The Effects of Gender and Slaughter Weight on the Growth Performance, Carcass Traits, and Meat Quality Characteristics of Heavy Pigs

Introduction

Traditionally, pig production in Spain has been based on crossing Landrace × Large White dams with lean sire lines, such as Pietrain and then slaughtering gilts and intact males at approximately 100 kg. Yet, over the past 15 yr, the trend has been to increase slaughter weight (SW) to reduce costs and improve marbling (Barton-Gade, 1987; Wood, 1993) and sensory characteristics of pork (Candek-Potokar et al., 1997; Beattie et al., 1999). However, increasing SW has proven to have economic disadvantages resulting from reductions in pig performance (especially feed efficiency), extension of the feeding period, and the production of overly fat pigs (Richmond and Berg, 1971).

Spain is the world leader in the production of drycured hams, with 33 million pieces cured in 2001 (MAPA, 2002). A recent regulation (DOCE, 1999) has established a minimum fresh ham weight of 9.5 kg for hams destined for the production of “Serrano” hams; therefore, an increase in SW to approximately 120 kg is needed to produce hams of proper weight to meet these regulations. At this weight, the productivity of current crosses needs to be reevaluated, and males will have to be castrated to avoid boar taint. Therefore, the objective of the present trial was to study the influence of gender and SW on performance and carcass characteristics, and meat quality of crossbred pigs intended for the production of dry-cured hams.

Materials and Methods

Growth Performance

Crossbred barrows and gilts (n = 192), weighing 75 ± 1.3 kg at 110 ± 3 d, were used in the present study. All pigs were progeny of the mating of Pietrain × Large White sires (Gene+, Erin, France) to Landrace × Large White dams (Copese, Segovia, Spain). Pigs were allotted to pens (eight pigs/pen) on the basis of age, and pens of pigs were assigned randomly to one of six treatments arranged in a 2 × 3 factorial design with two genders (barrows or gilts) and three slaughter weights (116, 124, or 133 kg). The study consisted of four replications of each treatment.

Pigs were placed in a naturally ventilated, curtainsided finishing barn on completely slatted, concrete floors, with a space allowance of 1.0 m2/pig. All pigs had ad libitum access to water and a pelleted, barleywheat- corn-soybean meal diet throughout the finishing period. The diet was formulated to meet or exceed NRC (1998) requirements of finishing swine for lysine, CP, energy, and other nutrients (Table 1). Live weight and feed consumption were recorded at the beginning and end of the study, as well as at 14-d intervals, to calculate ADG, ADFI (as-fed basis), and gain-to-feed ratio (G:F). Pigs were slaughtered when the planned slaughter weight (SW) was achieved. Therefore, all the pigs within each treatment were slaughtered the same day. The animal care and experimental procedures used in this study conformed to regulations and guidelines of BOE (1988).

The Effects of Gender and Slaughter Weight on the Growth Performance, Carcass Traits, and Meat Quality Characteristics of Heavy Pigs - Image 1

Carcass Quality

When pigs had achieved the appropriate SW (116, 124, or 133 kg), they were transported approximately 85 km to a commercial pork packing plant (Alfrese, Segovia, Spain), where they were afforded a 15-h rest period with full access to water, but not feed. Pigs were electrically stunned (225 to 380 V, 0.5 A, for 5 to 6 s), exsanguinated, eviscerated according to standard commercial procedures, and split down the center of the vertebral column. Hot carcass weights were recorded and used to calculate dressing percent. At 45 min postmortem, carcass length (from the posterior edge of the symphysis pubis to the anterior edge of the first rib), ham length (from the anterior edge of the symphysis pubis to the hock joint), ham circumference (at its widest side), fat thickness over the gluteus medius (at the thinnest point), and backfat depth (between the 3rd and 4th last ribs on the midline of the carcass) were measured on left sides of each carcass using a ruler with a precision of 0.5 mm. Furthermore, an incision was made into the semimembranosus (SM), and initial muscle pH was measured with a Crison pH meter (CRISON 507; Crison Instruments, S.A., Barcelona, Spain) equipped with a glass electrode (Model No. 52-11, Crison Instruments, S.A., Barcelona, Spain).

Carcasses were fabricated by dissecting the legs, loins, shoulders, bellies, tenderloins, neck fat, and backfat according to the simplified EC-reference method (Branscheid et al., 1990). Four carcasses from each pen were selected at random, and chops were collected from these carcasses. A 250 ± 15 g longissimus muscle (LM) section was excised at the level of the last rib from left sides after carcass data collection, weighed, stored in individual plastic bags, and frozen at −20°C for subsequent pork quality analyses. The remaining primal cuts from carcasses were chilled for 24 h at 4°C suspended in the air, and, at 24 h postmortem, pH of the SM was measured according to the previously described procedure. Also, weights of untrimmed ham and shoulder were recorded before and after chilling, and the difference in weights was divided by the “prechilling” weight to estimate drip loss percentage. At 48 h postmortem, hams and shoulders from each carcass were trimmed of external fat and weighed, and differences in primal cut weights were used to calculate trimmed ham and shoulder yields.

Meat Quality

The LM samples were frozen for 15 d, thawed for 24 h at 4°C, blotted dry, and weighed. The difference between the fresh and thawed sample weights was divided by the initial (fresh) weight as a measure of thaw loss percentage. Additionally, objective measures of pork color (L*, a*, and b*) were collected on thawed LM samples with a CM 2002 colorimeter (Minolta Camera, Osaka, Japan) using illuminant D65, 10° standard observer, and CIE (1976) color scale. Colorimeter was previously calibrated with a pure white color tile, and samples of LM were not allowed to bloom. An average of three random readings on the LM were used to measure lightness (higher L* value is indicative of a lighter color), redness (higher a* value is indicative of a redder color), and yellowness (higher b* value is indicative of a more yellow color). Additionally, chroma (c*) values (a measure of the total color or vividness/intensity of the color) were calculated as c* = (a*2 + b*2)¹⁄2. Myoglobin content of the LM was measured in two minced slices from each chop with a spectrophotometer (Beckman DU-640; Beckman Instrument, Fullerton, CA) according to the method of Hornsey (1956) as modified by Boccard et al. (1981). Briefly, a 5-g sample of minced LM was placed into an extraction vessel with 20 mL of acetone and 1 mL of water, and stirred for 30 s with a glass rod. Afterward, 0.5 mL of 12MHCl was added, the suspension was kept in sealed vessel overnight in the dark, filtered, and absorbance of the filtrate was measured at 510 nm. Each slice was evaluated two times and averaged. The concentration of myoglobin (milligrams per gram of fresh muscle weight) was obtained by multiplying the absorbance reading by the factor 8.816 obtained by calibration (Boccard et al., 1981).

Ether extract, CP, and moisture content of the 100 ± 5 g loin samples were determined by using a nearinfrared transmittance meat analyzer (Infratec 1265; Tecator, Ho¨gana¨ s, Sweden). The monochromator contained a 50-W tungsten lamp and a diffraction grating that created monochromatic light. The measured spectra were separated into the range from 800 to 1,100 nm. Chops were trimmed of all visible external fat, minced, and then distributed in the cup ring equipped with a plastic bottom plate (100 mm in diameter and 15 mm deep). Five subsamples were measured for each sample.

Cooking losses were determined by the method described by Honikel (1998). A fresh 15-mm-thick LM slice from each chop was weighed (80 ± 5 g), placed in a plastic bag, and cooked to an internal temperature of 70°C in a 75°C water bath (Precisterm; J.P. Selecta S.A., Barcelona, Spain). Internal temperature was monitored during cooking with a hand-held temperature probe (Hanna Instruments, Woonsocket, RI). Cooked samples were allowed to cool for 30 min, blotted dry, and weighed. The difference between pre- and postcooking weights was divided by the precooked weight to calculate cooking loss percentage. Then, samples were cut parallel to the long axis of the muscle fibers into 10- × 10-mm thick and 30-mm long slices. Slices (eight per chop) were sheared once, perpendicular to the fiber orientation, with a Warner-Bratzler shear force device attached to a texture meter (TA-XT2; Stable Micro Systems, Surrey GU 1YL, U.K.) equipped with a 5-kg load cell and a crosshead speed of 6 mm/s.

Statistical Analyses

The experimental unit for growth performance and carcass data was a pen of eight pigs, whereas the experimental unit for pork quality measures comprised four pigs chosen at random from the eight pigs penned together. Data were analyzed as a completely randomized design with treatments arranged factorially using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC). The model included gender and SW, as well as the gender × SW interaction, as main effects. The regression procedure of SAS was used to calculate the linear responses to SW. Actual means were computed and separated by a t-test, and P < 0.05 was classified as a significant difference, whereas a P-value between 0.05 and 0.10 was classified as a tendency. No (P > 0.10) gender × SW interactions were detected for any measured trait; thus, only differences among the main effects are reported in the text.

Results

Growth Performance

The effects of gender and SW on growth performance are presented in Table 2. At the beginning of the trial, barrows were heavier (P < 0.01) than gilts because pigs were allotted to treatments on the basis of age. From 75 to 116 kg, barrows had greater ADFI (P < 0.001) and ADG (P < 0.01) than gilts; however, G:F was not (P > 0.10) affected by gender. From 116 to 124 kg, barrows had higher (P < 0.01) ADFI than gilts, and there was a tendency for barrows to have higher (P < 0.10) ADFI than gilts from 124 to 133 kg. At the end of the trial, barrows had greater (P < 0.001) ADFI and ADG but lower (P < 0.05) G:F than gilts. There was a linear relationship between SW and ADG (R2 = 0.59; P < 0.01), indicating that ADG was decreased by 38 g/d for every 10-kg increase in SW above 116 kg. Even though ADFI was not (P > 0.10) affected by SW, G:F decreased linearly (R2 = 0.61; P < 0.001) with increasing SW at a rate of 0.01 kg for every 10 kg above 116 kg.

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Carcass Traits

The influence of gender and SW on carcass characteristics is exhibited in Table 3. Even though carcasses of barrows had more (P < 0.001) backfat and fat over the gluteus medius than carcasses from gilts, dressing percent was lower (P < 0.01) in barrows than in gilts. Carcass and ham lengths did not differ (P > 0.10) between barrows and gilts, but ham circumference was greater (P < 0.05) for barrows than for gilts. Carcasses of gilts had higher ham (P < 0.001) and shoulder (P < 0.10) yields than barrows. Both initial (P < 0.10) and ultimate (P < 0.01) pH values were lower in the SM of gilts than barrows, but carcass shrinkage was not (P > 0.10) affected by gender.

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Dressing percent, backfat depth, and fat over the gluteus medius increased (P < 0.001) linearly as SW increased from 116 to 133 kg (R2 = 0.60, 0.79, and 0.76, respectively). According to calculated prediction equations, dressing percent, backfat depth, and fat over the luteus medius increased 0.6 percentage unit, 2.4 mm, and 2.3 mm, respectively, for every 10 kg of extra BW at slaughter. Additionally, carcass length and ham circumference increased (P < 0.001) linearly (R2 = 0.82 and 0.80, respectively) with increasing SW, indicating that for every 10-kg increase in SW carcass length and ham circumference increased 2.0 cm. Ham length was linearly related to SW (R2 = 0.85; P < 0.001) and increased at a rate of 1.1 cm for each 10-kg increase in SW above 116 kg. Weight of trimmed ham and shoulder increased (P < 0.001) with weight, but trimmed cut yields were not (P > 0.10) affected by SW. Even though initial (45-min) pH of the SM from pigs slaughtered at 133 kg was higher (P < 0.05) than pigs slaughtered at 116 or 124 kg, ultimate (24-h) pH of the SM was not (P > 0.10) affected by SW.

Meat Traits

The effects of gender and SW on meat traits are displayed in Table 4. The LM from barrows and gilts had similar (P > 0.10) moisture and intramuscular lipid contents; however, the protein content of the LM from barrows was less (P < 0.05) than the LM from gilts. Gender had no (P > 0.10) effect on LM color or thaw loss percentage, but pork from barrows had lower (P < 0.05) cooking loss percentages and tended to have lower (P < 0.10) Warner-Bratzler shear force values than the LM from gilts. Lightness (L*) decreased (P < 0.001), and redness (a*) and myoglobin increased (P < 0.01 and P < 0.001, respectively), linearly (R2 = 0.21, 0.28, and 0.43, respectively) as SW increased from 116 to 133 kg. Thaw loss from the LM decreased at a rate of 1.2 percentage units for every 10-kg increase in SW from 116 to 133 kg (R2 = 0.45); however, SW did not (P > 0.10) affect cooking loss percentage or shear force values.

The Effects of Gender and Slaughter Weight on the Growth Performance, Carcass Traits, and Meat Quality Characteristics of Heavy Pigs - Image 6

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Discussion

Growth Performance

Barrows had greater ADFI and ADG, and lower G:F than females, which agrees with previous reports (Kanis et al., 1990; Augspurger et al., 2002; Latorre et al., 2003a). This information is consistent with the higher backfat thickness and fat at gluteus medius observed for barrows by Nold et al. (1997) and Hamilton et al. (2000).

Consistent with the results of Ellis et al. (1996) and Candek-Potokar et al. (1997), ADG decreased with increasing SW. Moreover, growth rate was decreased by 38 g/d for every 10-kg increase above 116 kg in the present study, which is slightly higher than the 35 and 24 g/d reduction in ADG of barrows and gilts, respectively, with increasing BW from 90 to 115 kg observed by Castaing and Leuillet (1976). Within the weight range of 110 to 140 kg, Leach et al. (1996) reported that ADG decreased at a rate of only 19 g/d for every 10-kg increase in BW, and Albar et al. (1990) observed that ADG decreased at a rate of 23 g/d from 125 to 135 kg. In contrast, Johnston et al. (1993) and Cisneros et al. (1996) failed to denote an effect of SW on growth rates in pigs from 100 to 120 or 160 kg, respectively. Conflicting results may be explained by the genetic propensity for growth rate of pigs used in the experiments. In the present study, Pietrain-cross sires were used in the production of pigs for the experiment, and Pietrain is an early-maturing swine breed whose ADG declines rapidly at BW in excess of 100 kg. Yet Johnston et al. (1993) and Cisneros et al. (1996) worked with Duroc, which is known to reach mature weights at later ages.

Contrary to the results of some researchers (Kanis et al., 1990; Lebret et al., 2001), SW did not influence ADFI in the present study. Both Albar et al. (1990) and Cisneros et al. (1996) reported that ADFI increased 100 g/d for every 10-kg increase in BW, and Weatherup et al. (1998) observed an increase in ADFI of 200 g/d for barrows and 120 g/d for gilts as BW increased from 92 to 125 kg. Pigs in the present study were fed in a commercial facility during the months of March through September, when temperatures often reached 33°C. This warm rearing environment may be partially responsible for the observed reduction in ADFI, as well as decreasing ADG, during the last period of the trial. Also, rearing environment (stocking density, health of the pigs, and humidity) might explain, in part, some of the discrepancies.

Results from the regression analysis indicate that G:F ratio was impaired by 0.01 kg for every 10-kg increase in SW, which is consistent with results of Weatherup et al. (1998) with pigs of 92- to 125-kg BW. Castaing and Leuillet (1976) and Candek-Potokar et al. (1997) observed an increase in G:F of around 0.018 kg for every 10-kg increase in BW from 32 to 130 kg BW.

Carcass Traits

Barrows produced heavier carcasses than gilts, but dressing percent was greater in gilts than barrows, which is in agreement with Langlois and Minvielle (1989) and Ellis et al. (1996). However, neither Cisneros et al. (1996) nor Weatherup et al. (1998) noted an effect of SW on carcass weight or dressing percent. In agreement with the results of Ellis et al. (1996), Leach et al. (1996), and Hamilton et al. (2000), carcasses from barrows were fatter than carcasses from gilts. Carcass length was similar between genders, which is in line with Cisneros et al. (1996) and Hamilton et al. (2000). The proportion of lean cuts was lower for barrows than for females, which agrees with Unruh et al. (1996) and Latorre et al. (2003b).

In the present study, initial and ultimate pH values were higher in the SM of barrows than gilts, which contradicts previously published results indicating that gender had no impact on muscle pH (Cisneros et al., 1996; Leach et al., 1996). Conditions imposed during transportation and lairage at the slaughterhouse are the main factors influencing pH of the carcass, and stress might affect barrows and females differently (Pineiro, 2001). No differences were detected for cooler shrinkage between genders, which disagrees with Cisneros et al. (1996) and Lebret et al. (2001), who reported lower drip loss in barrows than in gilts, which was attributed to differences in carcass fatness. Yet, in the present study, cooler shrinkage was not affected by gender even though the backfat depth of barrows was 4.9 mm thicker than gilts.

Dressing percent increased by 0.6 percentage unit per each 10 kg of extra BW, a value that is within the range reported in the literature (Castaing and Leuillet, 1976; Albar et al., 1990; Cisneros et al., 1996) for pigs of similar age. This observation was expected because the rate of growth with age is greater for the carcass than for the whole body (Gu et al., 1992).

Backfat depth and fat thickness at gluteus medius muscle increased by 2.4 and 2.3 mm, respectively, for each 10-kg increase in SW above 116 kg. Also, Castaing and Leuillet (1976) observed a linear increase in backfat thickness of 1.6 mm for barrows and 2.0 mm for gilts per 10-kg increase in BW, whereas Cisneros et al. (1996) reported an increase of 1.8 mm/10 kg BW, regardless of gender. Also, carcass length increased at a rate of 2.0 cm for every 10 kg of extra BW, which is within the range (1.9 to 2.1 cm) reported by Martin et al. (1980) and Cisneros et al. (1996).

In the present study, ham weight increased 1.6 kg, and shoulder weight by 0.9 kg, for each 10-kg increase in SW. This is consistent with the work of Martin et al. (1980), who found increases of 0.8 and 0.9 kg for hams and shoulders, respectively. Trimmed shoulder yield was not affected by SW in the present study, but trimmed ham yield was decreased 0.3 percentage unit for each 10-kg increase in SW. Cisneros et al. (1996) also observed a linear decrease of 0.19 percentage unit in ham yield in 100- to 160-kg pigs.

Most researchers have not found any effect of SW on pH (Garci´a-Maci´as et al., 1996; Leach et al., 1996; Monin et al., 1999), which agrees with our data for 24- h pH, but not for 45-min pH, which increased with weight. However, initial pH was in excess of 5.9 in all pigs, which reduces the possibilities of the formation of PSE pork.

Meat Traits

In accordance with Cisneros et al. (1996) and Hamilton et al. (2000), the moisture and intramuscular lipid content of theLMwas not affected by gender. However, Barton-Gade (1987) and Leach et al. (1996) reported more intramuscular lipid in muscles of barrows than gilts. The LM of gilts had more protein than the LM from barrows, which is consistent with the findings of Weatherup et al. (1998) and Beattie et al. (1999). In the present study, gender did not affect LM color. In previous studies, neither objective color (Unruh et al., 1996; Weatherup et al., 1998; Beattie et al., 1999), subjective color (Ellis et al., 1996; Nold et al., 1997), nor myoglobin content (Barton-Gade, 1987; Lindahl et al., 2001) were impacted by gender.

Cooking losses and shear force values were lower for the LM of barrows than gilts in the present study. However, other researchers have shown that gender had no effect on cooking loss percentages or shear force values (Cisneros et al., 1996; Ellis et al., 1996; Hamilton et al., 2000). Huff-Lonergan et al. (2002) found that marbling was negatively correlated with cooking losses, and positively related to cooked pork tenderness, which is consistent with results from the present study.

Chemical composition of LM was not affected by SW, which agrees with Garcia-Macias et al. (1996) and Beattie et al. (1999). However, Candek-Potokar et al. (1998b) and Weatherup et al. (1998) reported an increase in intramuscular fat, and a decrease in protein content with increasing SW. The influence of SW on meat color is subject of debate. In agreement with Garcia-Macias et al. (1996), increasing SW to 133 kg, in the present study, produced a darker, redder LM with more myoglobin. Also, Johnston et al. (1993), Cisneros et al. (1996), and Nold et al. (1997) detected that meat color increased with age as measured by subjective methods. However, Unruh et al. (1996) and Weatherup et al. (1998) observed that pork color, as measured by objective parameters, was independent of SW. Thawing losses decreased but cooking losses were not modified by SW, in accordance with Candek-Potokar et al. (1998a) and Latorre et al. (2003a). Moreover, SW had no appreciable effects on shear force values of cooked pork (Weatherup et al., 1998; Beattie et al., 1999).

Implications

An increase in slaughter weight of pigs from Piétrain × Large White sires mated to Landrace × Large White dams from 116 to 133 kg impaired live pig performance and carcass characteristics without any major benefit on pork quality. Therefore, barrows and gilts from this cross can be used to produce fresh pork when slaughtered at 116 kg BW, yet they might not be adequate for the production of hams for the dry-cured industry, where heavier pigs are required.

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