J. Anim. Sci. 2006. 84:910-917
© 2006 American Society of Animal Science
Growth performance, carcass composition, quality, and enhancement treatment of fresh pork identified through deoxyribonucleic acid marker-assisted selection for the Rendement Napole gene1,2
C. C. Carr*,3,
J. B. Morgan
,
E. P. Berg*,
S. D. Carter and
F. K. Ray
Department of Animal Science, Oklahoma State University, Stillwater 74078;
and
* Department of Animal Science, University of Missouri, Columbia 65211
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Abstract
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Progeny (n = 70) from unrelated, DNA tested, Rendement Napole carrier (RN–/rn+) Hampshire sires, and DNA tested, Rendement Napole normal (rn+/rn+) Yorkshire dams were genotyped for the segregating RN– allele via DNA marker-assisted methodology. Six slaughter groups ensued, with littermates all being represented within the same slaughter group. Boneless pork loins were removed from right carcass sides after a 48-h chill at 2°C. The anterior portions of the loins were not enhanced, whereas the posterior sections were enhanced with a solution containing 0.5% sodium chloride and 0.5% sodium tripolyphosphate to 110% of their initial weight. Carcasses of carrier pigs had less (P < 0.05) 10th rib fat depth and a greater (P < 0.01) percentage carcass lean than carcasses of normal pigs. Postmortem LM pH of carrier pigs was lower (P < 0.002) at 3, 6, 12, and 24 h, and tended to be lower (P = 0.062) at 48 h compared with that of normal animals. Samples of LM from carrier pigs had greater (P < 0.01) glycolytic potential values, drip loss percentages, and a* values, and lower pH values at fabrication than LM from normal pigs. No genotype differences (P > 0.05) were found for LM lactate, L*, or b* values. Nonenhanced semimembranosus samples from carrier pigs exhibited greater (P < 0.05) purge loss percentages and L* values, and lower (P < 0.01) pH values than samples from normal pigs. Enhanced LM samples exhibited greater (P < 0.05) drip and purge loss percentages, greater pH, and lower L* values at fabrication, regardless of Napole status. These findings suggest that the Napole gene has a positive influence on carcass leanness but detrimental effects for lean quality, which were often further compounded when meat was subjected to enhancement treatment.
Key Words: enhancement • pork • quality • Rendement Napole
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INTRODUCTION
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Over the past 15 years, domestic and foreign fresh meat consumers have shifted from selecting fresh pork products on leanness to product quality and appearance (Brewer and McKeith, 1999
; Cravens, 2001; Norman et al., 2002). This shift in consumer dynamic has forced the US pork industry to make a concerted effort to characterize and improve pork lean quality.
As with all corrective procedures, substantial focus should be asserted on genetics, the initial source of pork quality deviations. The presence of the Rendement Napole gene (RN–) has a derogatory affect on numerous lean quality traits (Hamilton et al., 2000; Miller et al., 2000; Moeller et al., 2003). However, Smith et al. (1984) and Sutton et al. (1997) reported whole muscle products could have water-holding capacity and color properties improved through enhancement injection.
The present series of studies was conducted to investigate the growth performance, carcass composition, and lean quality characteristics of heterozygous carriers (RN–/rn+), and homozygous normal/recessive (rn+/rn+) pigs as determined by the DNA marker assisted test. Additionally, the impact of enhancement treatment in combination with RN– genotype was evaluated relative to lean quality characteristics of fresh pork cuts.
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MATERIALS AND METHODS
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Animals
All pigs were bred and reared at the Oklahoma State University Swine Teaching and Research Facility (Stillwater, OK) by artificially mating purebred Hampshire boars to purebred Yorkshire females. Semen was obtained from a commercial boar stud (Swine Genetics International, Cambridge, IA) from 3 unrelated sires, which were deemed carriers, (RN–/rn+) of the Napole gene by DNA-based PCR testing. All females were DNA genotyped (rn+/rn+) at the Rendement Napole locus.
Three days postpartum, ear tissue samples obtained during conventional ear notching were gathered for each test progeny, then individually sealed and frozen for subsequent assessment of RN– genotype status. Because of variable conception rates and semen availability, sire progeny groups included 24 pigs from 6 litters for 2 sires. The remaining 11 litters generated 56 pigs from 1 sire. All progeny of the 17 test litters with greater than 8 live pigs weaned were subjected to Rendement Napole DNA genotyping. All dam and progeny testing was completed by a commercial genetic laboratory (Geneseek, Lincoln, NE) according to protocol described by Milan et al. (2000).
Each litter was weaned at 18 to 22 d of age, allotted by litter to pens in a conventional climate-controlled nursery, and subjected to a 3-phase nutritional scheme during nursery growth. The initial diet contained 1.47% lysine and 3,415 kcal of ME and was provided for 5 to 7 d. The second phase diet, formulated with 1.32% lysine, 3,430 kcal of ME, was fed for 7 d, followed by the third phase diet, containing 1.12% lysine, 3,432 kcal of ME, fed for approximately 21 d.
At the conclusion of the nursery phase (approximately 35 d), pigs (n = 6) were allotted by litter into pens in a mechanically ventilated grower/finisher building with fully slatted floors. Pigs were allotted to pens by weight with at least one RN–/rn+ barrow and gilt, and one rn+/rn+ barrow and gilt represented in each pen, as sex and genotype constraints allowed. All pigs were fed a grower ration containing 1.00% lysine and 3,320 kcal of ME to approximately 84 kg of live weight; then transferred to a finishing ration containing 0.70% lysine and 3,338 kcal of ME until slaughter. All rations throughout production met or exceeded all NRC (1998) requirements for nursery, growing, and finishing swine.
Pigs were delivered to the Oklahoma State University Meat Laboratory abattoir for humane slaughter according to industry-accepted procedures in 6 separate slaughter groups (n = 8, 12, 12, 12, 16, and 10 pigs/slaughter group, respectively). Pigs were commingled during transport and preslaughter holding and provided ad libitum access to water during a 12-h lairage at the abattoir. Ultimately, 33 carrier (RN–/rn+) pigs (16 barrows and 17 gilts), along with 37 normal (rn+/rn+) pigs (18 barrows and 19 gilts), were slaughtered over a 4-mo period at an average weight of 112.3 kg (99.7 to 113.8 kg). Average live weights from the initial slaughter group were at the lower end of industry acceptability. When analyzed as a fixed effect, live weights from slaughter 1 were lower (P < 0.0001) than the 5 remaining groups; live weights between the remaining slaughter groups did not differ (P > 0.05). Slaughter group, sire, and litter were accounted for within the statistical model individually or as all possible 2-way interactions and found to be nonsignificant (P > 0.05). One normal (rn+/rn+) barrow weighed 2 SD lower than the mean live weight and was subsequently removed from the data set. Adjusted days to 113.4 kg live weight were calculated according to NSIF (1997).
Carcass Composition and Quality Assessment
Postmortem LM pH and temperature was measured between ribs of left sides at 45 min (10th and 11th ribs), 3 h (11th and 12th ribs), 6 h (9th and 10th ribs), 12 h (12th and 13th ribs), and 24 h (8th and 9th ribs) using a portable pH meter (pH Star, SFK Co., Cedar Rapids, IA) and temperature probe (Koch Equipment Co., Kansas City, MO). At 48-h postmortem, midline last rib backfat was measured on right sides of all carcasses. Sides were then fabricated into primal cuts according to the National Association of Meat Purveyors (NAMP) guidelines (NAMP, 1997). After carcass fabrication, the loin (NAMP #413) was cut between the 10th and 11th ribs into anterior and posterior loin sections for subsequent treatments. After a 15-min bloom period, LM area, 10th rib fat depth, and subjective lean color and marbling scores (NPPC, 2000) were assessed by trained university personnel at the 10th/11th rib interface. Both loin sections were then skinned, deboned, and trimmed to 6.35 mm of subcutaneous fat.
From the anterior loin sections, one 2.54-cm chop was removed from the 10th/11th rib interface. On the bloomed surface, ultimate (48-h) pH was collected with a pH Star, and objective lean color (L*, a*, b*) measurements were collected with a Hunter Mini Scan XE Plus (Hunter Associate Laboratories, Reston, VA) using illuminant D65. A 2.54-cm2 section was removed from the center of the chop for drip loss determination using a method modified from Honikel et al. (1987). Briefly, the initial sample weight was recorded, and the sample was pierced with a tagging gun and suspended in an inflated plastic bag at 4°C for 24 h. Then, each sample was reweighed, and drip loss percentage was calculated by dividing the difference between weights by initial sample weight. Additionally, LM from the remaining chop was immediately frozen in a Whirl-Pak bag for glycolytic potential analysis. The anterior loin sections were individually identified, vacuum packaged (bag no. B2650, Cryovac Sealed-Air Corp., Duncan, SC) and stored for 7 d at 4°C for assessment of purge loss, muscle pH, and objective lean color.
Product Enhancement
Posterior loin sections were enhanced with a solution containing 0.5% sodium chloride and 0.5% sodium tripolyphosphate (STPP). Loin sections were infused to 110% of fresh (green) weight using a mechanical injector (Formac-Reiser, Canton, MA) and allowed to equilibrate for approximately 1 h before further fabrication. After equilibration, one 2.54 cm-thick chop was cut from the anterior end, allowed to bloom, evaluated for post-enhancement pH and objective lean color, then sampled for drip loss assessment as described previously. The remaining posterior loin sections were handled and stored as previously described for the anterior sections.
The semimembranosus muscle (SM) was removed from the primal ham (NAMP #401) of right sides, and the gracilis muscle was removed. The external lean surface of the denuded SM was allowed to bloom for approximately 30 min. Ultimate pH as well as 3 objective color evaluations were made and averaged from the medial side of the SM. Each SM was individually identified, vacuum-packaged, and stored for 7 d at 4°C for assessment of purge loss, muscle pH, and objective lean color.
At the conclusion of the storage period, purge loss was determined by weighing the initial package, dispersing stored fluids, then reweighing. Purge loss percentage was calculated by the weight loss divided by initial weight. Day 7 objective color and pH assessment were taken on all stored samples identical to previous protocol after an approximate 30 min bloom.
Glycolytic Potential and Lactate Analysis
Glycolytic potential (GP) was assayed on nonenhanced LM sample frozen at 48-h postmortem according to procedures described by Miller et al. (2000). Glycogen, glucose, and glucose-6-phosphate concentrations were quantified simultaneously by the change in absorbance at 340 nm (Dalrymple and Hamm, 1973; Keppler and Decker, 1974), whereas muscle lactate content was measured by the change in spectrophotometric absorbance of NADH at 340 nm (Bergmeyer, 1974). Values for GP were calculated using the formula of Monin and Sellier (1985), in which GP = 2 x ([glycogen] + [glucose] + [glucose-6-phosphate]) + [lactate]. Values were presented as micromoles of lactate equivalent per gram of wet tissue (µmol/g). Lactate analysis was obtained from the lactate fraction of the GP assessment and is reported in µmol/g of wet tissue.
Statistical Analysis
All performance, carcass composition, and lean quality data from both the LM and SM were analyzed using the mixed model procedure of SAS (SAS Inst., Inc., Cary, NC) as a randomized complete block design, with blocks based on litter of origin, and individual pigs as the experimental unit. Fixed effects included were genotype and sex; slaughter date, litter of origin, and sire were included as random effects. Sampling time postmortem was utilized as a fixed effect for the repeated measures analysis of carcass pH and temperature, whereas storage day (0 vs. 7) was a fixed effect in the repeated measures analysis of L*, a*, b*, and pH data from the LM and SM. Additionally, enhancement was a fixed effect in the ANOVA of LM pork quality data. Least squares means were calculated for main and interactive effects and separated statistically using pair-wise t-tests (P-DIFF option of SAS) when a significant (P < 0.05) F-test was detected. Additionally, the SE for each main effect was reported.
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RESULTS AND DISCUSSION
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Growth Performance and Carcass Characteristics
The documented effects of the RN gene on live pig growth performance and carcass characteristics have been inconclusive (LeRoy et al., 1996; Hamilton et al., 2000). Even though weight per day of age, days to 113.4 kg, hot carcass weight, last rib backfat depth, and LM area did not (P 
0.05) differ between genotypes, carcasses from RN– carrier pigs had less (P = 0.014) 10th rib fat depth, resulting in and greater (P = 0.004) lean percentages than carcasses from normal pigs (Table 1). These findings, relative to the Napole gene, are the most persuasive regarding a carcass composition advantage for Napole carriers. Published reports have shown advantages for carriers compared to normal pigs for carcass leanness (LeRoy et al., 1996) and loin eye area (Lebret et al., 1999).
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Table 1. Main effects of sex and Rendement Napole genotype on pig performance, pork carcass characteristics, and LM quality
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It should be noted that all literature published before 2002 phenotypically segregated individuals based on glycolytic potential as a predictor of Napole gene status. The present research initiative has shifted with the findings of Moeller et al. (2003), who segregated the test population for the RN gene by DNA assessment. Even so, our findings suggest that the RN– allele could have some influence on carcass leanness and red meat yield. Barrows had a greater (P < 0.05) weight per day of age, heavier live weights, and took fewer days to reach a live weight of 113.4 kg, as well as tending to have heavier (P = 0.068) HCW (Table 1
). Conversely, carcasses from gilts tended to have less (P = 0.088) fat opposite the last rib and greater (P < 0.001) estimates of lean percentage than carcasses of barrows. Differences between sexes concurs with results of the terminal line program sex (NPPC, 1995), as well as several other studies (Latorre et al., 2003; See et al., 2004).
Carcass pH and Temperature
Consistent with previous studies by Monin and Sellier (1985), no difference (P = 0.425) in 45-min LM pH was observed between carrier and normal pigs; however, carcasses from carrier pigs had lower (P 0.002) LM pH values at 3, 6, 12, and 24 h postmortem than carcasses from normal pigs (Table 2). Additionally, ultimate (48-h) pH tended to be lower (P = 0.062) in the LM from carrier than normal pigs, and this may be attributed to a lack of bilateral symmetry within carcasses (Stahl et al., 2000). The initial pH values are in agreement with Josell et al. (2003), who reported that LM from Napole carrier individuals, though still having a postmortem pH decline deemed normal for nonHalothane genotypes, possessed a significantly faster decrease in pH during the initial 5-h rigor onset compared with LM from normal individuals. Furthermore, the low ultimate pH value associated with RN– carrier pigs has been well documented (Hamilton et al., 2000; Miller et al., 2000; Moeller et al., 2003).
No interactions were identified (P 
0.114) for LM temperature relative to genotype or sex; however, LM temperature of barrow carcasses was greater (P = 0.031) for carcasses of barrows than gilts (15.4 vs. 14.9°C; results not shown). This may be attributed to barrow carcasses tending to be heavier with more last rib fat thickness, potentially delaying carcass chilling (Jones et al., 1987b).
Glycolytic Potential and Lactate
Glycolytic potential values were greater (P < 0.001) for LM samples from carrier than from normal pigs (Table 2). Hamilton et al. (2002) and Moeller et al. (2003) found similar results, suggesting the efficacy of the DNA test to distinguish GP values within a population in which the RN– allele is segregating. Similar to findings from Monin and Sellier (1985), no genotype differences were distinguished for lactate values from LM samples (Table 2). Thus, it could be hypothesized that the lower ultimate LM pH values from carrier pigs could be attributed to the compiling of intermediate glycogen metabolites within the postmortem muscle.
Physical Water-Holding Capacity
Longissimus muscle samples from RN– carriers had greater (P = 0.007) drip loss percentages than LM samples from normal pigs (Table 3). Furthermore, vacuum-packaged SM from carcasses of carrier pigs had greater (P < 0.001) purge loss percentages than the SM of normal pigs after 7 d of storage. The results of the present study are in accordance with the findings of multiple authors (Miller et al., 2000; Hamilton et al., 2000; Moeller et al., 2003), who reported greater percentages of moisture loss from muscles from carriers than from normal pigs. Moreover, purge loss results are in agreement with Bidner et al. (1999) and Moeller et al. (2003), who found that loins from RN– carrier pigs had greater purge losses than loins from normal pigs.
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Table 3. Main effects of sex, Rendement Napole (RN) genotype, and enhancement treatment on measures of water-holding capacity of the LM and semimembranosus (SM)
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Longissimus muscle samples from barrow carcasses had greater (P = 0.045) drip loss percentages than samples from gilt carcasses. A contributing factor to greater drip loss percentages of LM samples from barrow carcasses could be the tendency for barrows to exhibit a greater (P = 0.095) GP than gilts (272.3 vs. 245.6, results not shown). Miller et al. (1998) and Hamilton et al. (2003) reported that drip loss was positively correlated with GP within nonHampshire-influenced populations with normal GP values (GP < 160). Another contributing factor could be the findings that barrow carcasses had greater (P = 0.031) LM temperatures early postmortem compared with gilt carcasses (data not provided in table).
Enhanced loin sections had a greater (P < 0.001) drip and purge loss percentages compared to nonenhanced loin sections (Table 3). After 7 d of storage, enhanced loins from carcasses of carriers had purge loss percentages 5 times greater than nonenhanced loins from carcasses of normal pigs (genotype x enhancement treatment, P = 0.092; Figure 1). The tendency for this interaction for purge loss percentage suggests that pork from RN– carriers possess a different ultra structure than pork from normal pigs. Estrade et al. (1993) demonstrated that lean tissue from carriers possessed a more open protein lattice structure and larger sarcoplasmic compartment compared with lean tissue from normal pigs. Thus, the open contractile protein lattice of carriers provides less hydrostatic binding properties and a lower capacity to bind exogenous water, regardless of the presence of phosphate in an enhancement solution. Additionally, the lower water-holding capacity of the LM from RN– carrier pigs may also be due to the observations that ultimate LM pH values were close to the isoelectric point of the LM (approximately 5.2), where water holding capacity is lowest (Forrest et al., 1975).
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Figure 1. Interactive effect of Rendement Napole genotype and enhancement treatment on purge percentages of vacuum packaged loin sections after 7 d of storage (P = 0.092).
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Subprimal pH Assessment
The pH of LM and SM sections from RN– carrier pigs (5.48 and 5.44, respectively) was lower (P = 0.001) than LM and SM sections from normal pigs (5.60 and 5.62, respectively) after removal from vacuum packages (results not shown), which is consistent with previous research (Lebret et al., 1999; Bidner et al., 1999; Moeller et al., 2003). Conversely, sex had no (P = 0.324) effect on LM and SM pH after 7 d of storage (results not shown). The pH of enhanced loin sections was greater (P = 0.025) than that of nonenhanced loin sections on d 0 of vacuum-packaged storage; however, pH values were similar (P = 0.105) between nonenhanced and enhanced loin sections after 7 d of storage (treatment x storage time, P = 0.002; Figure 2). Moreover, pH values of nonenhanced loin sections were greater (P < 0.001) on d 7 than on d 0. The effect of STPP to increase muscle pH is supported by numerous authors (Smith et al., 1984; Jones et al., 1987a; Banks et al., 1998). Forrest et al. (1975) reports that greater muscle pH should either stay constant, as found in enhanced samples, or slightly increase during postmortem aging due to lactic acid leaching from muscle.
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Figure 2. Interactive effect of enhancement treatment and storage day on LM pH values (P = 0.002). Bars lacking a common superscript letter differ (P 0.025).
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Subprimal Objective Lean Color Assessment
Genotype had no (P = 0.207) effect on L* values of the LM at fabrication (Table 4). Similar to current results, Miller et al. (2000) reported L* values did not differ between RN– carrier and normal pigs, but Hamilton et al. (2000) and Moeller et al. (2003) observed that the LM from carriers was lighter (greater L* values) than the LM from normal pigs. Longissimus muscle from RN– carrier pigs was redder (greater a* values; P < 0.029) than the LM from normal pigs at fabrication, and d 7 a* values were greater (P < 0.001) than initial (d 0) values, regardless of RN– genotype (genotype x storage time, P = 0.014; Figure 3). After 7 d of storage, b* values for the LM from carrier pigs were greater (P < 0.001) than those of normal pigs, and d 7 b* values were greater (P < 0.001) than initial (d 0) values, regardless of RN– genotype (genotype x storage time, P = 0.002; Figure 3).
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Figure 3. Interactive effect of Rendement Napole genotype and storage day on LM redness (a*) values (P = 0.014) and yellowness (b*) values (P = 0.002). Bars lacking a common superscript letter differ (P < 0.029).
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The SM from carcasses of carriers was lighter (greater L* value; P = 0.001) than the SM from carcasses of normal pigs (Table 4). Moreover, the SM from RN– carriers tended to be redder (greater a* values; P = 0.102) and more yellow (greater b* values; P = 0.088) than that from normal pigs. Both the SM and LM from barrows had greater (P 0.017) L* values than the SM and LM from gilts (Table 4), and the increase in lightness may be attributed to the noted elevation in LM temperature in barrow compared with gilt carcasses. Redness (a*) values of the LM (P = 0.367) and SM (P = 0.406) were similar between sexes. Additionally, b* values for the SM did not (P = 0.138) differ between barrows and gilts; however, the LM from barrows was more yellow (P = 0.037) than the LM from gilts.
On d 0 of storage, enhanced loin sections were darker (P < 0.001) than nonenhanced loin sections and darker (P < 0.001) than enhanced or nonenhanced loin sections on d 7 (enhancement treatment x storage time, P = 0.002; Figure 4). This is consistent with the work of Banks et al. (1998) and Sutton et al. (1997), who found that pork loins enhanced with a solution containing 0.4% STPP had lower L* values than nonenhanced pork loins. Furthermore, a* values of enhanced loin sections were greater (P < 0.001) on d 7 than on d 0 and greater (P < 0.001) than nonenhanced loin sections on d 0 and 7 (enhancement treatment x storage time, P < 0.001; Figure 4). Yellowness (b*) values were similar (P = 0.162) between nonenhanced and enhanced loin sections on d 0, but lower (P < 0.001) than on d 7 of storage. Additionally, nonenhanced loins were more (P < 0.001) yellow than their enhanced counterparts on d 7 (enhancement treatment x storage time, P = 0.014; Figure 4).
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Figure 4. Interactive effect of enhancement treatment and storage day on LM lightness (L*) values (P = 0.002), redness (a*) values (P < 0.001), and yellowness (b*) values (P = 0.014). Bars lacking a common superscript letter differ (P < 0.002).
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IMPLICATIONS
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The findings of the present research indicate that the Rendement Napole carrier genotype (RN–/rn+) possesses carcass composition advantages, potentially validating the continued utilization of Rendement Napole carrier seedstock. However, the negative effect of Rendement Napole carrier genotype on pork quality characteristics cannot be ignored, especially considering that enhancement treatment did not alleviate the lean quality problems associated with the Rendement Napole gene. Further research should be conducted to identify the biological processes in the lean tissue of Napole carriers, whereby a steady postmortem pH decline resulting in a significantly lower ultimate pH is achieved independent of intramuscular lactate accumulation. The understanding of this glycolytic aberration could lead to potential corrective procedures and genetic selection, which could substantiate the utilization of carrier Hampshire seedstock.
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Footnotes
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1 This research was supported by the Oklahoma Pork Producers Council, Oklahoma City. 
2 The authors wish to thank L. Gunther and K. Novotny for their laboratory assistance, as well as C. Goad, D. Buchanan, K. Stalder, and M. Ellersieck for their statistical advice.
3 Corresponding author: cccktb{at}missouri.edu
Received for publication December 27, 2004.
Accepted for publication November 7, 2005.
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Abstract
INTRODUCTION
