The effects of two methods of increasing age at slaughter on carcass and muscle traits and meat sensory quality in pigs

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1 Animal Science 2001, 72: /01/ $ British Society of Animal Science The effects of two methods of increasing age at slaughter on carcass and muscle traits and meat sensory quality in pigs B. Lebret 1, H. Juin 2, J. Noblet 1 and M. Bonneau 1 1 INRA, Unité Mixte de Recherches sur le Veau et le Porc, Saint-Gilles, France 2 INRA, Domaine du Magneraud, BP52, Surgères, France Abstract The aim of this study was to investigate the effects of a 30-day increase in age of pigs slaughtered at 110 kg body weight (BW) on carcass and m. longissimus dorsi (LD) and m. biceps femoris (BF) traits, and meat sensory quality. A total of 60 pigs from two genotypes: synthetic line (Large White Landrace) (SL) and Duroc (Large White Landrace) (D) were used, each genotype containing five groups of six littermates (three castrated males and three females). At the average BW of 30 kg, littermates of the same sex were allocated to three groups. Pigs of group AL were offered ad libitum a standard growing-finishing diet (13 6 MJ/kg digestible energy, 9 5 g/kg of lysine) from 30 up to 110 kg BW. The R1 pigs received the same diet at 0 75 of the ad libitum intake of their AL littermates. The R2 pigs were submitted to both energy and protein restrictions in order to get the same growth rate as the R1 pigs and the same body composition as the AL pigs. Results were similar in both genotypes. In agreement with the protocol, age at slaughter was increased by 30 days in R1 and R2, and AL and R2 pigs had comparable carcass composition. Compared with AL, average daily gain was decreased in R1 and R2 pigs, and food efficiency was decreased in R2, but remained unaffected in R1 pigs. Intramuscular fat (IMF) concentration was decreased in the R1 pigs, especially in BF (15 5 v mg/g), while it was increased in the LD of the R2 pigs, particularly in the D animals (24 2 v mg/g), compared with AL pigs. Meat quality parameters (rate and extent of ph fall, reflectance and drip loss) were similar in the three feeding regimens. The taste panel did not find any significant difference between feeding regimens for tenderness, juiciness, flavour, flour sensation after mastication and mouth coating of the meat, despite the differences reported in IMF concentration. This suggests that, for the genotypes used in this experiment, an increase of 30 days in the age at slaughter greatly influences the carcass and/or the muscle chemical composition, depending on the feeding strategy applied to reduce the growth rate but does not strongly modify the meat eating quality. Keywords: age at slaughter, carcass composition, growth rate, meat quality, pigs. Introduction It is widely accepted that increasing the age of pigs at slaughter would result in an improvement in meat sensory quality. This can be achieved by increasing weight at slaughter. However, the market for heavier carcasses is limited to specific products such as drycured ham. For a given body weight (BW) at slaughter, increasing age implies a reduction in the growth rate of pigs. This can be achieved during the growing-finishing period through food restriction, or through lower protein or lysine/energy ratio in the diet. The former has been shown to reduce intramuscular fat (IMF) concentration (Wood et al., 1996; Candek-Potokar et al., 1998) and could thus 87 alter meat eating quality (Cannon et al., 1995; Ellis et al., 1996; Blanchard et al., 1999), whereas the latter increases IMF level and may improve sensory quality but gives fatter carcasses (Castell et al., 1994; Goerl et al., 1995; Blanchard et al., 1999). In that latter case, lean tissue development is limited by low protein intake, particularly during the early growing period when the protein requirement compared with energy is higher than during the finishing period. The aim of this study was to investigate the effects of a 30-day increase in age of pigs from two different genotypes and slaughtered at 110 kg BW, on carcass and muscle traits and meat sensory quality.

2 88 Lebret, Juin, Noblet and Bonneau Compared with pigs offered, ad libitum, a normal growing-finishing diet, increasing age was obtained according to two strategies: (1) restricted energy level and (2) restricted energy and protein levels. The latter strategy was expected to give the same carcass composition as in the pigs given food ad libitum. Material and methods Animals A total of 60 pigs from two genotypes were involved in the experiment: 30 synthetic line (Large White Landrace) crossbreds (SL), and 30 Duroc (Large White Landrace) crossbreds (D). For each genotype, five groups of six littermates (three castrated males and three females) were chosen on the basis of their growth rate from birth up to 25 kg BW. All the boars and sows were free of the halothane-sensitive allele (n) at the HAL/RYR1 locus, based on the DNA test described by Dalens and Runavot (1993), and all SL boars were noncarriers of the rrn - allele, determined on the basis of the muscle glycolytic potential according to Monin and Sellier (1985). Littermates of the same sex were allocated, at the average BW of 30 kg, to following groups: AL: ad libitum feeding of a standard growing-finishing diet; R1: distribution of the standard diet at 0 75 of the ad libitum intake of the AL littermates (on a BW basis), in order to increase the age at slaughter by about 30 days (Quiniou et al., 1995); R2: restricted energy and protein intakes, by distribution of a blend of the standard diet and of a low-protein diet, in order to obtain similar daily weight gains in R1 and R2 pigs and to give similar carcass composition in R2 and in AL pigs. For calculations of food allowances, it was assumed that lysine requirement was 21 g/kg of BW gain (Noblet and Quiniou, 1999) and that energy requirements for maintenance were 1 MJ metabolizable energy per kg body weight 0 60 (Noblet et al., 1999). Table 1 presents the composition and the nutritional traits (determined as described by Noblet et al. (1989)) of the two diets. The feeding schemes were calculated weekly within blocks of three littermates on the basis of BW, food intake of the AL pigs, and growth rate of the R1 pigs. All pigs were slaughtered at 110 (+5) kg BW, in compliance with the current national regulations applied in slaughterhouses. Animals were transported separately over a short distance and kept separately for at least 3 h at the abattoir, before slaughtering by electrical stunning at low voltage and exsanguination. Carcass traits Carcass weight, lean meat content (calculated from the carcass linear measurements) and mean back fat depth (mean of the measurements taken at the 3rd/ 4th lumbar vertebra and the 3rd/4th last rib levels), were measured on the day of slaughter. The weight of the fresh carcass and of the wholesale cuts of the left side were recorded on the following day. Muscle composition The day after slaughter, slices of m. longissimus dorsi (LD) (3rd lumbar vertebra level) and m. biceps femoris (BF) were taken, trimmed of external fat and epimysium, minced, freeze-dried, pulverized and kept at 20 C under vacuum before chemical analyses. Dry matter was determined from the weight of minced muscles before and after freezedrying. Lipid concentration was determined according to Folch et al. (1957). Protein concentration was determined from nitrogen concentration (Dumas method, Association of Official Analytical Chemists, 1990) using a multiplication factor of 6 25 and collagen concentration was determined from hydroxyproline determination (Bergman and Loxley, 1963) using a multiplication factor of 7 14 (Etheringthon and Sims, 1981). Heat solubility of collagen was determined according to Hill (1966). Table 1 Composition of diets (g/kg) Standard Low protein Ingredients Barley Wheat Maize Wheat bran 50 0 Soya-bean meal Animal fat 20 0 Treacle 30 0 Bicalcium phosphate Calcium carbonate Sodium chloride L-lysine 0 7 Trace minerals and vitamins mixture Chemical composition Dry matter Crude protein Crude fat Crude fibre Starch Ash Lysine Digestible energy (MJ/kg) Estimates from tables of Institut National de la Recherche Agronomique (1989).

3 Increasing age at slaughter and pork quality 89 Muscle quality parameters Forty-five min after slaughter, 2 g samples of LD (3rd/4th lumbar vertebra level) and BF were taken for ph determination after homogenization in 18 ml of 5 mmol/l iodoacetate (ph 1 ). The ph was measured using a combined glass electrode (Xerolyt, Ingold) connected to a portable ph-meter (Knick, Berlin, Germany). The day after, on both muscles, ultimate ph (ph u ) was measured on the carcass using the same apparatus as described before, and reflectance was determined using a reflectometer (Retrolux). A 1 5 cm thick slice of LD (3rd/4th lumbar vertebra level) was also taken, trimmed of external fat and epimysium, weighed and kept at 4 C in a plastic bag for determination of drip losses at 3 and 7 days post mortem, according to Honikel (1998). Sensory quality traits The day after slaughter, the loin piece of the right side was taken, trimmed of external fat and stored at 4 C. The following day, the loins were deboned (muscles and adipose tissue next to the LD were kept) and the resulting roasts were put under vacuum and stored at 20 C. Sensory analyses were performed by a trained sensory panel of 11 members. Three preliminary sessions on non-experimental pork meat were performed. Then, roasts were tested in nine sessions, each containing six samples tested 3 at a time, corresponding to one block of three castrated males and one block of three females from the same litter. Meat from the two genotypes was evaluated in two different periods (five consecutive sessions for SL and four for D). After slow thawing (24 h at 4 C), roasts were cooked in an oven by dry and then humid heat up to 70 C internal temperature. Tenderness, juiciness, flavour, flour sensation after mastication and mouth coating were scored on a scale from 0 (absent) to 10 (high). Statistical analysis Growth performance, carcass traits, muscle composition and quality parameters were submitted to an analysis of variance (GLM procedure, Statistical Analysis Systems Institute (SAS), 1989) including the effects of feeding regimen, genotype, sex, feeding regimen genotype, feeding regimen sex, genotype sex, and litter (intra-genotype). Sensory quality traits were submitted to an analysis of variance (GLM procedure, SAS, 1989) including the effects of feeding regimen, sex, panel member, and the confounding effects of litter and session. Where applicable, Tukey's test was used for multiple comparison of the means. For muscle composition, meat quality parameters and sensory quality traits, the data from one carcass (group R2, SL crossbreed) which exhibited a pale, soft, exudative (PSE)-type meat (ph 1 <5 90 in LD), were excluded from the statistical analysis. Results of growth performance, carcass traits, muscle chemical composition and quality parameters are expressed as means by feeding regimen (three levels), genotype (two levels) and sex (two levels). For meat sensory traits, means of the three feeding regimens are reported for each genotype, since the pattern distribution of the samples did not allow comparisons of means by sex or genotype to be performed. Results Growth performance and carcass traits Since no significant interaction effect between feeding regimen and genotype or sex was noticed on any of the growth performance and carcass traits data (Table 2), only the three main effects will be discussed. The feeding regimen greatly influenced growth performance and carcass traits of pigs. At slaughter at 110 kg BW, final age of the R1 and R2 pigs was increased by 30 days, in connection with a 0 26 proportional reduction of daily BW gain between 30 and 110 kg BW (665 v. 894 g/day). Energetic restriction (R1 v. AL) did not significantly influence food efficiency but led to leaner carcasses, i.e. lower back fat depth and perirenal fat weight, and higher lean meat content. Carcass dressing proportion was decreased, whereas hot carcass weight and drip loss remained unaffected in R1 pigs, which also exhibited higher proportions of ham and loin, and lower proportions of belly and back fat. In contrast, the energy and protein restrictions led to a lower food efficiency but similar carcass characteristics to the pigs given food ad libitum. Hot carcass weight, back fat depth, lean meat content and carcass composition were similar in these two groups, except that there was lower carcass drip loss and belly proportion in the R2 pigs. The similar composition of BW gain and reduced rate of gain of the R2 pigs was achieved through reductions in energy and protein intakes of 0 20 and 0 40, respectively, compared to ad libitum feeding. Moreover, in the former the lysine/digestible energy ratio (g/mj) was progressively reduced throughtout the experiment from 0 59 to 0 50, leading to a lower total protein intake in the R2 than in the AL pigs (36 2 v kg crude protein, respectively, P < 0 05), the R1 pigs exhibiting a similar protein intake to the AL pigs. The two genotypes showed similar growth performance but differences in carcass traits, the SL pigs exhibiting lower dressing proportion, leaner carcasses (higher lean meat content and lower back fat depth and relative weight) and higher proportion

4 90 Lebret, Juin, Noblet and Bonneau Table 2 Growth performance and carcass traits Feeding (F) Genotype (G) Sex (S) Significance Residual AL R1 R2 SL D CM F s.d. F G S No. of animals Growth performance Initial live weight (LW)(kg) 30 9 b 29 1 a 29 3 a * Initial age (day) Final LW Final age a b b d c 4 9 *** * Average daily consumption Diet (kg) 2 41 c 1 77 a 1 92 b c 2 00 d 0 07 *** ** Digestible energy (MJ) 32 7 c 24 0 a 25 5 b e 27 0 d 0 92 *** ** Lysine (g) 22 9 c 16 8 b 13 7 a e 17 7 d 0 7 *** * Average daily gain (g/day) 894 b 673 a 657 a *** P = 0 06 Food efficiency b b a c d 0 02 *** *** Carcass traits Hot carcass weight (kg) P = 0 08 Dressing (g/kg) 787 b 777 a 780 ab 778 c 785 d * * Carcass drip loss (g/kg) 23 b 22 ab 21 a c 23 d 2 ** * Lean meat content 58 8 a 61 0 b 58 8 a 60 0 d 59 1 c 58 2 e 60 8 f 1 9 *** * *** Mean back fat depth (mm) 17 3 b 15 0 a 17 2 b 15 4 c 17 7 d 17 7 e 15 4 f 1 8 *** *** *** Perirenal fat (kg) 1 2 b 1 0 a 1 2 b d 1 1 c 0 2 *** ** Carcass composition (g/kg) Ham 249 a 259 b 254 ab 257 d 251 c 252 e 256 f 8 *** ** * Loin 268 a 279 b 273 a c 276 d 8 *** * Shoulder b 240 a 8 * Belly 139 b 128 a 130 a c 135 d 8 ** * Backfat 69 b 56 a 64 b 58 c 68 d 68 f 58 e 8 *** *** *** a,b,c,d,e,f Within row, feeding, genotype or sex means with different superscripts differed significantly (P < 0 05). Abbreviations: AL = ad libitum; R1 = 0 75 of ad libitum; R2 = restricted energy and protein intakes; SL = synthetic line cross; D = Duroc cross; CM = castrated male; F = female. There were no significant effects of feeding genotype, feeding sex, or genotype sex interactions. Dressing proportion = (hot carcass weight/slaughter live weight) Proportion of left side. Table 3 Chemical composition of longissimus dorsi and biceps femoris muscles (mg/g) Feeding (F) Genotype (G) Sex (S) Significance Residual AL R1 R2 SL D CM F s.d. F G F G S G No. of animals M. longissimus dorsi Water b 739 a *** * Protein 231 b 230 b 221 a *** * IMF 15 8 b 12 6 a 20 6 c 14 5 d 18 0 e *** *** * * Collagen a 4 9 b * Heat-soluble a 1 0 b ** Heat-stable a 3 9 b * M. biceps femoris of ham. In Water b 749 a ** Protein 220 b 220 b 213 a *** IMF 19 7 b 15 5 a 18 7 b 16 6 c 19 4 d *** ** Collagen a 8 4 b *** * Heat-soluble a 1 6 b ** Heat-stable a 6 8 b *** * a,b,c,d,e Within row, for each main effect, means with different superscripts differed significantly (P < 0 05). See abbreviations in Table 2. There were no significant sex effects (P > 0 05). F G, S G : effects of interactions of feeding genotype and sex genotype, respectively. Intramuscular fat.

5 Increasing age at slaughter and pork quality 91 IMF (mg/g) AL R1 R2 AL R1 R2 SL D Figure 1 Influence of the feeding regimen (AL, R1 or R2) on the intramuscular fat concentration (IMF, mg/g) of the longissimus dorsi muscle in SL and D crossbreeds. spite of lower food intakes, females showed higher growth performance (lower age at slaughter and higher food efficiency) and leaner carcasses with higher proportions of ham and loin, than castrated males. Muscle chemical composition Muscle chemical composition was greatly influenced by feeding regimen (Table 3). The R1 feeding regime led to a decrease in IMF concentration in both LD and BF. In LD, this effect was more pronounced in D than SL genotype (P < 0 05) (Figure 1) and in castrated males than females ( 0 31 and 0 08, respectively, P < 0 05). In both muscles, water, protein, and total, heat-soluble and heat-stable collagen concentrations were similar in R1 and AL pigs. In contrast, the R2 pigs exhibited a higher IMF concentration than the AL pigs, particularly in the LD of D crossbreds (+0 40 and in D and SL genotypes, respectively) as opposed to the BF in which IMF concentration was not significantly modified. Compared with AL pigs, protein concentration in R2 pigs was decreased in both muscles, and water concentration was decreased in LD of D crossbreds but remained unchanged in LD of SL (P < 0 05). Amounts of total, heat-soluble and heat-stable collagen of LD were similar between R2 and AL pigs of both genotypes. There was also no difference in the BF of the D crossbreds for these traits, whereas for SL crossbreds, total and heatstable collagen concentrations of BF were increased in the R2 compared with AL pigs (7 9 v. 6 5 mg/g, P < 0 05). Chemical composition of LD and BF was different between genotypes, the D crossbreds exhibiting lower water and higher IMF and collagen but similar protein concentrations to SL. For collagen, differences between crossbreeds were due to both higher heat-soluble and heat-stable collagen levels in D, giving rise to a similar proportion of heat-soluble collagen (around 0 20) in the two muscles of the two genotypes. Chemical composition of LD and BF was not modified by sex in our experiment. Muscle quality parameters In both LD and BF muscles, ph 1 and ph u (rate and extent of ph fall respectively), reflectance and meat drip losses at 3 and 7 days were not affected by the feeding regimen (Table 4). Mean values of muscle quality traits were similar between genotypes and sexes, except for a slightly lower ultimate ph in LD of castrated males than females. Table 4 Quality parameters of longissimus dorsi and biceps femoris muscles Feeding (F) Genotype (G) Sex (S) Significance Residual AL R1 R2 SL D CM F s.d. S No. of animals M. longissimus dorsi ph ph u a 5 51 b 0 07 * Reflectance Muscle drip loss (g/kg) 3 days days M. biceps femoris ph ph u Reflectance a,b Within row, means with different superscripts differed significantly (P < 0 05). See abbreviations in Table 2. There was no significant effect of feeding, genotype or interactions (feeding genotype, feeding sex and sex genotype) (P > 0 05).

6 92 Lebret, Juin, Noblet and Bonneau Table 5 Sensory quality traits of cooked loin (scored from 0 = lack to 10 = high) from SL and D crossbreeds Feeding Residual AL R1 R2 s.d. SL No. of animals Tenderness Juiciness Flavour Flour sensation after mastication Mouth coating D No. of animals Tenderness Juiciness Flavour Flour sensation after mastication Mouth coating There were no significant effects of the feeding regimen (P > 0 05). Feeding treatments and crossbreeds are described in Table 2 footnote. Sensory quality traits In neither genotypes were there significant differences between the three groups AL, R1 and R2 for tenderness, juiciness, flavour, flour sensation after mastication or mouth coating (Table 5). Many studies have reported both higher IMF concentrations and higher back fat depth following the ad libitum feeding of a low-protein diet with a constant protein/energy ratio during all the growing-finishing period (Castell et al., 1994; Goerl et al., 1995; Blanchard et al., 1999) or the finishing phase (Cisneros et al., 1996; Witte et al., 2000). In our experiment, the higher IMF concentration of the energy and protein restricted pigs was obtained without increase in carcass adiposity but, compared with pigs given food ad libitum, the former were 30 days older at slaughter. Indeed, intramuscular adipose tissue exhibits a late development in pigs (Lee and Kauffman, 1974) and IMF concentration dramatically increases during growth (Lee and Kauffman, 1974; Lazo et al., 1994). Thus, the increase in age at slaughter obtained through the R2 feeding strategy led to an increase in IMF level, whereas the R1 feeding regimen did not have the same effect on IMF, because of its limiting dietary energy supply for adipose tissue development In conclusion, our study demonstrates that decreasing the growth rate of pigs during their growing-finishing period may either largely modify the carcass composition or not and, at the same time, decrease or increase the IMF concentration, depending on the feeding strategy applied to reduce the rate of BW gain. In other words, this means that it is possible to manipulate IMF concentration by nutrition, while controlling the carcass adiposity at acceptable values. Discussion Influence of the feeding regimen on growth performance, carcass traits and muscle composition The restricted feeding regimen applied in the experiment allowed the objective of the study to be achieved. In particular, it was possible to increase the age of pigs at slaughter by 30 days and at the same time obtain a similar carcass composition to the pigs given food ad libitum, through a reduction in protein and energy intakes during the growing-finishing period. As expected, the energy restriction increased carcass leanness and decreased IMF concentration, in accordance with previous studies on the influence of restricted feeding at 0 70 to 0 80 of the ad libitum level during the growing-finishing period (Quiniou et al., 1995; Affentranger et al., 1996; Ellis et al., 1996; Wood et al., 1996; Candek-Potokar et al., 1998; Blanchard et al., 1999). Water, protein and collagen muscle concentrations were not modified by the feeding restriction, in agreement with Wood et al. (1996), whereas Candek-Potokar et al. (1998) noticed a lower collagen concentration in m. longissimus dorsi of restricted pigs. Influence of the feeding regimen on meat quality parameters and sensory traits The two strategies used in the present experiment for increasing the age of pigs at slaughter did not modify the muscle quality characteristics. This is in agreement with Candek-Potokar et al. (1998) for food restriction, although Wood et al. (1996) reported a slightly lower drip loss in LD of restricted pigs. According to Goerl et al. (1995), the diet protein concentration does not influence either the ultimate ph or drip loss of the m. longissimus. The lack of difference in meat sensory traits between the different feeding regimens imposed here is in accordance with present results on muscle quality characteristics and with the previous observations of Wood et al. (1996) and Candek-Potokar et al. (1998), whereas Ellis et al. (1996) and Blanchard et al. (1999) reported a decrease in tenderness, juiciness and/or flavour of meat from restricted pigs. On the other hand, compared with a normal-protein diet, the ad libitum feeding of a low-protein diet has been shown to decrease shear-force (Goerl et al., 1995) and

7 Increasing age at slaughter and pork quality 93 increase meat tenderness and juiciness (Castell et al., 1994; Blanchard et al., 1999). A positive influence of IMF concentration on sensory traits of pork meat has often been reported (e.g. Barton-Gade and Bejerholm, 1985; De Vol et al., 1988; Touraille et al., 1989; Hodgson et al., 1991). However, the higher sensory quality may not be ascribed directly to IMF concentration, since in all these studies, different factors such as genetic type have been retained as source of variability in IMF concentration. Among pigs exhibiting a similar genetic background, a positive relation between IMF concentration and juiciness or flavour has also been observed (Eikelenboom et al., 1996; Fernandez et al., 1999). This was not the case in the present study, possibly because the range of IMF variation was much smaller in our experiment than in the others. Moreover, it has been suggested that there may be a threshold effect (around 20 to 25 mg/g) of IMF concentration for positive effects on pork meat sensory traits (Barton-Gade and Bejerholm, 1985; Fernandez et al., 1999). This may also explain our results on meat sensory traits because, even if they had exhibited a large variation, IMF values observed in the present study were also rather small and remained lower than 25 mg/g. Conclusions The different feeding strategies tested in this experiment led to similar results in both SL and D crossbreds. The two feeding strategies R1 and R2 led to an increase in age at slaughter of the pigs by 30 days at 110 kg BW, compared with pigs given food ad libitum. Food restriction alone (R1) increased carcass leanness and decreased IMF concentration, whereas combined energy and protein restrictions (R2) led to a similar carcass composition, but a higher IMF concentration in LD than ad libitum feeding. Despite its large influence on muscle composition, the feeding regimen did not modify the meat sensory traits. This means that within the range of concentrations observed in the present study, IMF had no influence on meat sensory traits. Finally, this suggests that, for the genotypes used in this experiment, a large increase in age at slaughter, even though it influenced muscle chemical composition, did not enhance the eating quality, and that other breeding strategies have to be found to achieve this objective. Acknowledgements The authors gratefully acknowledge the Association Elite for their financial contribution to this experiment. They also wish to thank the personnel of the experimental farm, the slaughterhouse and the laboratories of INRA, especially Nathalie Clochefert and Guy Malineau, for their excellent technical assistance. References Affentranger, P., Gerwig, C., Seewer, G. J. 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