as long as they were adjusted for age and sex. Duffy et al. (2001) suggested that normalizing BW for differences in length to give a measure of body density could provide a useful and accurate measure to compare data among studies that use animals of different size and/or weight, much as the BMI is used to evaluate obesity status in humans. No literature estimates of heritability of body length on the live animal or correlations with other traits were found. The objectives of this study were to estimate heritability for body length (with and without weight as a covariate) and two measures of body density involving body length and BW and to examine the relationships for these measures of body length and density with backfat and loin muscle area in Landrace, Yorkshire, Duroc, and Hampshire breeds of swine.
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Materials and Methods
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Data for this study consisted of performance test records of Landrace, Yorkshire, Duroc, and Hampshire pigs collected in a commercial swine operation (The Pork Group, a Division of Tyson Foods, Inc., Rogers, AR) from 1992 to 1999. These herds were established in 1992. Two indexes (breeding values) for each animal were calculated at birth for each breed separately. One was a maternal index based on number born alive, farrowing interval, and litter weaning weight. The other index was based on growth rate, leanness, and feed efficiency (Grow-Fin). The maternal index was computed using a three-trait model that included terms for the additive genetic effect, litter effects, and maternal genetic effects along with fixed effects for contemporary group (defined as farrowing within a month of each other at a location), parity, and birth year of the sow. The Grow-Fin index was computed using a model that included only additive genetic effects and fixed effects for contemporary group (defined as farrowing in the same week on the same farm) and sex. These two indexes were combined into an overall ranking depending on the breed. For Landrace, equal emphasis was given to both indexes; for Yorkshire, more emphasis was given to the maternal index; for Duroc, more emphasis was given to the Grow-Fin index; and for Hampshire, the emphasis was totally on the Grow-Fin index. Boars from approximately 60% of the litters were culled at weaning based on breed-specific index, which was a combined index for all breeds except Hampshire. Culled boars (barrows) were grown out and slaughtered. For economic reasons, these animals were not performance tested. Remaining boars and all females were grown to 100 d of age. At this time, all pigs were weighed (WT100) and a second culling event occurred with recalculated indexes using any new information collected on animals within each breed. Litter records for the individual would have been included in the maternal index, and the individual’s own record for WT100 would have been available. Leanness records for the individual would not have been available at this time; therefore, changes in the Grow-Fin index would have resulted from WT100 and additional performance records on relatives obtained since pigs were weaned. Fifty to sixty percent of the females and 20 to 25% of the remaining Yorkshire, Landrace, and Duroc boars were performance tested for approximately 77 d. A higher percentage (37%) of Landrace boars were performance tested. Females culled at 100 d of age were moved down the production pyramid and mated to produce crossbred offspring that then served as parents of commercial offspring. Females that were not culled at this time were retained and mated to purebred boars to produce progeny for the purebred nucleus herd.
Boars were individually penned in 2.79-m2 pens with slotted gating on slatted concrete floors. Barns were curtain-sided buildings that were tunnel ventilated in the winter. For all years, boars were given ad libitum access to a pelleted corn-soybean meal diet that was 1.14% lysine, 19% protein, and 3,344 mcal/kg of ME. This diet was fed for the entire test period. Exact composition of the diet varied due to ingredient cost. Gilts were fed this same diet in groups of 8 to 10 pigs in a pen providing 1.2 m2 of space per female. Different size pens were available in different facilities, so pens in some barns held eight pigs and in other barns 10 pigs. All pigs had ad libitum access to water. At the end of the 77-d performance test, all pigs were weighed (WT177) and body length (LEN) was measured from the tail head to the point of the shoulder when the head was down. At this time, backfat thickness (BF) and loin muscle area (LMA) were measured at approximately the 12th rib using B-mode ultrasound equipment. Within a contemporary group, the same technician did all the LEN measurements. Technicians were trained and not allowed to record this measurement until they had reached a high level of repeatability in taking the measurement. Body mass index, a ratio that is used in evaluating obesity in humans (Carmichael and McGue, 1995), was calculated as WT177/LEN2. The LEN measurement was used in place of the height measurement usually used in humans (Carmichael and McGue, 1995; Snijder, et al., 1999). Body weight was divided by LEN (in cm) to provide an estimate of overall density (DENS) as reported in mice by Duffy et al. (2001).
Contemporary group was defined as all pigs of the same sex reared in the same house and started on test within a 3-mo period. Data sets were edited to remove records of animals with missing sire or dam. Records were omitted if any trait measurement was greater than 4 SD away from the overall mean (Bryner et al., 1992). Some description of the data sets is given in Table 1, and means and standard deviations are given in Table 2. There were 15,594, 55,497, 12,267, and 9,782 observations at 100 d of age for Landrace, Yorkshire, Duroc, and Hampshire, respectively. Of these, 7,951 Landrace, 26,649 Yorkshire, 5,240 Duroc, and 3,615 Hampshire had ADG records, with approximately the same number of observations for LEN, LMA, BF, BMI, and DENS.
Genetic parameters were estimated for each breed and trait using an animal model and multiple-trait restricted maximum likelihood procedures (Boldman and Van Vleck, 1991; Boldman et al., 1993). A series of three-trait analyses was conducted using WT100 and combinations of two other traits in each analysis. Direct additive effects and litter effects were fitted for all traits. The only fixed effect was contemporary group, which was included in all analyses for each trait. The WT100 was included in each analysis in an attempt to remove bias due to selection at 100 d of age because not all pigs weighed at 100 d of age were performance tested. Initial test age (AGE100) was a covariate for WT100. Final test age (AGE177) was a covariate for all other traits. In a second series of analyses including LEN with LMA and BF, WT177 was added as a covariate for body length to adjust body length for weight (LENA). In the REML analyses, the convergence criterion (i.e., variance of the simplex values) for all runs was 10-9. The program was restarted with estimates at previous apparent convergence used as initial values until an apparent global maximum was found and estimates of genetic parameters did not change between runs. Standard errors for heritability estimates were estimated using single-trait models.
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Results and Discussion
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Average estimates of heritability, along with standard errors estimated from single-trait analyses, and the range of estimates are presented in Table 3. Average estimates of heritability (n = 9) of WT100 ranged from 0.12 for Duroc to 0.27 for Landrace, with standard errors of 0.02 or 0.03. For WT177, heritability estimates ranged from 0.15 ± 0.03 for Duroc to 0.27 ± 0.03 for Landrace. Estimates of heritability of WT100 from single- and two-trait models including maternal effects, as reported by Johnson et al. (2002), ranged from 0.05 for Duroc to 0.20 for Hampshire. No other literature estimates of heritability of weight at 100 or 177 d of age were found; however, Kuhlers and Jungst (1992a) reported a realized heritability of 0.13 ± 0.06 for 70-d weight in Landrace. This is lower than the 0.27 reported for Landrace at 100 d of age in this study.
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Table 3. Mean, standard error, and range of estimates of heritability for weight at 100 and 177 d of age, body length, loin muscle area, backfat thickness, and measures of body density for each breeda
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Average estimates of heritability for LEN and LENA were about the same for Yorkshire and Duroc, but were numerically higher for LEN than for LENA for Landrace and Hampshire (Table 3
). Estimates within breed were almost identical from one analysis to another and did not vary by more than 2%. Average heritability estimates for DENS were from 4 to 8% greater than those for BMI.
For LMA, average estimates of heritability (n = 4) ranged from 0.22 ± 0.03 for Duroc to 0.34 ± 0.03 for Landrace (Table 3). Comparable estimates were reported by Johnson et al. (2002) using a model that included maternal effects. A similar estimate (0.31) was also reported by Bereskin (1987) in live animals. An estimate of 0.24 was reported in Large White swine (Johnson et. al., 1999). Other researchers have reported higher estimates. Lo et al. (1992), using data from a 2 x 2 diallel mating system involving Landrace and Duroc pigs, estimated heritability of LMA measured ultrasonically at the last rib at 103.6 kg of BW to be 0.46 ± 0.08. Stewart and Schinckel (1990), using a weighted average of results reported in research papers from the United States and Europe, reported a heritability of 0.47 for LMA. Swiger et al. (1979), using carcass data from swine tested at the Ohio Swine Evaluation Station, estimated heritability of LMA to be 0.56 ± 0.06.
For backfat, average estimates of heritability (n = 4) ranged from 0.32 ± 0.05 for Hampshire to 0.47 ± 0.02 for Yorkshire (Table 3). Previous estimates including maternal effects in the model (Johnson et al., 2002) were higher for Landrace and Yorkshire (0.63 and 0.65, respectively) and similar for Duroc and Hampshire (0.35 and 0.31, respectively). Li and Kennedy (1994) reported estimates of 0.53, 0.55, 0.51, and 0.50 for Landrace, Yorkshire, Duroc, and Hampshire, respectively. Stewart and Schinckel (1990), using a weighted average of results reported in research papers from the United States and Europe, reported heritabilities of 0.41 for backfat and 0.52 for 10th rib fat. Other estimates of heritability of backfat in the literature ranged from 0.23 to 0.79 (Kuhlers and Jungst, 1983; Kennedy et al., 1985; Bereskin, 1986; Merks, 1988; McKay, 1990; Van Steenbergen et al., 1990; Bryner et al., 1992; Lo et al., 1992; Ferraz and Johnson, 1993; Mrode and Kennedy, 1993; ten Napel and Johnson, 1997).
As expected, estimates of genetic correlations of WT177 with LEN were high, ranging from 0.51 for Duroc to 0.75 for Landrace (Table 4). Estimates of genetic correlations between LMA and LEN were close to zero (0.08, 0.06, and 0.07) for Landrace, Yorkshire, and Hampshire, respectively, but when WT177 was added as a covariate for LEN, these correlations became negative (-0.16, -0.17, and -0.19, respectively). For Duroc, this genetic correlation was -0.19 without WT177 as a covariate for length and more negative (-0.32) when WT177 was added as a covariate for LEN. Genetic correlations between BF and LEN varied by breed, ranging from 0.10 for Duroc to 0.41 for Hampshire. When WT177 was added as a covariate for LEN, genetic correlations between LENA and backfat were low for all breeds, but positive (0.11) for Landrace and Hampshire and negative (-0.06 and -0.19, respectively) for Yorkshire and Duroc. It is not clear why these correlations should change so much, but a possible explanation could be that because LEN has a high correlation with WT177, longer pigs would tend to be heavier and fatter pigs (as evidenced by the genetic correlations of 0.10 to 0.41 reported here), but when LEN was adjusted to a constant weight (with covariate WT177), then there was little or no relationship between LEN and backfat. Estimates of genetic correlations between backfat and DENS were similar to or higher than those between backfat and BMI (and higher than correlations between backfat and LEN or backfat and LMA), indicating that either of these measures of body density would be a better selection tool for backfat than LEN.
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Table 4. Estimates of genetic correlations for measurements of body length and body density with loin muscle area and backfat thickness measured at 177 d of age for each breeda
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Genetic analyses with random litter effects included give an estimate of common environmental litter effects (c2). These are presented in Table 5. Common environmental litter effects explained 20 to 25% of the phenotypic variance for WT100 and slightly less (16 to 21%) of the phenotypic variance for WT177. Common environmental litter effects explained about the same amount (15 to 22%) of phenotypic variance for LEN and LENA, about 14% of the phenotypic variance for BMI, and slightly less for LMA, BF, and DENS (8 to 10%). Comparable estimates were reported for backfat by Li and Kennedy (1994) and Kennedy et al. (1985) for these same four breeds. Estimates by ten Napel and Johnson (1997) for Large White and Landrace were slightly lower. An earlier study by Ferraz and Johnson (1993) reported c2 values of 0.04 and 0.06 for backfat in Landrace and 0.05 for Landrace and Large White combined. Merks (1988) reported higher c2 effects for backfat for on-farm-tested Dutch Landrace and Dutch Yorkshire (0.20 and 0.21, repectively).
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Table 5. Average estimates and range of estimates for common environmental litter effects (c2) for weight at 100 and 177 d of age, body length, loin muscle area, backfat thickness, and measures of body density for each breeda
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No literature estimates of heritability of body length on live animals nor any correlations with backfat thickness or loin muscle area were found. Bereskin and Steele (1988), in a reciprocal recurrent selection experiment, reported an h2 of 0.71 ± 0.16 for carcass length, and genetic correlations between carcass length and average backfat and LMA of -0.73 ± 0.12 and -0.12 ± 0.21, respectively. Stewart and Schinckel (1990), using a weighted average of results reported in research papers from the United States and Europe, reported a heritability of 0.56 for carcass length and genetic correlations of -0.21 between 10th rib fat and carcass length and -0.18 between LMA and carcass length. These estimates of heritability of length of the carcass are higher than those found in the current study for body length of live animals; however, correlations of carcass length with LMA and BF reported by Stewart and Schinckel (1990) are fairly comparable to those found in the current study with LMA and BF and body length. Kuhlers and Jungst (1992b) reported that carcass length did not change significantly in response to selection for 200-d weight in Duroc swine, but that it did change with changes in weighted cumulative selection differentials for 200-d weight in Landrace pigs (Kuhlers and Jungst, 1993). Carcass length was less in the select line than in the control line. They also reported (Kuhlers and Jungst, 1992a) that carcass length decreased, but LMA and average BF did not change significantly in response to selection for 70-d weight in Landrace swine.
All analyses were done within breed. Each breed was a unique population (with its own set of gene frequencies for the traits considered), and therefore it would not be expected that variances and covariances would be the same (Falconer and Mackay, 1996). Selection procedures were not the same across breeds, and this may also have caused differences in relationships among traits examined. However, some trends are evident. Estimates of heritability of LEN, LENA, DENS, and BMI were high enough (0.16 to 0.32 for LEN; 0.12 to 0.26 for LENA; 0.23 to 0.33 for DENS; and 0.16 to 0.25 for BMI) to indicate that additive genetic variability does exist for each of these measures of body length or body density for all breeds. Genetic correlations between LEN and LMA were low positive or negative, and between LENA and LMA, were negative. It appears that adjusting body length for weight does have an affect on this correlation changing it from close to zero to negative. Genetic correlations between LMA and DENS were generally slightly higher than those between LMA and BMI. Genetic correlations between LEN and BF were higher than those between LENA and BF for Landrace, Yorkshire, and Hampshire. For Duroc, the correlation between LEN and BF was 0.11, and between LENA and BF, was -0.19. Adjusting body length for weight also had an effect on this correlation, changing it from a positive correlation, which indicated that longer pigs were fatter pigs, to a low correlation, which indicated no relationship. Genetic correlations between BF and DENS were higher than those between BF and BMI.
These results indicated that in these herds, there was not much relationship between backfat and body length at a constant weight and age, implying that body length would not be a good indicator of BF and that there is a negative relationship between LMA and LENA, implying that longer pigs have smaller LMA. Genetic correlations of both DENS and BMI with LMA and BF were moderate and fairly comparable, although those with DENS were generally somewhat higher, indicating that either of these measurements involving length might be a better indicator of BF or LMA than the LEN measurement alone.
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Implications
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Estimates of heritability of backfat thickness and loin muscle area in pigs were larger than those for body length (unadjusted for weight at 177 d of age), body length (adjusted for weight at 177 d of age), body density, or body mass index, implying that direct selection for these traits would be more effective than indirect selection using body length or any of the measures of body density. Genetic correlations of body density and body mass index with loin muscle area and backfat thickness, however, were sufficiently high to indicate that improvement in these traits could be accomplished with a measure of body density involving length and weight.
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Footnotes
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2 Current address: The Pork Group, a Division of Tyson Foods, Inc., Rogers, AR 72757. 
1 Correspondence—phone: 479-575-2983; fax: 479-575-7294; E-mail: zelphaj{at}uark.edu).
Received for publication October 11, 2002.
Accepted for publication May 9, 2003.
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Abstract
Introduction
