Effect of Halothane Genotype, Breed, Feed Withdrawal, and Lairage on Pork Quality of Belgian Slaughter Pigs 1

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1 Effect of Halothane Genotype, Breed, Feed Withdrawal, and Lairage on Pork Quality of Belgian Slaughter Pigs 1 S. M. De Smet*, H. Pauwels, S. De Bie, D. I. Demeyer*, J. Callewier, and W. Eeckhout *Department of Animal Production, Faculty of Agricultural and Applied Biological Sciences, University of Gent, Proefhoevestraat 10, 9090 Melle, Belgium; Ministry of Agriculture, Van Thorenburghlaan 14, 9860 Scheldewindeke, Belgium; Lokerse Vleesveiling cvba, Oude Bruglaan 53, 9160 Lokeren, Belgium; Department of Applied Mathematics, Biometry and Process Control, Faculty of Agricultural and Applied Biological Sciences, University of Gent, Coupure Links 653, 9000 Gent, Belgium; and National Institute of Animal Nutrition, Scheldeweg 68, 9090 Gontrode, Belgium ABSTRACT: A total of 434 Belgian Landrace ( B ) or Piétrain B (PB) pigs, of known halothane genotype (NN, Nn, and nn), were slaughtered in a commercial abattoir. Pigs were either fed until loading or deprived of food overnight before delivery. Upon arrival at the abattoir, pigs were slaughtered after different lairage times (within 1 h after arrival, after 2 to 3 h lairage, or 4 to 5 h lairage). Meat quality traits were measured on the carcass, as well as on a piece of loin. Halothane genotype was the predominant factor determining meat quality traits related to the PSE condition ( P <.001 for ph 40 min after death, internal reflectance, color L value; P <.01 for drip losses, transmission value). For these traits, nn pigs were always significantly different from Nn and NN pigs. Depending on the specific trait, Nn pigs were intermediate between NN and nn pigs, or close to NN pigs. For ph 40 min after death and drip losses, Nn and NN pigs were significantly different, whereas the difference between Nn and NN pigs was not significant for internal reflectance, color L value, and transmission value. Shear force and intramuscular fat content were apparently not related to the PSE condition and were not influenced by the halothane genotype ( P >.05). Differences in meat quality between B and PB pigs and between gilts and barrows were rather unimportant compared with the effect of halothane genotype. Overnight feed withdrawal had no effect on meat quality ( P >.05 for all PSE-related traits). On the other hand, holding pigs a few hours in lairage improved meat quality compared with immediate slaughtering ( P <.05 for ph and temperature in the loin 40 min after death, internal reflectance, color L value, transmission value, drip losses). This effect was more pronounced in stress-susceptible pigs than in stress-resistant pigs. Key Words: Pigs, Halothane, Genotypes, Meat Quality, Slaughter, Stress J. Anim. Sci : Introduction Pork quality is dependent on various genetic and environmental factors, and their possible interactions. In pig populations segregating at the halothane gene, the genotype of a particular animal has a major influence on various meat quality traits (Pommier and Houde, 1993; Casteels et al., 1995; De Smet et al., 1 This work was financially supported by IWONL, Brussels. Appreciation is expressed to Guido Geeroms and Daniel D Haese, who supplied the pigs; and to Annie Lejeune, Frederik Colin, Sabine Buyle and the staff of the abattoir Lokerse Vleesveiling cvba and the cutting plant Van Ruyteghem for their technical support. Received November 16, Accepted March 25, ). Breed differences in meat quality may be specific but are mainly associated with differences in the incidence of halothane susceptibility. In addition, additive genetic variance may be considerable for most traits (Sellier, 1988). In general, both across and within breeds, the halothane gene increases the amount of lean in a carcass but equally increases the risk of development of PSE meat. Many environmental factors, of which most are difficult to control, may interact with the genetic predisposition for meat quality (Warriss, 1987). It is generally accepted that minimizing stress in the preslaughter period reduces the risk of development of meat quality defects above a certain level determined by the genetic status of the animal. The purpose of this study was to compare meat quality of the three 1854

2 GENETICS, ENVIRONMENT, AND PORK QUALITY 1855 stress susceptibility genotypes and to examine the possible beneficial effects of feed withdrawal and various lairage times in the abattoir. Table 1. Distribution of the pigs according to halothane genotype, breed, sex, feed withdrawal, and lairage time Materials and Methods Halothane genotype a NN Nn nn Breed Belgian Landrace Piétrain Belgian Landrace Sex Gilts Barrows Feed withdrawal No Overnight Lairage time <1 h to3h to5h a NN = stress-resistant, Nn = stress-carrier, nn = stress-susceptible. Animals and Slaughtering. A total of 434 pigs were slaughtered in a commercial abattoir on 20 slaughter days in the period June 1992 till June The number of pigs per slaughter day varied between 13 and 35. The halothane genotype (NN, homozygous halothane-negative or stress-resistant; Nn, heterozygous halothane-negative or stress carrier; nn, halothane-positive or stress-susceptible) of all pigs was previously determined with a DNA test using a polymerase chain reaction technique as described by Coppieters et al. (1992). All pigs belonged to the Belgian Landrace ( B) breed or were Piétrain B ( PB) crosses and originated from two commercial farms. Breed, halothane genotype, and farm were partly confounded. One farm delivered all NN pigs belonging to the B breed and most of the Nn pigs, consisting partly of B and PB pigs. The second farm delivered most of the nn pigs. Because the original Belgian Piétrain breed is entirely stress-susceptible, PB crosses are either stress-susceptible or heterozygous stress-resistant (Hanset et al., 1983). The transportation time to the abattoir was approximately 50 and 10 min from farms 1 and 2, respectively. Pigs were not treated with sedatives, and electric prods were not used during loading and unloading. Feed withdrawal was varied between and lairage time within slaughter days. On 12 slaughter days, feed was withdrawn approximately 12 h before loading and transport, whereas pigs were given ad libitum access to feed until loading on 8 slaughter days. Within each slaughter day pigs were divided into three groups, which were slaughtered after intended lairage times of 0, 2, and 4 h. Due to practical constraints in the abattoir, the planned lairage times could not be maintained in all cases. Actual mean lairage times were 0, 2, and 4 h for pigs from farm 1 and 1, 3, and 5 h for pigs from farm 2. Hence, due to the confounding of farm and halothane genotype, mean lairage time of nn pigs was approximately 1 h longer than that of Nn and NN pigs. However, within slaughter days, the three groups were separated by a rather constant interval of 2 h. The distribution of pigs according to halothane susceptibility, breed, sex, feed withdrawal procedure, and lairage time is given in Table 1. Pigs from different pens of the same farm were mixed during transport and lairage, but pigs of different farms were never mixed. Pigs were stunned in a V-type restraining conveyor using a high-voltage electric apparatus. Pigs were bled in a lying position, and generally were stuck within 5 s after stunning. About 1 h after death carcasses were placed in a blastchilling room (1 h, 4 to 5 m/s, ± 0 C). Afterward, they were moved to a conventional chilling room ( 0 to 2 C), where they remained until the following morning. After chilling, carcasses were transported to a cutting room. Carcass and Meat Quality Measurements. Instrumental carcass grading was performed by means of a SKGII device (Eurocontroll Belgium, Vilvoorde, Belgium), which combines four physical measurements (fat thickness, ham width, waist width, angle of the ham) into an estimate of the carcass lean content and a conformation value ( a lower value means shorter and more muscular carcasses) using two standard multiple regression formulas (Casteels, 1989). Carcass measurements were performed on 310 pigs covering 15 slaughter days, because the SKGII device was not operational on five slaughter days. Cold carcass weight of all pigs was determined just before cutting. At 40 min after death ( PH1) and before cutting ( PH2), ph was measured in the loin ( L) around the 13th costa (m. longissimus thoracis) and in the ham ( H) (m. semimembranosus) of both carcass sides, using a Knick Portamess 654 ph meter (Knick, Berlin, Germany) equipped with an Ingold Xerolyt electrode (Mettler-Toledo, Greifensee, Switzerland) ( PH1L, PH1H, PH2L, PH2H). Temperature was measured at 40 min after death in the lumbar part of the loin at a depth of approximately 7 cm, using a Digitap LHM 900 apparatus (Digitap, Bornem, Belgium). Internal reflectance ( FOP, Fibre Optic Probe, Premier Electronics, West Yorkshire, U.K.) was measured before cutting in the loin and in the ham of both carcass sides ( FOPL, FOPH). The average value of ph and FOP measurements of both carcass sides per pigs was further used.

3 1856 DE SMET ET AL. A total of 421 carcasses were cut at 1 d after death and sampled for subsequent meat quality measurements. At cutting, a piece of the loin anterior to the last costa (approximate thickness 7 cm) was removed and transported to the laboratory in a cooling box. In the laboratory, the loin piece was deboned and sliced. One slice was used for various measurements. Color was assessed using the 6-point Japanese pork color scale ( JPCS) (Nakai et al., 1975) and by determining the CIELAB color co-ordinates (color L, A, B) in triplicate with a HunterLab MiniScan device after a 30-min blooming time (D65 light source, 10 standard observer, 45 /0 geometry, 1 in. light surface, white standard; Hunter, Reston, VA). The filter paper method of Kauffman et al. (1986) (FPM) was also applied to this slice at 15 min after slicing. The transmission value of Hart (1962) ( TRANS), referring to denaturation of sarcoplasmic proteins, was determined on a second slice of muscle. Intramuscular fat content was determined after Soxhlet extraction using petroleum ether on the remaining of these two slices. Drip losses ( DRIP) were assessed as the proportionate weight loss of a third slice of muscle that had been suspended in a plastic bag for 48 h at 2 C (Honikel, 1987). Hereafter, these samples were used for shear force determination after heating at 75 C during 50 min and cooling in tap water. Shear force of the cooked samples ( SHEAR) was determined on cylindrical cores (diameter 1.27 cm) taken parallel and sheared perpendicular to the fiber direction using an INSTRON 1140 device (load cell type , speed 200 mm/min; Instron, High Wycombe, U.K.) equipped with a triangular Warner Bratzler shear. Cooking before shear force determination was performed immediately after final weighing for drip losses, meaning that the entire aging period of these samples was 3 d. Statistics. Data were analyzed using the GLM procedure of SAS (1988). The model included the fixed effects of genotype (NN, Nn, nn), breed (B, PB), sex (gilts, barrows), feed withdrawal (no, overnight), lairage time (group 1, 2, 3), the random effect slaughter day (number = 20), and the interaction terms genotype sex, genotype feed withdrawal, genotype lairage time, breed sex, lairage time feed withdrawal, and lairage time slaughter day. Because halothane genotype, breed, and farm were partly confounded, the effect of farm and genotype breed were not included in the model. Cold carcass weight was included as a covariate. For early ph and temperature measurements in the abattoir, the exact time of measurement (ph: mean time 40 min after death, range 33 to 51 min; temperature: mean time 40 min after death, range 32 to 57 min) was included as an additional covariate. For carcass grading traits Table 2. Least squares means ± SE of carcass and meat quality traits according to halothane genotype Halothane genotype a Item NN Nn nn P-value Carcass quality traits No. of observations Lean, g/kg 591 ± 3.8 b 596 ± 3.1 b 614 ± 5.9 c.032 Conformation value 2.26 ±.063 b 2.08 ±.050 c 1.64 ±.096 d.000 Meat quality traits measured on the carcass No. of observations PH 40 min loin 6.25 ±.030 b 6.09 ±.021 c 5.76 ±.050 d.000 PH 40 min ham 6.30 ±.031 b 6.18 ±.022 c 5.93 ±.052 d.000 Temperature 40 min loin 39.5 ± ± ± PH 1 d loin 5.59 ± ± ± PH 1 d ham 5.72 ± ± ± FOP 1 d loin 32.9 ± 1.15 b 32.3 ±.83 b 42.6 ± 1.98 c.000 FOP 1 d ham 35.7 ± 1.27 b 37.5 ±.92 b 49.9 ± 2.21 c.000 Meat quality traits measured on a loin slice No. of observations Japanese pork color scale 2.95 ±.070 b 2.99 ±.050 b 2.49 ±.121 c.003 Color L value 50.8 ±.40 b 51.0 ±.29 b 54.7 ±.69 c.000 Color A value 7.6 ± ± ± Color B value 13.8 ±.11 b 13.7 ±.08 b 15.0 ±.20 c.000 Transmission value, % 4.9 ± 1.24 b 5.4 ±.90 b 13.8 ± 2.15 c.007 Filter paper method, mg fluid 56 ± 3.6 b 71 ± 2.6 c 99 ± 6.2 d.000 Drip losses, g/kg 38 ± 2.7 b 46 ± 1.9 c 60 ± 4.6 d.001 Shear force, N 52.2 ± ± ± Fat, g/kg 9.7 ± ± ± a NN = stress-resistant, Nn = stress-carrier, nn = stress-susceptible. b,c,d Means lacking a common superscript letter differ ( P <.05).

4 GENETICS, ENVIRONMENT, AND PORK QUALITY 1857 Table 3. Least squares means ± SE of carcass and meat quality traits according to breed and sex a Breed Sex P-value Item B PB Barrows Gilts Br S Interaction Carcass quality traits No. of observations Lean, g/kg 581 ± ± ± ± Conformation value 2.21 ± ± ± ± Meat quality traits measured on the carcass PH 40 min loin 6.04 ± ± ± ± PH 40 min ham 6.14 ± ± ± ± Temperature 40 min loin 39.6 ± ± ± ± PH 1 d loin 5.60 ± ± ± ± PH 1 d ham 5.73 ± ± ± ± FOP 1 d loin 35.1 ± ± ± ± FOP 1 d ham 39.0 ± ± ± ± Meat quality traits measured on a loin slice No. of observations Japanese pork color scale 2.77 ± ± ± ± Color L value 52.6 ± ± ± ± Color A value 7.0 ± ± ± ± Color B value 14.2 ± ± ± ± Transmission value, % 7.7 ± ± ± ± Filter paper method, mg fluid 73 ± ± ± ± Drip losses, g/kg 45 ± ± ± ± Shear force, N 55.0 ± ± ± ± Fat, g/kg 9.0 ± ± ± ± Br S* a B = Belgian Landrace, PB = Piétrain Belgian Landrace, Br = breed, S = sex. *P <.05.

5 1858 DE lairage time slaughter day interaction was not included. Results Overall mean cold carcass weight was 84.2 ±.37 kg. Mean carcass weight of pigs from farm 2 was higher than mean carcass weight of pigs from farm 1. Hence, mean carcass weight of nn pigs (90.4 ±.96 kg) was higher than that of Nn and NN pigs (respectively, 82.6 ±.44 and 82.0 ±.53 kg). Similarly, PB pigs were somewhat heavier at slaughter than B pigs (mean cold carcass weight, respectively, 86.5 ±.62 and 82.0 ±.39 kg). Carcass weight significantly affected lean content, PH2H, A, FPM, DRIP, and fat content. As expected, lean content and conformation value were significantly influenced by halothane genotype, breed, and sex (Tables 2 and 3). Lean content was higher and conformation value was lower for PB pigs and gilts than for B pigs and barrows, respectively. Lean content increased and conformation value decreased from NN to nn pigs, but the difference in lean content between NN and Nn pigs was not significant. There was a significant effect of halothane genotype on several meat quality traits: PH1L, PH1H, and JPCS values decreased and FOP, color L, color B, TRANS, FPM, and DRIP values increased as the number of n alleles increased (Table 2). For these traits, nn pigs always significantly differed from Nn pigs. Differences between Nn and NN pigs were significant for PH1L, PH1H, FPM, and DRIP but were smaller than differences between Nn and nn pigs. For FOP, JPCS, color L, color B, and TRANS, mean values for Nn and NN pigs were very similar and nonsignificantly different. Hence, Nn pigs were generally closer to NN than to nn pigs, although the differences varied according to the traits. The PB crosses did not really differ from B pigs with respect to meat quality, although PB crosses had clearly higher mean color A and lower mean SHEAR values compared with B pigs (Table 3). The PB crosses had also a slightly but significantly lower mean PH2H and higher mean FOPH value. There was a tendency for increased DRIP in PB pigs compared with B pigs. Barrows had significantly higher temperature, PH2H, FOPL, FOPH, color L, color B, FPM, and fat content values and significantly lower SHEAR values compared with gilts (Table 3). The lairage and feed withdrawal procedure had no effect ( P >.10) on lean content and conformation value (data not shown). The feed withdrawal procedure had no effect on most meat quality traits, except that overnight feed withdrawal was accompanied by a lower mean color A value and a higher mean SHEAR value (Table 4). A significant genotype feed withdrawal interaction was found for ultimate ph values in loin and ham. These interaction effects may SMET ET AL. probably be neglected. Differences between least squares means due to feed withdrawal within genotype were small and nonsignificant. Several meat quality traits were significantly affected by the time spent in lairage (Table 4). Mean temperature values decreased significantly with increasing lairage times. Mean FOPL, FOPH, color L, TRANS, and FPM values were lower and mean JPCS value was higher for pigs that were allowed to rest compared with pigs that received no lairage. For these traits, there was a clear difference between no lairage and 2 to 3 h of lairage, whereas two additional hours of lairage had no or only minor effects. For PH1C and DRIP, mean values for the zero lairage group were only significantly different from the longest lairage group. A significant effect of slaughter day was apparent for all meat quality traits. In addition, a significant slaughter day lairage time interaction was observed for several meat quality traits, meaning that the effect of lairage differed across slaughter days. Discussion Due to practical constraints, it was not possible to slaughter all pigs strictly after the planned lairage times. Slaughtering of nn pigs in particular was delayed on several days. This may have underestimated somewhat the effect of several hours of lairage in this genotype, because nn pigs that had to be slaughtered immediately after arrival in fact always received a certain resting period up to 1 h. It is not clear to what extent this might have influenced differences in meat quality between the lairage time groups, but it is improbable that this would have considerably changed the interpretation of the results. There are now numerous studies showing the large and opposite differences in meat quantity and meat quality between stress-susceptible and stress-resistant pigs (Webb, 1981; Webb et al., 1985; Sellier, 1987). Likewise, De Smet et al. (1995) reported on the inferior meat quality of halothane-positive B and PB compared with halothane-negative pigs. In their study, an inverse relationship between carcass and meat quality was apparent in both the halothanepositive and the halothane-negative group. However, the difference in meat quality between NN and Nn pigs was not assessed. In the present study, the difference between NN and Nn pigs depended on the trait considered. There was little or no difference between NN and Nn pigs for color-related traits such as FOP, JPCS, and CIELAB color co-ordinates, whereas there was a clear difference between NN and Nn pigs for PH1, FPM, and DRIP. These results greatly agree with those of Casteels et al. (1995) on Belgian Landrace and Large White pigs. Lundström et al. (1989) also found no difference in mean FOP

6 GENETICS, ENVIRONMENT, AND PORK QUALITY 1859 Table 4. Least squares means ± SE of carcass and meat quality traits according to lairage time and feed withdrawal before slaughter a Lairage time, h Feed withdrawal P-value Item <1 2 to 3 4 to 5 No Overnight L F D Interactions Meat quality traits measured on the carcass No. of observations PH 40 min loin 5.98 ±.026 b 6.04 ±.025 bc 6.08 ±.023 c 6.01 ± ± PH 40 min ham 6.10 ± ± ± ± ± L D* Temperature 40 min loin 40.4 ±.09 b 39.6 ±.09 c 39.1 ±.08 d 39.6 ± ± L D*** PH 1 day loin 5.59 ± ± ± ± ± F G* PH 1 day ham 5.69 ± ± ± ± ± F G** FOP 1 day loin 39.2 ±.97 b 35.1 ±.99 c 33.5 ±.87 c 36.4 ± ± FOP 1 day ham 43.7 ± 1.08 b 39.8 ± 1.11 c 39.7 ±.97 c 42.7 ± ± L D* Meat quality traits measured on a loin slice No. of observations Japanese pork color scale 2.68 ±.059 b 2.89 ±.060 c 2.87 ±.053 c 2.91 ± ± Color L value 53.0 ±.34 b 51.8 ±.35 c 51.8 ±.30 c 51.7 ± ± L D** Color A value 7.5 ± ± ± ± ± L D* Color B value 14.3 ± ± ± ± ± L D** Transmission value, % 11.3 ± 1.05 b 7.0 ± 1.08 c 5.9 ±.95 c 8.9 ± ± L D* Filter paper method, mg fluid 82 ± 3.1 b 72 ± 3.1 c 72 ± 2.7 c 78 ± ± L D**, L F* Drip losses, g/kg 52 ± 2.3 b 47 ± 2.3 bc 45 ± 2.0 c 47 ± ± L D** Shear force, N 52.4 ± ± ± ± ± Fat, g/kg 9.4 ±.40 b 9.3 ±.42 b 8.4 ±.37 c 9.0 ± ± a L = lairage time, F = feed withdrawal, D = slaughter day, G = genotype. b,c,d Means within lairage time lacking a common superscript letter differ ( P <.05). *P <.05. **P <.01. ***P <.001.

7 1860 DE SMET ET AL. values and color but a significant difference in drip losses between homozygous and heterozygous halothane-negative pigs. Similarly, Jensen and Barton-Gade (1985) found no difference in color but a clear difference in PH1 values and water-holding capacity between Nn and NN pigs. Sather et al. (1991) found a significant difference for both color L value and drip losses, but the latter difference was more marked than the former. Hence, the inheritance of the n allele with respect to meat quality is not consistently recessive or partly recessive, but may vary according to the specific trait. In most studies mentioning similar meat quality traits, an intermediate position of heterozygous pigs or a position closer to the NN pigs was reported (Sellier, 1987), although Murray et al. (1989) concluded from their results that the Nn genotype was closer to the nn genotype for all objective quality measurements. Our finding that halothane susceptibility strongly influences the rate of ph fall and the concomitant development of PSE but is not related to ultimate ph agrees with most authors (Jensen and Barton-Gade, 1985; Lundström et al., 1989; Casteels et al., 1995). In line with Boles et al. (1992) and De Smet et al. (1995), no differences in shear force values due to halothane susceptibility were seen. On the other hand, higher shear values and lower tenderness panel scores for stress-susceptible pigs were mentioned by Murray et al. (1989) and Boles et al. (1991). Casteels et al. (1995) also concluded that selection against the halothane gene would positively influence sensory meat quality traits. Food deprivation for 16 to 24 h before delivery was recommended by Eikelenboom et al. (1991) in order to reduce the frequency of PSE meat. Murray et al. (1989) concluded that an off-feed treatment of 24 or 48 h seriously reduced the frequency of PSE in nn and Nn pigs. In our study no indications emerged for a positive effect of overnight feed withdrawal on PSErelated traits. This is in accordance with Fischer et al. (1988), who concluded that extended fasting periods up to 72 h did lower the carbohydrate reserves in the muscle somewhat, but did not provide a useful way of reducing the PSE problem in meaty animals. It is possible that in our study the period of feed withdrawal was too short to markedly affect the muscle glycogen stores and(or) meat quality. Total time off feed, including transportation and lairage times, was never longer than 18 h. Because all animals were given ad libitum access to feed throughout the fattening period and the feed was withdrawn at night, it may be argued that differences in feed intake between the fed and the fasted group were minimal. Indeed, it was shown that feeding of group-housed slaughter pigs predominantly takes place during the diurnal period, with the largest peak occurring in the afternoon (de Haer and Merks, 1992; Labroue et al., 1995).

8 GENETICS, ENVIRONMENT, AND PORK QUALITY 1861 By depleting muscle glycogen stores, feed withdrawal is sometimes seen as a means of raising ultimate ph (Warriss, 1982; Eikelenboom et al., 1991). No main effect of overnight fasting on ultimate ph was seen in our study, but a genotype feed withdrawal interaction was apparent. However, least squares means of fasted vs non-fasted pigs within genotype were not significantly different. Murray et al. (1989) found no difference in ultimate ph in the loin between the three genotypes in fasted pigs, whereas in non-fasted pigs ultimate ph of nn pigs was lowered. Eikelenboom et al. (1991) found increased ultimate ph values after a fasting period of at least 24 h, but not after 16 h. As discussed above, the fasting time in our study may have been too short to raise ultimate ph consistently. The only effect of feed withdrawal was seen on color A and SHEAR values, which decreased and increased, respectively, after overnight fasting, and which were not affected by genotype in our study. This is in complete contradiction to Murray et al. (1989), who found no effect of fasting but increasing color A and shear values with increasing stress susceptibility. These opposite effects are difficult to explain. In accordance with De Smet et al. (1995), PB pigs had lower shear force values compared with B pigs, in spite of their lower overall meat quality with respect to PSE. A time interval of a few hours between arrival at the abattoir and the moment of slaughter should allow the animals to recover from the transportation stress, with beneficial effects on meat quality (Fortin, 1989; Eikelenboom and Bolink, 1991; Warriss et al., 1992). This was also apparent from our study. A lairage time of 2 to 6 h compared with immediate slaughtering improved meat quality, based on lower DRIP, FOP, and color L values and higher PH1L values. Lairage may have lowered the body temperature, as indicated by lower temperature values in the carcass, which has been shown to positively influence meat quality (Klont et al., 1994; Klont and Lambooy, 1995). Although stress-susceptible pigs may react more strongly to environmental stress than stress-resistant pigs, Barton-Gade (1984) suggested that meat quality of stress-resistant pigs will particularly benefit from low environmental stress conditions, because stresssusceptible pigs yield meat of inferior quality anyway. In our study, it was observed that stress-susceptible pigs needed more time to recover from transportation stress than stress-resistant pigs, but apparently this did not result in a significant genotype lairage time interaction in the model used. However, the results of the statistical analysis indicated a significant lairage time slaughter day interaction for several traits (see Table 4). When this interaction term was omitted from the model for these traits, a significant interaction between genotype and lairage time emerged. Because on many slaughter days either stress-resistant or stress-susceptible pigs were processed, there was some confounding between slaughter day and genotype. Hence, the interaction between lairage time and either slaughter day or genotype may have the same background. Least squares means for the genotype lairage time interaction for some selected traits (PH1C, color L, DRIP) are presented in Table 5, based on the statistical model without the lairage time slaughter day interaction. In this case, the beneficial effect of lairage time on PSE-related traits was more pronounced for stress-susceptible pigs than for stress-resistant pigs. In nn pigs, there was a significant difference between the zero and the longest lairage group for the traits mentioned. For Nn and NN pigs, differences were smaller, and there was only a significant difference between the zero and the longest lairage group for PH1C in Nn pigs and for DRIP in NN pigs. Hence, nn pigs may benefit most from a certain resting period, at least under the circumstances employed in this study. Indeed, the impact of lairage may vary according to the specific situation in each abattoir, and in relation to the amount of stress experienced during transportation. An absolute prerequisite for a beneficial effect of a certain lairage time on meat quality is that animals be allowed to rest and to recover from transport. Especially when animals from different farms are mixed upon arrival at the abattoir, lairage may be a subsequent stressor. In our study, pigs remained in their group after transport and were seen to rest in the lairage pens after an investigative behavior of approximately half an hour. Hence, as also pointed out by Warriss (1987), holding pigs for a certain time in lairage compared with immediate slaughtering upon arrival at the abattoir may be beneficial as well as without effect or detrimental to meat quality, depending on the type of pigs and the previously experienced stress. Although transportation stress was minimal in our study, the stress-susceptible nature of our extremely muscled pigs made lairage beneficial. In general, halothane susceptibility was the most important factor determining meat quality in this study. However, not all traits measured were affected. Halothane genotype and lairage affected approximately the same traits, which can be seen as PSErelated traits. These are PH1, which describes the rate of ph fall; FOP, JPCS, color L and TRANS, which all relate to the paleness of the meat and the denaturation of proteins; and FPM and DRIP, which are a measure of the water-holding capacity of meat. Other traits, such as ultimate ph, shear force (tenderness), and intramuscular fat content, may behave independently of PSE traits and were apparently affected by other factors such as breed and sex in this study. Similar observations were made by De Smet et al. (1995), who stated that shear force values are not a suitable indicator of the soft component of PSE meat. In addition, color and water-holding capacity of meat

9 1862 DE SMET ET AL. Table 6. Distribution of the meat samples according to the 50th percentiles of color L value (51.4) and drip losses (4.42%) Color L value Drip losses, % n % > > >51.4 > do not necessarily change in a similar way by genetic or environmental factors. In other words, the term PSE is not adequate anymore, and the pale color and exudative condition of meat do not need to occur simultaneously. In fact, meat quality traits vary on a continuous scale, and fresh meat samples cannot easily be classified into good or aberrant quality. In this context, Kauffman et al. (1993) and van Laack et al. (1994) distinguished several descriptive but arbitrary quality groups. Based on drip losses and color L values, these authors classified samples in either an ideal quality category (RFN, red, firm and nonexudative) or in one of four aberrant quality categories (PSE, pale, soft and exudative; RSE, red, soft and exudative; PFN, pale, firm and non-exudative; DFD, dark firm and dry). However, we believe that a classification should primarily be based on subjective scores or objective standards determined by representatives from the processing, the wholesale trade, and(or) the retail industry, because meat quality defects first directly have an effect in these sectors. Standards could be based on consumer perceptions, for instance, which may depend upon the region, and therefore no fixed standards can be recommended. In addition, methodology should be standardized before using threshold barriers (e.g., we experienced that color L values may differ by almost three units according to the type of colorimeter used), as was also suggested by van Laack et al. (1994). For these reasons, no attempt was made in this study to classify samples into different categories. Dark, firm and dry meat was certainly not a worry, as can be seen from the low mean ultimate ph values. Ultimate ph values higher than 6.2 were recorded in none and in 1.7% only of the cases, when measured in the loin and in the ham, respectively. Using ph 40 min after death as a criterion for PSE development (Bendall and Swatland, 1978), a proportion of 16.8% and 35.5% for PH1C < 5.8 and < 6.0, respectively, was found. In order to get an idea of the association between color and water-holding capacity, samples were distributed into four categories based on the 50th percentiles of color L value and DRIP (Table 6). If the association between color and water-holding capacity were perfect, the samples should be equally distributed into the combination of the two lower and the two upper 50th percentile groups. However, a large discrepancy was seen in this study. Approximately one-quarter of the samples was found in each of the four categories, meaning that a large number of samples had a relatively pale color in combination with relatively low drip losses, and vice versa. A loose correlation between reflectance and drip losses was also described by Warris and Brown (1987) and van Laack et al. (1994). Both authors reported a biphasic relationship between reflectance and drip losses, but their results were not identical. At first sight no biphasic relationship between color L value and drip losses was apparent in our data. Anyway, our data confirm the suggestion made by van Laack et al. (1994) that color brightness and water-holding capacity are, at least to some extent, the results of independent pre-rigor biological phenomena. Implications In a Belgian slaughter pig population, it was shown that the halothane genotype was by far the most important factor determining meat quality traits related to the PSE condition. Hence, most improvements in meat quality may be obtained by controlling the halothane gene. 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