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Correlated response to selection for litter size in pigs: II. Carcass, meat, and fat quality traits¹

J. Estany^*,,2, D. Villalba, M. Tor, D. Cubiló^*, and J. L. Noguera^*

^* Area de Producció Animal, Centre UdL-IRTA, Universitat de Lleida (UdL)— Institut de Recerca i Tecnologia Agroalimentàries (IRTA) and and Departament de Producció Animal, UdL, Rovira Roure 177, 25198 Lleida, Spain

² Correspondence:
phone: 34 973 70 28 93; fax: 34 973 70 28 74; E-mail:
jestany{at}prodan.udl.es.

	Abstract

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Data on a pig line selected for litter size (H) and a control line (C) were used to estimate the correlated responses to litter size in carcass, meat, and fat quality traits. The differences between the genetic means of animals from line H and line C were used to estimate correlated responses. No differences were found between the two lines in carcass measurements except backfat depth, which was higher (P < 0.05) in line H (0.69 ± 0.28 mm). This led to a decrease (P < 0.05) in predicted carcass lean content (-6.0 ± 2.7 g/kg). Differences in joint weight distribution between lines were primarily due to belly weight, which was higher (P < 0.05) in line H (6.3 ± 1.2 g/kg). There were no important changes in meat quality traits. Chemical composition of semimembranosus muscle (SM) and subcutaneous backfat (SB) differed between lines only for DM in SB, which was higher (P < 0.05) in line H (15.1 ± 7.1 mg/g), and for the fatty acid composition of intramuscular fat. The fatty acid profile in line H showed a lower (P < 0.01) proportion of polyunsaturated fatty acids (-14.7 ± 4.8 mg/g FA), particularly with regard to the content of linoleic acid (-12.5 ± 3.9 mg/g FA). It is concluded that selection for litter size reduced the lean content in the carcass but the proportion of high-priced cuts and meat quality traits were not affected. However, selection may lead to changes in the composition of intramuscular fat lipids towards a lower content of polyunsaturated fatty acids. The observed correlated effects can be interpreted assuming that selected pigs are more mature at the same weight, though the underlying genetic and physiologic processes that cause them are unknown. The results of this experiment indicate that the metabolic pathways taking part in fat metabolism should be considered first.

Key Words: Correlated Responses • Fats • Litter Size • Meat Quality • Pigs • Selection

	Introduction

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Litter size is a major component of the breeding goal in pig dam lines. There is experimental evidence that litter size can be substantially improved by selection, as some recent publications have documented (Sorensen et al., 2000; Noguera et al., 2002). However, in order to optimize selection decisions, the progress achieved in litter size has to be balanced against correlated genetic changes occurring in other traits of interest.

Information in the literature concerning relationships between litter size and performance and carcass traits has been recently reviewed (Clutter and Brascamp, 1998; Rothschild and Bidanel, 1998). It should be noted, however, that available estimates of genetic correlations and correlated responses involving carcass traits are very limited. In recent years, particular emphasis has been put on meat and fat quality traits. It has been shown that selection for carcass lean-to-fat ratio can have detrimental effects on meat and fat quality (for a review see Sellier, 1998), but little is known about the correlated response to selection for litter size on qualitative properties of muscle and fat tissues.

The purpose of the present work was to examine the correlated responses in carcass, meat, and fat quality traits after one cycle of intense selection for litter size (Noguera et al., 2002). In this experiment, average litter size of the selected line was about one-half liveborn piglets per litter higher than a contemporary control line. Estimated correlated response in growth, fat deposition, and feeding behavior traits were presented in a companion paper (Estany et al., 2002).

	Materials and Methods

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Animals

Pigs were from the two Landrace lines (line H and line C) used in the experiment of selection for litter size described in Noguera et al. (2002). Line H was the result of a single cycle of intense selection for number of liveborn piglets, whereas the foundation stock for line C was a random sample of animals from the population before selection was applied. Details of the establishment of lines H and C, as well as direct and on-farm correlated genetic responses, were given by Noguera et al. (2002). Sows in the first generation of line H showed an average genetic superiority of 0.46 piglet born alive compared with contemporary sows from line C. The data used in these analyses were gathered from the same random sample of pigs that were used in Estany et al. (2002) to examine the correlated response in growth, backfat deposition, and feeding behavior traits. A total of 115 (57 intact males and 58 females) and 72 pigs (36 intact males and 36 females) from the progeny of the first generation of lines H and C were used, respectively. Pigs were farrowed at Nova Genética breeding farm in Solsona, Spain. After weaning at 27.2 d of age (SD 5.6), pigs were weighed and delivered in three batches to the early-weaning unit associated to the IRTA swine test station in Monells, Spain. At about 25 kg they were moved to a unit equipped with the IVOG automatic feeding system (De Haer et al., 1992). After a period of adaptation, the pigs were individually put on test at an average weight of 29.3 kg (SD 7.8). Then pigs were allocated in pens with 12 individuals of the same sex in each pen and were given ad libitum access to a pelleted finishing diet. The ingredients and the estimated nutrient content of the diet used throughout the trial period are shown in Table 1.

The pigs were slaughtered at a weight of 102.0 (SD 9.1) kg in a commercial slaughterhouse. Live weight was recorded 24 h prior to slaughter. Hot carcass weight was used to calculate the dressing percentage and joint weight proportions. After slaughter, subcutaneous backfat and loin muscle thickness at 6 cm off the midline between the third and fourth last ribs were obtained using the SFK Fat-o-Meater (FOM) probe (SFK, Herter, Denmark). Carcass conformation was evaluated with a scale from 1 (good) to 4 (poor) using a photographic model of the former EC Pig Grading Grid for type (EC Regulation No. 2760175, Council of European Communities). Carcass length was measured from the anterior edge of the symphysis pubic to the recess of the first rib. After chilling for 24 h at 2°C, both sides of each carcass were divided into standardized commercial joints. Weight of untrimmed hams and shoulders (whole leg) and weight of trimmed loin, belly, and ribs were collected according to customary procedure used in Spanish slaughterhouses. Carcass weight distribution was calculated as weight of individual joint per unit carcass weight. A muscle sample of each carcass was used to determine the halothane genotype at Universitat Autònoma de Barcelona, following the method described by Otsu et al. (1992). Only homozygous stress-resistant and stress-carrier pigs were identified in the animals used in this study.

Meat and Fat Quality Variables

At 45 min and 24 h postmortem, muscle pH (Scharlau Hl-9025, Crison Instruments, Alella, Spain) and electrical conductivity (PQM, Quality Meter, Giralda, Munchen, Germany) were measured in longissimus (LM) and semimembranosus (SM) muscles. Muscle color measurements were assessed at 24 h postmortem on the exposed cut surface of the ham using a Minolta Chromameter CR-200 (Osaka, Japan) (L*: luminance; a*: redness; and b*: yellowness, CIE, 1976). The hue angle (H⁰ = arctan (b*/a*)) and chroma (Chroma =((a*)² + (b*)²)^0.5) parameters were calculated.

Immediately after quartering a slice of about 200 g of SM, together with a cut of approximately 100 g of subcutaneous backfat (SB), were taken from the left ham of the carcass. Both samples were vacuum packaged in different bags and stored in a deep freeze until required. Frozen samples were removed from the freezer 18 h prior to laboratory analyses. Once defrosted, vacuum drip losses were eliminated and both SM, after trimmed of excess of fat, and SB were minced. In both samples DM was determined by drying 16 to 18 h at 100 to 102 °C in air oven. Four representative aliquots from freeze-dried SM and SB were used for chemical analyses. The first three aliquots were used to determine OM in SB and OM, CP, and intramuscular fat (IMF) in SM. Crude protein content was determined by the Kjeldhal method and fat content by ether extraction in a Soxhlet apparatus. These chemical analyses were conducted according to the AOAC (1990).

The fourth aliquot was used for fat analysis. Total lipids were extracted using the method of Hanson and Olley (1963). Methyl esters of fatty acids were obtained by heating the extract with boron trifluoride 20% in methanol. Analysis of fatty acids methyl esters was performed by gas chromatography with a capillary column (SP2330, Supelco, Inc., Bellefonte, PA) and a flame ionization detector. The identification of fatty acid methyl esters was made by mass spectrometry. The quantification was carried out through area normalization with an external standard mixture of fatty acid methyl esters (Sigma Chemical Co., St. Louis, MO). Fatty acid composition was calculated as the percentage of each individual fatty acid relative to total fatty acids. Other details of the fat analysis are in Tor (1997). Several indexes were obtained from individual fatty acid percentages. In particular, the proportion of PUFA, monounsaturated (MUFA), and saturated (SFA) fatty acids were calculated. The average chain length of fatty acid composition was calculated as ACL = (F_ni x n_i)/100, where F_ni is the percentage of fatty acids with a chain length of n_i number of carbon atoms. The double bond index was calculated as DBI = (UF_bi x b_i)/100, where UF_bi is the percentage of unsaturated fatty acids with b_i number of double bonds. The unsaturated index was calculated as UI = DBI/SFA.

Statistical Analyses

The data were fitted to an individual animal model. Fixed effects included line (H, C), sex (boar, gilt), halothane genotype (NN, Nn), and batch (1 to 3), with carcass weight within sex added as a covariate. The model for carcass weight included the same fixed effects but used live weight as a covariate. Random effects were the animal additive genetic value and the residual, which were assumed to be independent. Additive genetic values are multivariate normally distributed N(0, A_a²), in which A is the numerator relationship matrix and _a² is the additive variance. A three-generation pedigree was used to calculate A. Residuals were assumed uncorrelated with variance _e². The variance-covariance matrices were estimated by REML using the EM algorithm, as applied in the REMLF90 programs (Misztal, 1999). Iterations continued until the criterion of convergence was less than 1 x 10^-7. Estimated fixed effects and predicted random effects were obtained regarding the estimated variance-covariance parameters as the true parameters. Henderson’s mixed model equations were solved by direct inversion of the coefficient matrix. The correlated response for a trait was estimated as the genetic difference between pigs from the selected line H and pigs from the control line C. The genetic value of an animal was estimated by the line effect plus the additive genetic value within line. Line means were estimators of the line marginal means that would be expected had the experimental design been balanced. Standard errors of correlated responses were calculated using the (co)variances of line effects and estimated breeding values. These latter values were obtained from the inverse coefficient matrix from the associated mixed-model equations. Tests of hypotheses were carried out using the t-test.

	Results

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Estimates of line effects will be presented adjusted for sex, batch, and halothane genotype. Line means for carcass measurements are shown in Table 2. No differences were found between the two lines in any of the carcass traits recorded except backfat depth measured through the FOM probe. Line H had 0.69 ± 0.28 mm more (P < 0.05) subcutaneous backfat depth on the carcass than line C. This result is in accordance with estimated differences taken on the live animal, where ultrasonic backfat depths at 100 kg taken at different points were about 1 mm higher in line H than in line C (Estany et al., 2002). According to the officially approved equation used in Spain to estimate carcass lean proportion (Gispert and Diestre, 1994) from backfat and loin measured by FOM, selected pigs have a decrease (P < 0.05) in predicted carcass lean content (-6.0 ± 2.7 g/kg). Two carcass joints showed differences in weight between lines (Table 3). Although line C showed a tendency to have higher (P = 0.10) proportion of high-priced cuts in the carcass (ham + shoulder + loin), differences in joint weight distribution between lines were due to shoulder weight and belly weight. Pigs from line H had less (P < 0.05) proportion of shoulder (-1.9 ± 0.7 g/kg) and higher (P < 0.01) proportion of belly (6.3 ± 1.2 g/kg) than pigs from line C.

	Discussion

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At a fixed carcass weight, pigs from line H had a lower carcass lean content than pigs from line C. This result agrees with the prediction made from ultrasonic measurements taken on the live animal (Estany et al., 2002; Noguera et al., 2002) and is comparable with the one obtained by Herment et al. (1994) in French Large White and Landrace pigs, where a significant decrease in meat percentage was also found (-0.2 to -0.6%). Selected pigs had a higher proportion of belly and a lower proportion of shoulder. There is also evidence that belly weight increased after selection for weight at a fixed age (Kuhlers and Jungst, 1992, 1993) and after selection for lean growth and feed conversion efficiency (Godfrey et al., 1991). Taken as a whole these results suggest that the joint weights located in the dorso-ventral axis are sensitive to selection. Nonetheless, this is not necessarily consistent with increased fatness because it has been reported that heavier bellies seemed to be primarily due to increased muscle (Godfrey et al., 1991; García-Macías et al., 1996).

Meat quality traits and chemical composition were not substantially affected by selection for litter size. This can be illustrated with the lack of response in ultimate pH, a trait that is genetically associated with all components of meat quality (Sellier, 1998). Herment et al. (1994) also did not observe any correlated response for the meat quality index used in France, which was calculated from ultimate pH, color, and water-holding capacity. However, it appeared that pigs from line H showed higher values for a* and Chroma in SM (Table 4), as well as a higher proportion of DM in SB (Table 5). Both results would confirm the general trend pointed out in Estany et al. (2002) that pigs selected for litter size might be more mature at the same age. First, the higher values of a* and Chroma can be interpreted in light of the results that showed that pig meat becomes more red with maturity, most likely as a consequence of increased concentration of pigments and myoglobin (García-Macías et al., 1996). Secondly, a higher DM in SB will result in fat depots with a lower water:lipid ratio, which has been associated with physiologically more mature pigs (Wood et al., 1983).

Selection for litter size increased subcutaneous fat without changing its fatty acid profile. Instead it modified the composition of lipids in the IMF without changing its content. It is not a simple matter to disentangle these correlated effects because it would require a more comprehensive knowledge of the energy metabolism in both lines. These results, however, indicate that selection for litter size might prompt a shift in the genetic and physiological factors that regulate the mechanisms of extra and intramuscular fat storage, which in turn operate largely independently of each other. Thus, backfat thickness only showed a small correlation with IMF, in agreement with the review of Sellier (1998), who concluded that part of the genetic variation of IMF is independent of overall lipid content of the carcass.

The fatty acids that make up IMF were less polyunsaturated than those found in SB, following the general trend pointed out by Lawrence and Fowler (1997). It is known that PUFA content in the adipose tissue decreases with age (Nürnberg et al., 1998) and that mature adipose tissue shows a higher proportion of triglycerides, more saturated, than phospholipids (Lawrence and Fowler, 1997). Accordingly, the lower proportion of PUFA in line H may reflect a greater maturity of IMF in this line. This hypothesis is consistent with the results obtained for parameters a* and Chroma in SM, DM in SB, and those reported in Estany et al. (2002). In contrast, the subcutaneous backfat composition did not differ between lines. However, in pigs there are no important changes in the fatty acid composition of backfat after 180 d of age (Nürnberg et al., 1998). It can be argued then that in both lines subcutaneous fat would have reached such a degree of development that no more changes in composition are expected. This would not be the case for IMF yet because it tends to develop later than SB.

The lower proportion of PUFA in the IMF of line H was mostly due to linoleic acid concentration. This difference was maintained even after adjusting linoleic acid concentration for backfat thickness and IMF content. It appears that selection for litter size exerts an effect upon the regulation of IMF acid composition. Thus, the endogenous synthesis of IMF will be more strongly developed in line H than in line C. Instead, line C will direct lipogenesis to assimilate dietary linoleic acid, provided that linoleic acid is preferentially deposited compared with other fatty acids (Lawrence and Fowler, 1997). This may be seen as the basis for the positive relationship of IMF content and percentage of linoleic acid in line C and why IMF and linoleic acid are unrelated in line H. The use of glucose by the muscle could be one of the physiological causes behind this phenomenon, as intramuscular fatty acids are synthesized from glucose rather than other lipogenic precursors (Hocquette et al., 1998). In fact, selection for lean growth has been shown to increase the use of glucose by the muscle, to the detriment of its use for the synthesis of IMF (Hocquette et al., 1998).

The results of this paper, together with those presented in Estany et al. (2002), have shown that litter size and production and quality traits are probably not independent. Even though the short-term effects were not very dramatic, the observed tendencies give some insight into what would happen following selection for litter size. As correlated effects mainly occurred in the timing and pattern of fat deposition, the major consequences of selection for litter size might be expected in the physiological pathways associated with fat metabolism. Broadly, correlated effects can be interpreted assuming that selected pigs are more mature at the same age, but little is known regarding the actual genetic and physiologic processes that originate the change.

	Implications

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The results of this study confirm that selection for litter size can reduce the lean content in the carcass, though little or no short-term effects are expected concerning the joint weight distribution and meat and fat quality traits. However, evidence for selected pigs having less polyunsaturated fatty acids in intramuscular fat was found. This highlights the fact that the major effects of selection for litter size on carcass and meat traits would be associated with fat metabolism. Overall, the pattern of correlated changes can be interpreted as selected pigs being more mature at the same age, though the underlying genetic and physiologic processes that originate them are unknown.

	Footnotes

 
¹ Financial support was provided by CICYT (grant No. AGF94-1016), Paeria de Lleida (grant No. X0106), and INIA (grant No. 9084), Spain. The authors would like to thank A. Diestre and M. Gispert for technical advice, M. Arqué and A. Ñaco for laboratory analyses, and the staff of Nova Genètica S.A., Copaga Coop., and Leridana de Congelación S.A. for cooperating in the protocol of the experiment. SEDAI Anàlisi Cromatogràfic, UdL, is greatly thanked for its help in the laboratory analyses.

Received for publication August 21, 2001. Accepted for publication June 6, 2002.

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