Influence of Slaughter Weight on Growth and Carcass Characteristics, Commercial Cutting and Curing Yields, and Meat Quality of Barrows and Gilts from Two Genotypes

Introduction

Slaughter weights (SLW) for swine in the United States have been increasing steadily over recent years and currently average approximately 112 kg for slaughter barrows and gilts (Morgan et al., 1994). The potential advantages of producing heavier pigs are widely recognized by the slaughter/processing industry, and this sector would like to increase slaughter weights above current levels. Traditionally, the major limitation to increasing slaughter weights has been the high carcass fat levels observed at heavier weights and the associated deterioration in feed efficiency. These two factors have moderated the rate of increase of slaughter weights over time. However, the genetic potential of commercial pigs has changed dramatically over recent years, particularly in terms of lean growth rates. Modern, high-leangain genotypes should have the potential to be slaughtered at heavier weights with less effect on carcass merit and(or) feed conversion efficiency compared with low-lean-gain genotypes. In addition, attention is increasingly being focused on meat quality (Morgan et al., 1994), and any consideration of increasing slaughter weights requires the evaluation of the impact of increased weight on meat quality, as well as on growth and carcass characteristics. The objective of this study was to carry out an integrated evaluation of the influence of heavier slaughter weights on growth, carcass merit, carcass cutting and curing yields, and meat quality in modern genotypes.

Materials and Methods

On-Farm Trial. This study was carried out to evaluate the performance of two genotypes of pigs at five live weight end points (100, 115, 130, 145, and 160 kg). A breeding company commercial hybrid ( BCH) and a three-breed cross of Yorkshire ´ Duroc dams with Hampshire sires (HYD) were compared. One hundred sixty pigs consisting of equal numbers of barrows and gilts of each genotype were used in the study. They were reared in a controlled-environment finishing barn in like-sex, like-genotype groups of four pigs. The pen dimensions were 1.8 ´ 2.6 m, giving a space allowance of 1.17 m2/pig. The pen floors were partially slotted. Pigs were randomly allotted to slaughter weight groups from outcome groups of similar weight within genotype and sex. The average starting weight was 60 ± .9 kg and the average age at the start was 114.5 ± 9.5 days. Pigs were given ad libitum access to feed and water. A corn-soybean meal diet formulated to meet NRC (1988) requirements was used for both genotypes and the two sexes (Table 1). Live weight and feed consumption were recorded at 14-d intervals.

Influence of Slaughter Weight on Growth and Carcass Characteristics, Commercial Cutting and Curing Yields, and Meat Quality of Barrows and Gilts from Two Genotypes - Image 1

Carcass and Meat Quality Evaluation. When the average weight of the pen reached the target end point weight ( ± 2.5 kg), two pigs were chosen at random from each pen and were slaughtered for carcass and meat quality evaluation. Pigs were transported to the Meat Science Laboratory, University of Illinois approximately 18 h before slaughter and were held off feed and provided water during lairage. Pigs were slaughtered using commercial procedures in a federally inspected facility. Live weights were recorded immediately before slaughter and hot carcass weights were taken approximately 1 h postmortem. Longissimus lumborum pH was evaluated at 45 min postmortem on each carcass. Muscle samples (approximately 3 g) were obtained immediately posterior to the last rib, using a cork bore (8 mm diameter), and the sample was homogenized in 10 mL of .005 M iodoacetate. The pH was evaluated using a pH meter (Orion model 720 A) fitted with a glass electrode (Ross Sure Flow 81-72).

At 24 h postmortem, carcass characteristics were evaluated on the right side of the carcass (first rib, last rib, last lumbar, and 10th rib fat thickness; longissimus muscle area at the 10th rib; longissimus thoracis color, firmness and marbling; carcass length) using the procedures described by NPPC (1991). Ultimate pH was evaluated on the longissimus lumborum using the procedures previously described. For drip loss determination, a 2-cm-thick chop was removed from the longissimus thoracis anterior to the last rib, weighed, and suspended in a plastic bag in a 4°C cooler for a 24-h period and then reweighed. The left side of the carcass was weighed and fabricated and weights were recorded for carcass cutting yields. Carcasses were fabricated using the procedures described by NAMP (1992). Untrimmed wholesale cuts, trimmed wholesale cuts (trimmed to 3 mm of subcutaneous fat), and trimmed, boneless wholesale cuts were obtained, and the weights were recorded. A 15-cm section of longissimus lumborum muscle was taken posterior to the last rib for sensory evaluation, shear force determination, and chemical analysis. The section was vacuum-packaged, aged for 7 d in a 4°C cooler, and then frozen ( -20°C) for subsequent evaluation.

The trimmed, boneless ham and belly were identified, vacuum-packaged, and frozen ( -20°C) and held for subsequent curing yield evaluation. After all samples were accumulated, they were thawed (at 4°C for 72 h) and pumped to 110% of their weight with a commercial cure solution (sodium chloride, 2.0%; sodium tripolyphosphate, .5%; sodium erythorbate, 500 ppm; sodium nitrite, 150 ppm for the ham, and 120 ppm for the belly) using an injector (Smith Pokomat, Clifton, NJ). Bellies were allowed to equilibrate for 24 h and were cooked and smoked in a smoking house (Voctron model TR, Beloit, WI). The cooking cycle for bellies included changes in the dry bulb temperature (50, 55, 60, and 70°C) and the wet bulb temperature (0, 0, 49, and 57°C) every 2 h. The smoke generator was on at all times and the last setting was maintained until the internal temperature of the bellies reached 60°C. Hams were tumbled for 4 h in a Zuber (Minneapolis, MN) tumbler (13 rpm) at 25 mm of Hg vacuum. Hams weighing less than 5.4 kg hams heavier than 5.4 kg were placed into 17-cm diameter fibrous casings. The cooking cycle for hams included changes in the dry bulb temperature (55, 65, 74, 77, and 82°C) and the wet bulb temperature (0, 0, 62, 66, and 72°C) every 2 h, the smoke generator was switched on after the first 2 h of cooking, and the last temperature setting was held until the internal temperature of the ham reached 66°C. Ham and belly fresh weights and cooked, chilled weights were recorded to calculate curing yields. Sensory Evaluation. Chops (2.5 cm thick) from the longissimus lumborum sample were cut with a band saw from the frozen section to ensure a uniformity of thickness. Chops for sensory evaluation were thawed for 24 h in a 4°C cooler. They were cooked on Farberware open-hearth grills (Model 155N, Walter Kidde, Bronx, NY) to an internal temperature of 70°C, monitored using copper constantan thermocouples and a recording thermometer (Campbell Scientific, Logan, UT). When the internal temperature of the chops reached 35°C, they were turned to prevent charring.

Tenderness, juiciness, and off-flavor intensity were evaluated by a six-member, trained panel using a 15-cm structured line scale with anchors and a midpoint (0 cm = extremely dry, tough, and intense offflavor; and 15 cm = extremely moist, tender, and no off-flavor). Water was provided to panelists to cleanse the palate. Ten panelists (faculty members and graduate students in the Meat Science Laboratory) were trained according to the procedure described by the American Meat Science Association (1978), and six of the panelists participated in each panel. Chops from the longissimus lumborum sample for Warner-Bratzler shear force determination were prepared using the same procedure described for sensory chops. Cooking loss was evaluated using raw and cooked weights. After cooling to 25°C, five cores 1.3 cm in diameter were removed parallel to the muscle fibers and sheared using an Instron model 1122 Universal Testing Machine (Instron, Canton, MA) fitted with a Warner-Bratzler shear attachment. The full scale load was set at 10 kg and the chart drive and crosshead speeds were 200 mm/min.

Chemical Analysis. Duplicate samples of longissimus lumborum were analyzed for moisture (AOAC, 1984), and lipid content was determined by repetitive extraction of moisture-free samples with an azaeotropic mixture of warm chloroform and methanol (4:1) as described by Novakofski et al. (1989). Statistical Analysis. The design of this study was a completely randomized 2 ´ 2 factorial, two genotypes and two sexes, with slaughter weight considered as a covariable. For growth performance, the pen was used as the experimental unit, and the individual animal was used as the experimental unit for the remainder of the data. Results were analyzed using the GLM procedure of SAS (1985). The model used in the analysis included the effects of genotype, sex, genotype ´ sex interaction, and slaughter weight fitted as a covariable. Initially, the model including linear and quadratic effects of slaughter weight and two- and three-way interactions of slaughter weight with sex and(or) genotype was considered for all variables.

This will be referred to as the full model. Subsequently, simpler models were derived from the full model by dropping higher-order regression terms. The adequacy of the simpler models was tested by a lack of fit test (Neter et al., 1985) and the simplest model without lack of fit, compared to the full model, was derived for each variable.

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Results and Discussion

Results for the growth performance of pigs are presented in Table 2. Genotype BCH had higher feed intakes ( P < .05) and daily gains ( P < .05) than HYD but a similar gain:feed ratio. In addition, barrows grew faster than gilts ( P < .05); however, there was an interaction between genotype and sex for days on test and daily gain ( P < .05). The interaction resulted from no difference in growth rate between the sexes for HYD (daily gain = .811 and .809 kg for barrows from BCH grew faster than gilts (daily gain = .930 and .817 kg for barrows and gilts, respectively). This interaction between sex and genotype for growth rate resulted mainly from differences in feed intake, which were similar for barrows and gilts of HYD (3.01 vs 2.96 kg/day, respectively, SD .108) but differed for BCH (3.40 vs 3.17 kg/day, respectively, SD .107). However, there was no sex ´ genotype interaction for feed intake ( P = .24). Other studies have generally shown faster growth rates for barrows than for gilts resulting largely from higher feed intakes for the former (Kanis et al., 1990; Friesen et al., 1994).

Hansson (1974) reported similar growth rates between 25 to 130 kg live weight for barrows and gilts that were fed a constant amount of feed after 80 kg. The results of the current study suggest that sex differences in growth rate may vary with genotype. There were no quadratic effects between SLW and growth traits and no evidence that the relationships differed between genotypes or sexes (Table 2). As expected, days on test and average daily feed intake increased linearly with slaughter weight; pigs required an extra 1.2 d to reach end weight ( P < .001) and consumed an extra 10 g of feed per day ( P < .001) for every kilogram increase in slaughter weight. Our results suggest that overall growth rates and feed efficiencies are largely unaffected by slaughter weights between 100 and 160 kg (Table 2). The lack of a slaughter weight effect on growth rate agrees with the results of Neely et al. (1979), who found no significant difference in daily gain in pigs from 68 kg to 100, 113, or 127 kg. Kanis et al. (1990) reported that barrows and gilts grew substantially more slowly between 60 kg and 140 kg that between 60 and 100 kg. It has been suggested that factors such as decreased pen space, increased fat deposition, and the onset of puberty in the gilt may contribute to the lack of response in growth rate at heavier weights. A study carried out by the NCR-89 Committee (1993) suggested that growth rates for pigs slaughtered at 113 kg was maximized at a space allowance of .93 m2/pig. The floor area per pig in this study was 1.17 m2 for all slaughter weight groups, and it would seem unlikely that inadequate space allowance restricted growth until the upper end of the weight range investigated in this trial. The regression of gain:feed on slaughter weight was not significant. This is in contrast to the findings of Kanis et al. (1990), who observed a 16% reduction in feed efficiency for pigs slaughtered at 140 kg compared to 100 kg. However, as previously discussed, these workers also observed a reduction in growth rate to the heavier slaughter weight, which explains the deterioration in feed efficiency.

The growth responses of pigs at all weights, in terms of feed intake, live weight gain, and feed efficiency, are strongly influenced by the nutritional program used and, particularly, the energy and protein supply. The use of one diet throughout this study for both genotypes and sexes is likely to have resulted in nutrients being under- or oversupplied for parts of the growth period and may have limited the growth responses observed. Estimated daily lysine intakes for barrows and gilts, based on the diet composition (Table 1) and feed intake data (Table 2), were approximately 24 and 25 g, respectively. Hahn (1994), in a study carried out at this center, proposed lysine requirements for pigs between 50 and 114 kg of approximately 23 g/d for barrows and 25 g/d for gilts. Thus, it seems likely that the diet used in this study met or exceeded the lysine requirements of these animals for at least a significant part of the growth curve. In any event, there is a need for further research to establish the nutrient requirements of various genotypes and sexes at heavier weights. Least squares means for slaughter and carcass characteristics for genotype and sex and regressions on slaughter weight are presented in Table 3. There were no effects of genotype or sex on live weight at slaughter, hot or cold carcass weights, or dressing percentage. However, carcass weights and dressing percentage increased linearly ( P < .001) with slaughter weight, equivalent to 8.1 kg and .32 percentage units increase in hot carcass weight and dressing percentage, respectively, per 10-kg increase in slaughter live weight. This increase in carcass yield is consistent with other studies. For example, Albar et al. (1990) found that dressing percentage increased by .5 percentage units for each 10-kg increase in slaughter weight between 105 and 125 kg. In addition, Gu et al. (1992) showed that the growth coefficient of the carcass relative to live weight was significantly greater than unity, and Tess et al. (1986) found a similar result for the growth coefficient of carcass relative to empty body weight.

Carcass cooler shrink (percentage weight loss from the carcass in the first 24 h postmortem) was higher for gilts than for barrows ( P < .05). In addition, there was a genotype ´ sex interaction ( P < .05) for this trait; BCH barrows lost less weight than gilts (2.09 and 3.17%, respectively). However, there was no difference in cooler shrink between barrows and gilts for HYD (3.00 and 2.99%, respectively). Carcass length was similar across genotypes and sexes and this trait increased linearly ( P < .001) with slaughter weight (Table 3). Genotype BCH had less subcutaneous fat at the last rib and last lumbar ( -.26 and -.38 cm, respectively) but had a smaller loin eye area ( -3.51 cm2) than HYD. Gilts had less 10th rib fat ( -.27 cm) and a larger loin eye area (+2.79 cm2) than barrows. Other studies have shown that gilts are leaner than barrows at slaughter weights above 100 kg (Geri et al., 1990; Boland et al., 1993). The genotypes evaluated in this study were from markedly different genetic backgrounds and may have been expected to differ in lean growth potential. This trait was not measured directly but was estimated using carcass data from equations provided by NPPC (1991). Predicted values for lean gain were .294 and .276 kg/d (pooled SEM .006) for genotypes BCH and HYD, respectively. Friesen et al. (1994) suggested that pigs with lean growth rates of this order could be considered as medium lean gain genotypes.

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slaughter weight between the sexes for 10th rib and last rib backfat and between the genotypes for 10th rib backfat (Table 3). However, these quadratic relationships were of limited practical significance. The regressions of first rib and last lumbar fat depths and loin eye area on SLW were linear with no effect of sex or genotype. These data suggest that the overall rate of backfat deposition did not increase with SLW. Gu et al. (1992) also found a linear effect of live weight on carcass length, backfat depths, and loin eye area in five genotypes of pigs slaughtered between 59 and 127 kg live weight. However, they also reported a curvilinear increase in carcass fat weight relative to SLW, suggesting an increase in fat accretion and carcass fat percentage with animal weight. This highlights the limitations with using linear measurements taken at a limited number of positions on the carcass to predict body composition.

Results for wholesale cut weights and percentages are presented in Table 4. Side weights were similar across all treatment groups. There were genotype differences for the weight of shoulder, boston butt, and neck bones, and the percentages of spare ribs, shoulder, boston butt, and neck bones. The sexes differed for the weight and percentage of ham and carcass trim and the percentage of shoulder and picnic (Table 4). In addition, there were genotype ´ sex interactions for carcass trim weight and percentage of picnic ( P < .001). Gilts from BCH had greater carcass trim than barrows (1.82 vs 1.62 kg, respectively, P < .001), whereas there was no difference between the sexes for HYD (1.76 vs 1.69 kg, respectively). Picnic percentage of HYD barrows was greater than for gilts (11.43 vs 10.57%, respectively, P < .05); however, there was little difference between the sexes for BCH (10.85 vs 10.90%, respectively). Overall, however, the genotype and sex differences and the quadratic regressions on SLW for cut weights and proportions observed in this study were of limited practical significance.

As expected, the weight of all of the wholesale cuts increased with slaughter weight (Table 4). In addition, the percentage of loin, clear plate, and jowl increased and ham, shoulder, picnic, and spare rib percentages decreased with slaughter weight. The percentage of belly, boston butt, and carcass trim did not change with slaughter weight (Table 4). Overall, the changes in wholesale cut percentages with SLW observed in this study were relatively small. A number of other studies have also found little effect of slaughter weight on primal cut distribution (Martin et al., 1981; Albar et al., 1990).

Treatment effects on weight and percentages of trimmed wholesale cuts and trimmed, boneless wholesale cuts are summarized in Tables 5 and 6, respectively. Differences between the genotypes and sexes for trimmed cut weights and percentages were generally small (Table 5). However, genotype BCH had a greater weight and percentage of trimmed, boneless boston butt and a higher percentage of trimmed, boneless ham than genotype HYD (Table 6). There were, however, no effects of genotype or sex on the total yields of trimmed or trimmed, boneless cuts (Tables 5 and 6). A number of studies have shown an advantage in lean cut yields for gilts compared with castrates (Fortin, 1980; Geri et al., 1990).

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The weight of trimmed cuts and trimmed, boneless cuts increased linearly with slaughter weight (Tables 5 and 6). The regression of trimmed ham weight was related curvilinearly to SLW ( P < .05, Table 5). The percentage yields for trimmed cuts decreased with increasing slaughter weight (Tables 5). In addition, the percentage of trimmed, boneless boston also decreased with increasing weight, but there was no change in the percentage of the other trimmed, boneless cuts (Table 6). Overall, total side trimmed and trimmed, boneless cut weights increased by 1.8 and 1.4 kg, respectively, for each 10-kg increase in SLW weight. However, when expressed as a percentage of side weight, these changes were equivalent to decreases of .65 and .32 percentage units, respectively.

Carr et al. (1978) and Neely et al. (1979) also reported a small decrease in percentage trimmed, boneless yields in pigs slaughtered between 45 and 136 kg and 100 and 127 kg, respectively. Other workers have shown little change in percentage lean cut yield with slaughter weight (Fortin, 1980; Martin et al., 1981). The results of this experiment, in which pigs were taken to heavier weights than in other studies, suggests that reductions in percentage yields of the shoulder, loin, and ham with SLW will be relatively small.

Curing yields for the ham and belly are presented in Table 7. There were no differences between the genotypes or sexes for ham or belly yield. In addition, there was no effect of slaughter weight on ham yields (Table 7). However, belly yields increased ( P < .001) with slaughter weight; the increase was equivalent to .83 percentage units increase in yield for each 10-kg increase in slaughter weight. Fredeen (1980) suggested that carcass weight was the major determinant of belly curing yields and that increases in yield with slaughter weight were primarily the result of an increase in the fat content of the belly. The range of meat quality measurements that were taken on the longissimus thoracis et lumborum muscle are presented in Table 8. Subjective color, firmness, and marbling scores were higher ( P < .05) for barrows, indicating that they had darker, firmer muscle with more visible marbling compared to that of gilts. There were no differences between the sexes for any of the other muscle quality traits measured. Most other studies that have compared muscle quality in barrows and gilts have shown little difference between them (Barton-Gade, 1987). There was an effect of genotype on 45-min pH ( P < .01); HYD had higher values. However, there were no genotype effects for any other trait (Table 8). Increasing slaughter weight was associated with reductions in longissimus thoracis color ( P < .05) and firmness ( P < .01) scores and lower 24-h pH ( P < .05), tenderness scores ( P < .05), and moisture content ( P < .001) in the longissimus lumborum. However, longissimus thoracis drip loss and longissimus lumborum fat content increased (P < .05). The increase in drip loss was equivalent to around .3% for a 10-kg increase in slaughter weight, a change that would result in a significant increase in commercial loss at higher weights. These results suggest that heavier pigs may be more prone to the development of the pale, soft, and exudative (PSE) condition. The development of PSE is normally associated with a rapid drop in muscle pH immediately postmortem. This, however, was not the case in this study, as evidenced by 45-min pH values, which did not change with slaughter weight. On the other hand, the development of PSE is temperature-dependent, and elevated muscle temperatures can also produce this condition (Fernandez et al., 1994; McCaw, 1994). One possibility is that muscle cooling rates are slower in heavier carcasses and, thus, muscle temperatures remain high in the early postmortem period, leading to an increase in the incidence of PSE. Further work looking at muscle temperature changes in heavy carcasses immediately after slaughter is warranted. Other studies have generally observed limited effects of slaughter weight on meat quality parameters, including water-holding capacity (Shuler et al., 1970; Martin et al., 1981). Differences between studies in the changes in meat quality with increasing slaughter weight may reflect differences in genotypes and(or) pre- and post-slaughter handling procedures. These factors are known to influence meat quality and they may interact with slaughter weight.

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The changes in longissimus lumborum composition with increasing slaughter weight noted here are consistent with the findings of Shuler et al. (1970), who reported that ether extract fat increased and moisture content decreased with slaughter weights between 45 and 113 kg. However, Martin et al. (1981) found no change in these two components with increasing slaughter weights between 73 and 137 kg. Meat tenderness, as determined by a trained taste panel, decreased with slaughter weight in the present study (Table 8), which is in agreement with the findings of Ellis et al. (unpublished data), who showed an increase in taste panel assessed toughness and in shear force with increases in slaughter weight from 80 to 120 kg. However, in the present study, there was no change ( P = .18) in shear force values with weight and the change in taste panel scores with weight ( -.015 units/kg, P < .02) was relatively small. Overall, the results of this study suggest limited changes are observed in most muscle quality parameters with increased slaughter weight.

Implications

This study evaluated slaughter weights in excess of current commercial levels in two genotypes with medium lean growth potential reared on a single diet and under space allowances in excess of those used in commercial practice. The results suggest limited effects of slaughter weight on growth rate, feed efficiency, and cutting and curing yields. The tendency for meat quality to deteriorate with slaughter weight noted in this study, particularly in terms of drip loss, is of concern and merits further research.

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