J. Anim Sci. 2008. 86:2357-2368. doi:10.2527/jas.2008-0893
© 2008 American Society of Animal Science
Effect of litter size and birth weight on growth, carcass and pork quality, and their relationship to postmortem proteolysis1
J. Bérard*,
,
M. Kreuzer and
G. Bee*,2
* Agroscope Liebefeld-Posieux, Research Station ALP, 1725 Posieux, Switzerland; and
ETH Zurich, Institute of Animal Science, 8092 Zurich, Switzerland
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Abstract
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The objective of the study was to test the hypothesis that birth weight (BtW) influences growth, carcass characteristics, meat quality, and postmortem (pm) proteolysis differently when pigs originate from small (S) or large (L) litters. Swiss Large White barrows (60) used in this study originated from 20 litters with either less than 10 (S) or more than 14 (L) piglets born per litter. Within each of the S and L litters, 3 barrows were selected at birth: the lightest (L-BtW), the heaviest (H-BtW), and the one with a BtW nearest to the average BtW of the litter (M-BtW). The BW and total feed intake of the individually penned pigs were determined weekly. At slaughter, carcass characteristics were assessed. Meat quality traits were determined in the LM and dark portion of semitendinosus muscle. Titin, nebulin, desmin, and integrin proteolysis were evaluated by SDS-PAGE and Western blot technique, and µ - and m-calpain activities were monitored using casein zymography. Litter size affected BtW of L-BtW and M-BtW but not of H-BtW barrows (BtW x litter size interaction; P = 0.07). From weaning to slaughter, L-BtW barrows grew slower (P < 0.01), ingested less feed (P < 0.01), and were less efficient (P < 0.01) than H-BtW and M-BtW barrows. The carcass yields were greater (P < 0.01), and livers and kidneys were lighter (P 
0.01) in L-BtW compared with H-BtW barrows. Regardless of BtW, barrows from S litters had greater percentages of shoulder (P = 0.02) and lower percentages of omental fat (P = 0.06) than barrows from L litters. Compared with the LM of H-BtW barrows, the LM of L-BtW barrows was redder (P < 0.01). The semi-tendinosus muscle of M-BtW barrows was more (P < 0.01) tender than that of L-BtW and H-BtW barrows. The extent of titin and nebulin proteolysis at 24 and 72 h pm was greater (P 0.07) in the LM of H-BtW than in L-BtW barrows. At 72 h pm, integrin of the LM had been less (P = 0.08) degraded in barrows originating from S than from L litters. These results confirm the known effect of BtW on growth performance, whereas its effect on carcass characteristic and meat quality traits could only be partially demonstrated. Although litter size affected average BtW of the L-BtW and M-BtW barrows, its effect on growth performance, carcass characteristics, and meat quality was minor. The almost complete absence of significant BtW x litter size interaction indicates that litter size affects swine growth and carcass and meat quality through its inverse relationship with BtW.
Key Words: birth weight • litter size • meat quality • pig • proteolysis
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INTRODUCTION
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Genetic selection strategies toward greater prolificacy resulted in increased ovulation rate and greater embryonic survival, without a concomitant improvement of the uterine capacity (Foxcroft, 2007
). Père and Etienne (2000) showed that the uterine blood flow increases to a lower extent than the number of fetuses, resulting in a reduced uterine blood flow and, thus, a lower nutrient supply per fetus in larger litters. This might explain why fetal development gets more and more impaired with increasing litter size. This is obvious from the lower average litter birth weight (BtW) and the greater number of low-BtW piglets (Quiniou et al., 2002). Town et al. (2004) found that intrauterine growth retardation at d 30 and 90 of gestation negatively affected placental and fetal development and the number of secondary myofibers. Recently, Rehfeldt and Kuhn (2006) showed that, compared with their high-BtW littermates, low-BtW pigs grow slower and are fatter at slaughter. They assumed that, due to lower myofiber hyperplasia, more rapid hypertrophy occurs in low-BtW pigs, and the plateau of myofiber growth is attained earlier than in high-BtW pigs. Consequently, dietary energy is available earlier for extensive fat deposition (Rehfeldt and Kuhn, 2006). Moreover, low-BtW pigs were found susceptible to impaired meat quality as expressed in greater drip loss and lower tenderness scores than their heavier siblings (Gondret et al., 2005; Rehfeldt and Kuhn, 2006). The item that is not clear yet is at which extent litter size and BtW independently contribute to changes in growth, carcass composition, and meat quality. Thus, the aim of the present study was to test the hypothesis that effects found for BtW on the aforementioned traits were caused by litter size and not by the affected BtW per se. Building on evidence that early postmortem (pm) proteolysis is related to pork quality (Huff-Lonergan and Lonergan, 2005), pm proteolysis in 2 different muscles was also monitored.
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MATERIALS AND METHODS
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The Swiss Federal Committee for Animal Care and Use approved all procedures involving animals.
Animals, Treatments, and Slaughtering Procedure
The 60 Swiss Large White barrows used in this study originated from 10 litters with 10 (S) and 10 litters with 
14 (L) born piglets per litter. Within each of the S and L litters, 3 barrows were selected: the lightest (L-BtW), the heaviest (H-BtW), and the one with a BtW nearest to the average BtW of the litter (M-BtW). Piglets of large litters were cross-fostered from sows with small litters according to usual practice. From the time of weaning at d 35 of age, when reaching 9.1 ± 0.60 kg of BW, piglets were reared in individual pens (2.6 m2/pig) on a concrete floor in environmentally controlled buildings (22°C and 60 to 70% relative humidity). They had free access to standard starter (9 to 27 kg of BW), growing (27 to 63 kg of BW), and finishing diets (63 to 105 kg of BW) until slaughter (Table 1). The barrows were switched from the starter to the growing diet and from the growing to the finishing diet when the barrows reached 25 and 60 kg of BW, respectively, the day of weighing. Feed analysis had been performed as described previously (Bee et al., 2004). Weekly, BW, and total feed intake were determined. Feed was withheld from the barrows 12 h before transport to the research station abattoir. Slaughter and dissection of the left carcass side at 1 d pm were carried out according to the Swiss Pig Performance Testing Station (SUI-SAG, Sempach, Switzerland) meat-cutting standards, as described previously (Bee et al., 2004). In addition, 30 min after exsanguination, the weights of the heart, liver, and kidney were assessed, and 2 muscles were removed from the right side of each carcass, including the LM and the semitendinosus muscle (ST). Weight, length, and girth of the ST and loin eye area at the 10th-rib level were determined. Furthermore, the carcass yield, expressed as the proportion of the HCW over the BW at slaughter, was calculated.
Meat Quality Measurements
In the LM, pH and temperature were monitored at 30 min, 3 h, and 24 h pm, using a pH meter (WTW pH197-S, WTW, Weilheim, Germany) equipped with a WTW Eb4 electrode (WTW). These sets of measurements were obtained at the 10th-rib level inside of the intact left carcass side. In addition, the ST of the right carcass side was excised 30 min pm, and the pH and temperature were measured in the semitendinosus muscle (STD).
One day after slaughter, two 1.5-cm-thick LM chops were removed from between the 10th to 12th rib of the left carcass side, and subcutaneous adipose tissue was removed. Furthermore, the ST was removed also from the left carcass side, and the ultimate pH was determined in the STD. Subsequently, 1 slice (approximately 70 g) was obtained. After a 10-min bloom period, L* (lightness), a* (redness), and b* (yellowness) values for the LM and STD were measured using a Minolta Chroma Meter (CR-300, Minolta, Dietikon, Switzerland) and illuminant D65. In addition, the chroma values were calculated according to the formula chroma = 
a*2 + b*2. Three replicated measurements were performed on each muscle sample. Drip losses from the LM and STD were measured as the proportions of purge generated during storage for 48 h at 2°C (Honikel, 1998). Subsequently, muscle samples were vacuum-packaged and stored at –20°C for later (approximately 3 to 4 mo) Warner-Bratzler shear force determination. The frozen vacuum-packaged samples were kept for 24 h at 2°C and then weighed for thaw loss determination. The LM chops and STD slices were then kept at room temperature for 1 h, weighed, and cooked on a preheated (190 to 195°C) grill plate (Beer Grill AG, Zurich, Switzerland) to an internal temperature of 69°C. After cooking, samples were reweighed to calculate cooking loss percentage. Shear force was determined on the cooked samples cooled to ambient temperature, from 10 cores of the LM chops (5 per chop) with a diameter of 1.27 cm each and from 5 STD strips of a size of 10 x 10 x 30 mm. The shear force was measured perpendicular to the fiber direction using a Stable Micro System TA.XT2 Texture Analyzer (Godalming, Surry, UK) equipped with a 2.5-mm-thick Warner-Bratzler shear blade. Shear force for 10 cores (LM) or 5 strips (STD) was recorded.
Proteolysis and µ - and m-Calpain Activity
Samples were prepared from the LM and STD collected at 30 min, 24 h, and 72 h pm for SDS-PAGE analysis of titin and nebulin and for Western blotting of desmin and integrin. Whole-muscle sample preparation, SDS-PAGE (titin and nebulin), protein transfer, and chemiluminescent detection (desmin and integrin) were carried out as described by Bee et al. (2006b). Gels were loaded with 20 and 80 µ g of total protein per lane for desmin and integrin, respectively. Primary antibodies included monoclonal antidesmin (clone DE-U-10; Sigma, Saint Louis, MO: diluted 1:10,000) and monoclonal antiintegrin beta1D (clone CD29: Chemicon, Temecula, CA: diluted 1:5,000). Secondary antibodies included goat anti-mouse IgG peroxidase conjugate (N° A2554; Sigma: diluted 1:10,000 for integrin and desmin). Desmin and integrin degradation ratio was indicated by a decrease in intensity of the 55- and 110-kDa bands, respectively. Desmin and integrin degradation ratio was calculated as the intensity of each immunoreactive desmin and integrin band over the intensity of the respective protein bands in a reference sample (porcine LM sample sampled 30 min pm) that was loaded on each gel.
Activities of µ - and m-calpain were determined in sarcoplasmic fractions of LM and STD samples collected at 30 min, 24 h and 72 h pm using casein zymography. The sarcoplasmic protein extraction procedure and the casein zymography technique used were described previously by Melody et al. (2004). Gels were loaded with 240 µ g of total protein per lane, and µ - and m-calpain activities were determined as the intensity of clear zones over the intensity of clear zones of a reference sample (LM sample collected at 30 min pm) that was loaded on each gel. Because calpain loses activity after extensive autolysis, loss of calpain activity during pm aging of meat indicates prior activation. Calpain that is prevented from being active in the tissue will not fully autolyze and will thus be able to be activated once the conditions for activity are satisfied such as, for example, ample calcium and reducing conditions as in the casein gel assay (Huff-Lonergan and Lonergan, 2005). To estimate the changes in activity, the differences in the intensities of the clear zones between either 30 min and 24 h or 30 min and 72 h pm were calculated.
Statistical Analysis
Data were analyzed using the MIXED procedure (SAS Inst. Inc., Cary, NC). The model included BtW, litter size, and the respective interaction as fixed effects and litter as random effect. When the BtW effects were statistically significant at P < 0.05, least squares means were separated using the PDIFF option and indexed in the tables. Because BtW x litter size interactions were almost never significant, only the P-value of the interaction but not the interaction means were reported in the tables. Pearson correlation coefficients were calculated to determine the relationship between different meat quality traits, protein degradation pattern, and calpain activity.
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RESULTS
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Growth Performance and Carcass Characteristics
The barrows selected from L litters were lighter (P = 0.02) at birth than those from S litters (Table 2). As expected, average BtW differed (P < 0.01) among the BtW groups. The litter size x BtW interaction (P = 0.07) indicated a lower BtW for L than S litters in L-BtW (1.23 ± 0.25 vs. 1.60 ± 0.24 kg in L vs. S) and M-BtW barrows (1.60 ± 0.28 vs. 1.88 ± 0.21 kg in L vs. S) but not in H-BtW barrows (1.90 ± 0.31 vs. 2.02 ± 0.18 kg in L vs. S). The difference persisted after the 35-d lactation period among the BtW but not between litter size groups, because L-BtW barrows were lighter (P = 0.01) at weaning than H-BtW barrows, with intermediate values for M-BtW barrows. Regardless of litter size, L-BtW barrows grew slower (P < 0.01) in the growing and finishing period, were older (P < 0.01), and, due to the greater (P < 0.01) total feed intake in the growing period, they were less efficiently utilizing feed (lower G:F ratio; P < 0.01) than M-BtW and H-BtW barrows in the finishing period, in the growing and finishing period together, and from weaning to slaughter at 105 kg of BW. Litter size had no (P 0.21) effect on these traits, and the BtW x litter size interaction was also not significant. The carcass yields were greater (P < 0.01) and kidneys were lighter (P < 0.01) in L-BtW and M-BtW compared with H-BtW barrows (Table 3). Liver weights were lower (P = 0.01) in L-BtW compared with H-BtW barrows with intermediate values in M-BtW barrows. Hearts were heavier in L litters of H-BtW than of L-BtW barrows (0.43 vs. 0.39 kg) with intermediate values for M-BtW barrows (0.41 kg), whereas in S litters, heart weights did not differ among BtW groups (litter size x BtW interaction; P = 0.04). Regardless of the BtW, barrows from S litters had greater (P = 0.02) percentages of shoulder and lower (P = 0.06) percentages of omental fat than barrows from L litters. The weight, length, and girth of the ST were neither affected by the BtW nor by the litter size (Table 3). By contrast, LM area was smaller (P = 0.06) in barrows from S than L litters.
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Table 3. Effect of birth weight and litter size on carcass characteristics, organ weights, and morphometric muscle measurements in barrows
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Meat Quality Traits
The pH of the LM, measured at 30 min, 3 h, and 72 h pm, was neither affected by the BtW nor by the litter size (Table 4). By contrast, in the STD, the pH at 24 h pm tended to be greater (P = 0.06) in M-BtW than L-BtW and H-BtW barrows. Regardless of litter size, the LM was darker (P = 0.09), redder (P < 0.01), and, consequently, had a greater (P < 0.01) chroma value than the LM from H-BtW barrows with intermediate values found in the LM of M-BtW barrows. The color of the STD was not (P 0.28) affected by the BtW. However, the STD of barrows originating from L litters was more yellow (P = 0.06) and had a greater (P = 0.08) chroma value than barrows from S litters. Water-holding capacity and shear force values of the LM were not (P 0.12) affected by the BtW and the litter size. However, drip loss percentage tended to be lower (P = 0.07) in the STD of L-BtW than that of M-BtW and H-BtW barrows. Moreover, the STD of M-BtW barrows had lower shear force values (P < 0.01) than the STD of L-BtW and H-BtW barrows. No BtW x litter size interactions (P 0.10) were observed for these traits.
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Table 4. Effect of birth weight and litter size on meat quality traits of the LM and of the dark portion of the semi-tendinosus muscle (STD) in barrows
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Except for the pH at 30 min pm and thaw loss of the STD, pH measured at 30 min and 24 h pm in both muscles were negatively (P < 0.05) correlated with drip and thaw loss percentage (Table 5). The pH at 24 h pm in the LM was positively correlated (P < 0.01) with shear force, whereas in the STD, a negative (P < 0.01) correlation coefficient was found.
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Table 5. Correlation coefficients of pH values with drip and thaw loss percentage and shear force values determined in the LM and in the dark portion of the semitendinosus muscle (STD) in barrows
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Proteolysis
The factors BtW and litter size had only a small effect on pm proteolysis in the LM and none in the STD (Table 6). Independent of the BtW groups, the relative abundance of intact integrin in the LM at 72 h pm was 47% greater (P = 0.08) in barrows originating from S than L litters (Table 6; Figure 1A), whereas no differences in the integrin degradation pattern were observed in the STD. Proteolysis of titin at 24 h pm tended (P = 0.06) to be 44% greater in the LM of H-BtW and M-BtW barrows than in the LM of L-BtW barrows (Table 6, Figure 1B). At 72 h pm, nebulin tended to be degraded at a greater (55%; P = 0.07) extent in the LM of H-BtW compared with M-BtW barrows (Figure 1B). Intermediate values were observed in the LM of L-BtW barrows. The litter size x BtW interaction (P = 0.02) indicated that in S litters, titin degradation at 30 min pm was greater in H-BtW and M-BtW than in L-BtW barrows as indicated by the lower relative abundance of intact titin (H-BtW: 2.04; M-BtW: 2.17 vs. L-BtW: 4.02), whereas in L litters, titin proteolysis did not differ among BtW groups (H-BtW: 3.02; M-BtW: 3.28; L-BtW: 2.50). In both muscles, the extent of desmin degradation at 24 and 72 h was not affected by BtW and litter size (Table 6; Figure 1C).
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Table 6. Effect of birth weight and litter size on degradation of the intact desmin, integrin, titin, and nebulin determined in the LM and in the dark portion of the semitendinosus muscle (STD) in barrows1
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In the LM, intact integrin abundance at 24 h pm was negatively (P < 0.05) correlated with drip (r = – 0.34) and thaw loss percentage (r = – 0.26), and in the STD, integrin abundance at 72 h pm was negatively (r = – 0.31; P < 0.01) correlated with drip loss percentage. Thus, along with greater integrin proteolysis, water-holding capacity of the LM and STD decreased. The negative (r =– 0.28; P = 0.03) correlation between intact integrin abundance at 30 min pm and shear force in the LM, but not in the STD, suggested that loin tenderness increased when integrin was less degraded. Conversely, when intact nebulin (72 h pm) was degraded to a greater extent in the LM, this was correlated with a lower shear force (r = 0.27; P < 0.05). Furthermore, in the STD, the relative abundances of intact nebulin at 24 h pm (r = 0.31) and intact titin at 72 h pm (r = 0.33) were positively (P < 0.05) correlated with drip loss percentage.
The µ - and m-calpain activities at 30 min, 24 h, and 72 h pm were not affected (P > 0.10) by BtW and litter size (Table 6
; Figure 2). Negative (r = –0.31; P < 0.01) correlation coefficients suggested that with greater changes in the intensity of the clear band, indicating greater µ-calpain activity from 30 min to 72 h pm, shear force values decreased in the LM.
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Figure 2. Casein zymography gels depicting µ- and m-calpain activity in sarcoplasmic extracts of the LM and dark portion of semitendinosus muscle (STD) at 30 min, 24 h, and 72 h postmortem of a representative barrow in the study. Each lane was loaded with 240 µ g of protein. The uppermost clear zone indicates µ-calpain activity, and the bottom clear zone indicates m-calpain activity.
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DISCUSSION
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Growth Performance and Carcass Characteristics
The results of a recent study demonstrated that the average BtW may decrease and the percentage of low-BtW pigs may increase with increasing litter size (Quiniou et al., 2002). These findings have been associated with the effect known as intrauterine crowding, which, together with genetic and epigenetic factors, influences angiogenesis, growth, and vascularization of the placenta. Consequently, nutrient and oxygen supply of the fetuses and, ultimately, their growth and development are affected (Town et al., 2004; Wu et al., 2004, 2006). Furthermore, Père and Etienne (2000) reported that when litter size increases, the uterine blood flow increases, but to a lower extent than the number of the fetuses. This results in reduced uterine blood flow per fetus, which then might affect fetal nutrient supply. Consistent with that, in the present study the BtW of the selected barrows from the L-BtW and M-BtW groups were lighter in L compared with S litters. By contrast, the lacking differences in BtW between the H-BtW barrows originating from L or S litters suggests that heavier pigs in L litters are less susceptible to intrauterine crowding than their lighter siblings. However, overall fetal growth retardation among the litter size groups was not severe enough to cause postnatal growth differences among the litter size groups.
Because the ADG in the preweaning period was similar in the 3 BtW groups, the initial differences in BtW still persisted after the suckling period, and regardless of litter size, L-BtW barrows were still lighter than H-BtW barrows at weaning. Our findings are in contrast with results reported by Quiniou et al. (2002) showing that during lactation heavier piglets grow faster than lighter piglets. These authors assumed that heavier piglets have a greater ability to occupy the best-performing teats, to stimulate and to drain them, thereby, to induce a larger milk flow. Apparently, in the present study, the differences in BtW of the selected pigs for each litter size group were not sufficiently pronounced to provoke slower growth of the lighter barrows during the preweaning period. Different from the suckling period, L-BtW barrows grew slower during the growing and finishing period compared with M-BtW and H-BtW barrows, which is in agreement with results obtained in recent other studies (Bee et al., 2006a; Gondret et al., 2006; Rehfeldt and Kuhn, 2006). Because feed efficiency did not differ among BtW groups in the growing period, the slower growth of L-BtW can be explained by the lower feed intake. This is not the case for the finishing period, in which the L-BtW pigs exhibited less efficient growth. A possible explanation for the latter could be related with the IGF system, described by Oksbjerg et al. (2004) as one of the most important systems that regulates prenatal muscle development and postnatal muscle growth. In fact, Gondret et al. (2005) reported lower circulating IGF-I concentration in low- compared with high-BtW pigs at slaughter. However, their results did not allow determination of whether the lower IGF-I concentrations in low- compared with high-BtW pigs reflected an alteration of the postnatal IGF-I axis maturation (Harrell et al., 1999), or an inadequate nutritional status of low-BtW pigs, because the IGF-I axis is highly responsive to the nutritional level (Thissen et al., 1994).
Lower BtW has also been associated with impaired myofiber hyperplasia (Wigmore and Stickland, 1983; Handel and Stickland, 1987). Because myofiber hyperplasia ceases at d 90 of gestation in pigs, at birth and at slaughter, high-BtW pigs display lower myofiber numbers, a lower secondary:primary myofiber ratio, and larger myofibers than low-BtW pigs (Wigmore and Stickland, 1983; Handel and Stickland, 1987; Dwyer and Stickland, 1991). Rehfeldt and Kuhn (2006) hypothesized that in low-BtW pigs, postnatal increase in myofiber size is faster because of the low myofiber number, and the plateau of myofiber growth is therefore attained earlier. Consequently, the dietary energy can no longer be used for muscle accretion but is mainly used to deposit fat, which might explain the fatter carcasses of low- compared with high-BtW pigs observed in various studies (Bee, 2004; Gondret et al., 2005, Rehfeldt and Kuhn, 2006). By contrast, in the present study, slower growth and lower feed efficiency of L-BtW compared with M-BtW and H-BtW barrows had no effect on body fat accretion. A possible cause for this lacking response could be that in the aforementioned studies, the average BtW of the low-BtW pigs was on average lower by 0.4 kg than that of the L-BtW barrows in the present study and that either both sexes or only gilts were taken into account. Furthermore, based on results of Poore and Fowden (2004), the effect of BtW on carcass fatness is less evident in barrows than in gilts. Nevertheless, it is worth mentioning that carcasses of barrows from L litters had numerically greater subcutaneous fat percentages and tended to have more omental fat than carcasses of barrows from S litters. Although the differences were small, these findings might suggest that the carcass compositions of all barrows from L litters are similar to those of low-BtW pigs reported in the aforementioned studies. Thus, one could hypothesize that intrauterine crowding impaired prenatal development, which then affects the carcass composition in all, heavier as well as lighter, barrows of larger litters. No references concerning the effect of the litter size on fat deposition are known to the authors, and further investigations will have to confirm or disprove this hypothesis.
The greater carcass yields found in the L-BtW and M-BtW barrows compared with H-BtW barrows can only partly be explained by the lower weights of inner organs. This indicated that the weight of the intestinal tract could have been markedly greater in L-BtW barrows, which might be responsible for lower feed conversion efficiency and therefore affecting their growth performance.
Meat Quality Traits and Proteolysis
The overall effects of BtW and litter size on meat quality traits were minor. As reported by Gondret et al. (2006) and Rehfeldt et al. (2008), these differences were also small when only BtW was taken into account. Furthermore, the few BtW effects described were not always consistent across studies. Rehfeldt et al. (2008) found a lower pH at 45 min pm and a tendency toward a greater drip loss in the LM of low and high compared with medium-BtW pigs. Gondret et al. (2006) reported the LM of low-BtW gilts to be lighter and less tender than that of gilts born with a high BtW. In the present study, the LM of L-BtW and M-BtW barrows was redder and showed a trend to be darker than the LM of H-BtW barrows, but did not differ in pH. Compared with the aforementioned studies, the average pH values at 30 min were similar, but at 24 h pm, they were markedly greater indicating a less distinct pH drop within the first 24 h pm. This could explain the overall darker LM in this study compared with the LM evaluated in the studies of Gondret et al. (2005) and Rehfeldt et al. (2008). In addition, Bee (2004) showed that, compared with high-BtW pigs, low-BtW pigs have a trend toward fewer glycolytic and, concomitantly, more oxidative-glycolytic myofibers. The latter are known to have a greater myoglobin content (Lefaucheur, 2001), consistent with the redder and darker LM of the L-BtW barrows. Furthermore, the differences in these traits could also be related to differences among pig breeds used in the different experiments and might be responsible for the lack of more marked effects of BtW in the present study carried out with the Swiss Large White breed. Interestingly, the effect of BtW on meat quality traits was less distinct in the LM than the STD. In effect, BtW did not affect water-holding capacity and tenderness, and the 24-h pm pH of the LM was only numerically greater in H-BtW compared with M-BtW and L-BtW barrows. The positive correlation between pH 24 h pm and shear force values is in accordance with results reported by Fernandez and Tornberg (1992) and Gondret et al. (2006). Furthermore, shear force values decreased with increasing µ-calpain activity from 30 min to 24 h, whereas pH at 24 h pm was not related to the µ-calpain activity. The lacking relationship suggested that other effects than pH influenced µ-calpain activity and meat tenderization. Conversely, in the STD, the greatest ultimate pH and the lowest shear force values were observed in M-BtW barrows, which closely corroborates with the negative correlation found between these 2 traits. However, contrary to the LM in the STD, µ-calpain activity from 30 min to 24 h and shear force were not related. In line with recent conclusions drawn by Rehfeldt et al. (2008) from results of their extensive study, the results of the present experiment suggest that pigs with the greatest BtW will not necessarily display a better pork quality. Furthermore, and in contradiction to the apparent negative effect of low BtW on pork quality, drip loss in the STD of L-BtW barrows was markedly lower than in M-BtW and H-BtW pigs.
As described by several authors in bovine (Huff-Lonergan et al., 1995, 1996), ovine (Wheeler and Koohmaraie, 1994), and porcine muscles (Jiang, 1998; Kristensen and Purslow, 2001; Zhang et al., 2006; Bee et al., 2007), increased early pm proteolysis of key myofibrillar and myofibrillar-associated proteins positively affects meat tenderness and water-holding capacity. These proteins, which have been shown to be related to these quality traits, are involved in inter- (e.g., desmin and vinculin) and intramyofibril (e.g., titin, nebulin, and troponin-T) linkages or in linking myofibrils and sarcolemma by costameres (e.g., vinculin and dystrophin). In vivo, these proteins maintain the structural integrity of myofibrils. Their degradation will, therefore, cause weakening of myofibrils and consequently enhance meat tenderization (Jiang, 1998). Moreover, degradation of intermediate filaments and costameric proteins has been suggested to contribute to removing the force causing the water flow from the cell to the extracellular space, water that is responsible for the drip loss (Huff-Lonergan and Lonergan, 2005). Titin and nebulin proteolysis was found to be greatest in the LM of H-BtW compared with M-BtW or L-BtW barrows. This observation, together with the positive correlations found between nebulin abundance and shear force, would imply a more tender LM in the H-BtW barrows. However, no BtW-related differences in LM shear force values were observed, suggesting that the differences in the degree of degradation were not large enough to have an effect on meat texture. In contrast, in the STD, where no differences in the titin and nebulin degradation pattern over time were observed, drip loss differed among BtW. Nevertheless, the positive relationship between titin as well as nebulin abundance and drip loss indicated that part of the differences observed in drip loss were caused by differences in proteolysis. Although it has been shown in numerous experiments (Kristensen and Purslow, 2001; Melody et al., 2004; Zhang et al., 2006) that proteolysis of the intermediate filament protein desmin is closely related to water-holding capacity, in the present study, no differences in the desmin degradation pattern and no relationship between desmin abundance and drip loss in STD and LM were found. By contrast, the proteolysis of integrin, a heterodimeric cell adhesion molecule that links the extracellular matrix to the cytoskeleton (Lawson, 2004), was negatively correlated with drip loss of both muscles and was degraded to a greater extent in the LM of barrows originating from L than S litters. This negative relationship found in the present study is consistent with the hypothesis that a greater integrin proteolysis increases the formation of drip channels between the cell and cell membrane, resulting in greater drip loss in pork (Lawson, 2004; Zhang et al., 2006). Again, the differences between litters were not large enough to have an effect on drip loss. It is noteworthy to mention that the relative abundance of integrin at 72 h pm was 3 times greater in the STD compared with the LM, which could partly help explain the marked differences in drip loss between the 2 muscles.
The calpain system has been described as one of the most important enzyme systems responsible for the pm protein degradation of the muscle during refrigerated storage (Goll et al., 2003; Koohmaraie and Geesink, 2006; Zhang et al., 2006). Proteins that are substrates of calpains include desmin, synemin, talin, and vinculin that form the cytoskeletal framework of the muscle cell (Huff-Lonergan and Lonergan, 2005). The total potential of in vitro µ - and m-calpain activity determined by casein zymography was not affected by BtW and litter size in both muscles. However, in accordance with results of other studies (Geesink et al., 2006; Koohmaraie and Geesink, 2006; Zhang et al., 2006), a greater µ-calpain activity was found in the correlation analysis to contribute to meat tenderization of the LM.
In conclusion, the present results confirm the marked effect of BtW on growth performance. However, although litter size affected average BtW of the L-BtW and M-BtW barrows, its effect on growth performance, carcass characteristics, and meat quality was minor, when considered independently of BtW. This, in conjunction with the almost complete lack of significant interactions between BtW and litter size, disproved our hypothesis that the effects of BtW on these traits depend on litter size studied in this experiment. In accordance with previous observations, the present study confirms the relationships between early pm proteolysis and pork quality traits. Furthermore, the present study revealed that the extent of proteolysis differently influenced meat quality and effects differed among muscles. Activities of the calpains added only few explanations for the overall weak effects of BtW and litter size on pork quality.
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Footnotes
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1 This study is part of the EU COST C05, 0126 grant and was supported by the State Secretariat for Education and Research. 
2 Corresponding author: giuseppe.bee{at}alp.admin.ch
Received for publication January 22, 2008.
Accepted for publication April 25, 2008.
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Abstract
INTRODUCTION
