EFFECTS OF EXOGENOUS PORCINE ...

Effects of exogenous porcine somatotropin (pST) administration on growth performance, carcass traits, and pork meat quality of Meishan, Pietrain, and crossbred gilts J. P. Bidanel, M. Bonneau, A. Pointillart, J. Gruand, J. Mourot and I. Demade J ANIM SCI 1991, 69:3511-3522.

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EFFECTS OF EXOGENOUS PORCINE SOMATOTROPIN (pST) ADMINISTRATION ON GROWTH PERFORMANCE, CARCASS TRAITS, AND PORK MEAT QUALITY OF MEISHAN, PIETRAIN, AND CROSSBRED GILTS‘ J.-P. Bidane12, M. Bonneau3, A. Pointilld, J. Gruands, J. M0urot3 and I. &made6 Institut National de la Recherche Agronomique (INRA) Jouy-en-Josas 78352 and St Gilles, L’hennitage 35590, France and SmithKline Beecham Animal Health Products, West Chester, PA 19380 ABSTRACT

Seventeen to twentythree females of lean (Retrain, PI), fat (Meishan, MS), or intermediate genotype (PI x [3/4Large White x 1/4MS]), referred to as crossbred (CR), were injected between 60 and 100 kg live weight with 6 mg/d of porcine somatotropin (pST) and compared to similar numbem of control females receiving the vehicle only. Average daily gain increased similarly in the three genotypes (125 g/d). Feed conversion ratio tended to decrease to a higher extent in MS (-2.0 kg of feed/kg of gain) than in the other two genotypes (-1.1 and -.9 kg of feed/kg of gain for CR and PI, respectively). A significant genotype x treatment interaction was also observed for backfat thickness (BF) and fat, muscle, and bone development. Effects of pST in PI, CR, and MS pigs were, respectively, -6.2,-9.6, and -16.1 mm for BF and 3.0,6.8,and 11.8% carcass muscle. The influence of pST on physical measurements of meat quality was rather low, although desirable effects (P c .05) were obtained on the reflectance and water-holding capacity of PI and CR. Intramuscular fat content was reduced by approximately 1% in MS and CR but not in PI. The metatarsals of pST-treated animals had a higher water content (except in PI), a lower mineralization, and a lower breaking strength (except in MS).The existence of breed variations in the response to pST might result in changes of the relative merit of crossbreeding schemes. Key Words: Pigs, Somatotropin, Genotypes, Body Composition, Meat Quality I. Anim. Sci. 1991. 69:3511-3522

Introduction

reduces feed intake, improves feed efficiency, The injection of porcine somatotropin and markedly alters body composition (Mach(pST) into growing pigs enhances growth rate, lin, 1972; Etherton et al., 1987; Bonneau, 1990). Somatotropin influences both fat deposition, by inhibiting the lipogenic action of insulin (Walton et al., 1986), and muscle ‘The authors acknowledge the technical assistance of growth, through complex mechanisms involvthe entire team operating at the INRA “Statim expcrimen- ing insulin-like growth factor I (Boyd and tale de stkction porcine,” Rouill6 P86480. Bauman, 1988). Comparisons between females ’INRA, Station de G6n6-e Quantitative et Appli- and castrated males (Campbell and Taverner, qu& Jouy-en-Josas F18352 Ctdex. 1988; Bonneau et al., 1989) and results %RA, Station de Rectmches Porcines, St Gilles, obtained in fat genotypes (McLaughlin et al., L’hermitage F35590. 1989; Van der Steen et al., 1989) suggest a %IRA - Laboratoire de Nulrition et Skurib?. h e n take Jouy-en-Josas F18352 Ctdex. higher response of fat animals to pST. The h R A , Station ExpCrimentale de Sdection Porcine, existence of a negative association between the Rouillt W6480. animal’s potential for lean tissue growth and 6SmithKhe Beecham Animal Health products, BNSthe amplitude of its response to pST would sels Bl050, Belgium. have important consequences on the genetic Received October 18, 1990. Accepted April 15, 1991. structure of crossbreeding schemes. Lower

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effects of pST in highly muscled sire lines than in conventional animals would reduce the economic value of these sire lines. Conversely, higher responses of fat genotypes to pST would boost the interest in highly prolific Chinese breeds whose use is now limited because of their poor growth and carcass performance (Legault et al., 1985; Bidanel, 1989). The objective of this study was to assess the effects of pST treatment on growth and carcass performance of very lean (Retrain) and very fat (Meishan) genetic types of pigs and to test the existence of an association between body composition and the amplitude of the response to pST administration. Materials and Methods

Animals. The effects of pST were compared in three genotypes. Two of them are extreme with respect to body composition: the Retrain (PI) breed has a high muscle development, and the Meishan (MS) b r e d has a high fat content. The third genetic type (CR) was obtained by crossing a Pietrain boar with a backcross 3/ 4-Large White x 1/4-Meishan sow. Body composition of CR pigs is similar to that of standard European slaughter pigs (Bidanel et al., 1989). Ekperimental Design. Two replicates of the same 3 x 2 (three genotypes, two treatments) factorial plan were conducted. The first replicate was in the autumn of 1988, the second in the summer of 1989. The average monthly temperature ranged from 5'C to 16'C during the first replicate and from 14'C to 20'C during the second one. In each replicate, 20 to 24 female pigs from each of the three genotypes were purchased at approximately 30 kg live weight and allotted to two pens in a semiopen building where they stayed until slaughter at 100 kg live weight. They were given ad libitum access from 30 to 100 kg live weight to a wheat-soybean based diet formulated to contain 3,180 kcal of DE, 19.5% CP, and 1.1% lysine (Table 1) and water. When they reached approximately 60 kg, gilts within a pen were ranked according to their live weight and were randomly allocated within pairs to control (CTRL) or pST treatment. Each of the 12 combinations (three genotypes x two pens x two treatments) so defined was randomly allotted to one finishing pen. The treatment consisted of daily i.m. administration of either somatotropin (6 mdpig) dissolved in sterile bicarbonate buffer or vehicle only. For

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"calculated nutrient composition: CP, 19.5%; DE, 3.18 Mcal/kg; digestible lysine. .%9% digestible methionine + cystine, ,5896; digestible threonine, .58%, digestible hyptophan, .21%, Ca. 1.1096; and P. .70%. bContributed the following per kilogram of diet: vitamin A, 16,000 W, vitamin D3, 2,400 IU; vitamin E, 20 mg, vitamin K , 1 mg; thiamin, 1 mg. riboflavin, 5 mg, pantothenic acid, 12 q, vitamin B12.20 mg, niacin, 20 mg. Co, . 5 q . Cy 20% Fe,200mg; I, .5 mg; MIJ, 62 mg, Se, 22 mg. and Zn, 160 mg.

each pair of gilts, administration of pST or vehicle started when a mean weight of 60 kg was reached and was continued until the day before slaughter. Measurements. Individual pig weights were measured weekly from about 50 kg to slaughter weight. Average daily gain was calculated during the period from the beginning of the treatment to the day before slaughter. Feed intake and feed conversion ratio measurements began when all pigs in a given pen were treated and was continued until the day before slaughter. Animals were slaughtered at 100 kg live weight and carcass weight as well as blood and selected organ weights (second replicate only) were recorded. The day after slaughter, carcass length between the atlas and the anterior edge of the pulvian symphysis and backfat thickness at the levels of the last lumbar vertebra (rump), last thoracic vertebra (back), and last cervical vertebra (neck) were measured. The right carcass side was weighed This was considered the net halfcarcass weight on which all subsequent calculations were based The kidney and leaf fat were removed and the half-carcass was divided into seven cuts. The front and back feet were separated from limbs at the levels of the carpal bones and of the tibio-tarsal joint, respectively. The ham was isolated along a first line parallel to the general direction of the sacrum and a second line perpendicular to the long axis of the carcass between the last lumbar and the 1st sacral vertebrae. The loin was separated from the belly and the shoulder with a cut starting


WFWX OF pST IN DIFpeRENT PIG GENOTYPES

under the psoas muscle at the level of the last lumbar vertebra and ending under the blade bone. The belly and the shoulder were separated with a cut perpendicular to the long axis of the carcass between the 5th and the 6th rib. The backfat was dissected from the loin, but the other cuts remained untrimmed. This cutting method was described in more detail by Ollivier (1970). Muscle content of the carcass was estimated from the weight of five cuts, expressed as a percentage of half-carcass weight, according to the following equation: percentage of muscle = -.75 + .80 (percentage of ham) + 1.06 (percentage of loin) + .48 (percentage of belly) -SO (percentage of backfat) -.66 (percentage of leaf fat). Carcass fat content was estimated using the following equation: percentage of fat = 43.7 + 127.5 (percentage of backfat) - 31.9 (percentage of ham) -75.3 (percentage of loin). Various meat quality criteria were also measured 24 h postmortem, including 1) ultimate pH on longissimus, adductor femoris, gluteus superficialis, and biceps femoris muscles, 2) water-holding capacity as assessed by the time (in tenths of seconds) necessary for a piece of pH paper to become wet when put on the freshly cut surface of biceps femoris and gluteus superficialis muscles (Charpentier et al., 1971), and 3) reflectance of biceps femoris and gluteus superficialis muscles at 630 nm, using a Manuflex7 reflectometer (scale 0 to 1,OOO). Lipid content in psoas major muscle was measured according to the method of Folch et al. (1957). At slaughter, the two main metatarsal bones were excised from the left hind legs (second replicate only). The separate bones were used to measure bending moments and apparent density. Ash and DM contents were measured on the whole bones and ash relative to bone volume was calculated, All bone measurements have been described in detail previously (pointillart et al., 1987). Statistical Analyses. Growth, carcass, and body composition traits were analyzed by mixed-model least squares procedures (Harvey, 1975). The model included the fixed effects of replicate (R), genotype (C), treatment (T),finishing pen nested within R, G, and T, interaction between G and T, the random effect of birth litter within genotype

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with zero mean and known variance, and slaughter weight as a covariable. A similar model, excluding the replicate effect, was used to analyze organ weights and characteristics of metatarsal bones. For meat quality traits, pen effect was replaced by the effect of slaughter date, nested within replicate. Litter variance was computed as one-half of the heritability (Le., .25 for meat quality traits, .30 for growth traits, and S O for carcass and body composition traits). The PROC HARVEY procedure of SAS (1986) was used to perform these analyses. Feed intake and feed conversion ratio were analyzed using PROC GLM of SAS (1985). The model included the fiied effects of R, T, and their interaction. The interactions between R and G or T were tested for all traits in preliminary analyses and excluded from the model when nonsignificant (P > .lo). Results

Seven PI pigs (three CTRL and four pST) died suddenly because of porcine stress syndrome, but these deaths were apparently not related to treatment effects. Two pens of MS pigs (one pST and one CI'RL) exhibited serious respiratory problems at the end of the second replication. The 10 other pens, which reached 100 kg earlier, were unaffected. Data for these two pens were excluded from the analysis. Performance was similar in the two replicates, except for growth rate and meat quality traits, which were higher in the first replicate. None of the traits exhibited any genotype x replicate or treatment x replicate interactions. Consequently, only pooled results from the two replicates will be presented. Growth, Feed Intake, and Eflciency. Genotype affected growth rate (P c .001), feed intake, and feed conversion ratio (P < .01) in CI'RL pigs. The CR animals grew faster and consumed more feed than MS; PI pigs were intermediate (Table 2). The MS pigs also had a much higher feed conversion ratio (P c .01) than CR or PI pigs, which did not differ significantly. Pigs treated with pST exhibited faster and more efficient gain (P c .001) and consumed less feed (P < .01) during the treatment period than CI'RL pigs (Table 2). The three genotypes tended to respond similarly to pST; no genotype x treatment interaction was observed. However, comparison of the means procedures indicated a larger (P c .lo) reduction of feed 'Model No. 2, Etablissements Nicou, Clamart, Frauce. intake in CR animals and a greater improve-


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ment of feed conversion ratio in MS pigs ( P < .05) than in the other genotypes. On the whole, pST-treated PI, CR, and MS pigs consumed, respectively, 35, 50, and 80 kg less feed than CTRL pigs. Carcass Traits. As expected, the three genotypes differed markedly with respect to carcass composition, as shown by the large deviations observed between MS and PI CTRL pigs for muscle percentage (more than 12 residual standard deviations), backfat thickness (7 standard deviations), and proportions of the carcass cuts (Table 3). The MS females also exhibited a lower (P< .001) dressing percentage and a higher head weight than PI pigs. Performance of CR was intermediate, though closer to that of PI. Administration of pST reduced ( P < .001) dressing percentage, backfat thickness, belly, leaf, and backfat percentages, whereas it increased (P < .001) muscle percentage and the proportions of loin, ham, shoulder, head, and feet (Table 3). Treatment x genotype interactions were observed (P < .05) for all traits except carcass weight, dressing percentage, carcass length, and shoulder percentage (Table 3), indicating variations in the response to pST according to pig genotype. Backfat percentage was reduced by approximately 40% in all three genotypes. On the other hand, leaf fat proportion decreased by 70%, 55%, and 25% in MS, CR, and PI, respectively. Similarly, pST effects on lean cuts proportions were high in MS (24 and 14%for loin and ham percentages, respectively), intermediate in CR (9 and 5%, respectively), and low in PI (3% for both joints). Blood and Organ Weights. Genotypes differed (P < .05) with respect to the weights of gastrointestinal tract organs, except liver and small intestine (Table 4). The MS pigs had a 55% heavier gastrointestinal tract than PI pigs, and CR pigs were intermediate (17% heavier than PI). On the other hand, there was no difference between genotypes for blood, tongue, heart, and lung weights. Pigs given pST had more blood and a higher gastrointestinal tract content (P < .01) than CIXL pigs (Table 4). Treatment with pST also increased (P < .05) organ weights, with the exception of tongue and large intestine, which were unaffected, and mesentary + pancreas weight, which was reduced (P < .001). However, genotype x treatment interactions were detected (P e .05) for most

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traits. Responses to pST were generally high in MS, low and nonsipficant in PI, and intermediate in CR pigs. The most striking differences were obtained for lungs, mesentary + pancreas, stomach, and small intestine weights (50, -43, 43, and 34%, respectively in MS; 12, -19, 18, and -1% in CR,7, -12, 18, and 13% in PI, respectively). Similar trends were observed for heart and liver weights. However, response to pST on spleen weight was threefold higher in CR than in MS or PI pigs. Muscle and Meat Characteristics. Although the overall genotype effect was nonsignificant, PI CIXL pigs exhibited a lower intramuscular fat content than CR (P .lo) and MS pigs (P < .05; Table 5). Genotypes also differed (P < .OS) in meat quality traits, except for the ultimate pH of adductor and biceps femoris muscles. The MS animals had a higher ultimate pH of longissimus and gluteus superficialis muscles (P < .001), a higher waterholding capacity (P < .00l), and a lower reflectance of biceps femoris (P < .01)and gluteus superficialis (P e .001) muscles than the other two genotypes (Table 5). The CR pigs were similar to PI with respect to ultimate pH but exhibited a lower reflectance (P< .OS) and tended to have a higher water-holding capacity ( P < .lo) than PI. Exogenous pST administration reduced (P < .Ol) intramuscular fat content in MS and CR pigs, whereas it had little effect in PI (Table 5). Ultimate pH of the four muscles was not altered in any genotype, but water-holding capacity tended to be improved by pST treatment in C R and PI pigs (Table 5). No significant effect of pST was observed in MS pigs. Bone Characteristics. Pig genotype had no significant effect on metatarsal bone characteristics, except on ash relative to bone DM (Table 6). The administration of pST did not alter fresh weights or total ash contents of metatarsals. However, it increased bone water content in MS and, to a lesser extent, in CR pigs. Mineralization was reduced in CR and PI pigs treated with pST, as demonstrated by decreased (P < .OS) density and ash relative to bone volume or DM. Bones from these groups tended to be less strong, as indicated by decreased (P < .lo) bending moments. Alteration of mineral content at bone tissue level (characterized by a decreased ash relative to bone DM ratio) was only significant in the CR group.


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Dlscusslon

Little information is available concerning pST effects on extreme genotypes (Demeyer et al., 1988; Van der Steen et al., 1989; Kanis et al., 1990; Pmnier et al., 1990). Moreover, except for the data of Demeyer et al. (1988), these results were obtained with lower daily doses of pST and(or) at a different treatment period, so that accurate comparisons are difficult. The lack of significant interaction between genotype and pST administration for growth rate was in good agreement with the results of Kanis et al. (1990) and Verstegen et al. (1990) with Pietrain, Duroc, and Landrace x Yorkshire pigs, of Campbell and Taverner (1988), Bark et al. (1389), and Campbell et al. (1990) with lean and fat lines of pigs, and of Skaggs et al. (1989a) with three genotypes differing for the halothane gene. These studies tend to indicate that pig genotype is not a major factor of variation of pST effects on growth rate. The improvement of feed efficiency was higher than most previous results, except those of Campbell and Taverner (1988) and Campbell et al. (1990) in Large White x Landrace crossbred pigs and McLaughlin et al. (1989) with a Chinese composite line. The tendency for a negative association between spontaneous feed efficiency in a given genotype and the extent of its improvement after pST treatment was in agreement with observations of Campbell and Taverner (1988), Bark et al. (1989), and Skaggs et al. (1989a), who also obtained lower responses in the more efficient genotypes. Conversely, the improvement of feed conversion ratio was independent of genotype in a recent trial reported by Campbell et al. (1990). The reduction in dressing percentage tended to enlarge from PI to MS. It was partly due to a wellestablished increase in heart and liver weights @vock et al., 1988; Bonneau et al., 1989; McLaughlin et al., 1989; Campbell et al., 1990) and also to positive effects of pST on the development of lungs, stomach, and small intestine weights, on the quantity of blood, and on large intestine and stomach content (mostly in MS animals). Fat, muscle, and bone development all exhibited differential responses to pST according to genotype. The most dramatic effects were observed for fatty tissue, with large differences between anatomical locations. The administration of pST caused substantial re

ductions in subcutaneous fat weight in all three genotypes with a larger effect in MS than in CR or PI pigs. A similar interaction between genotype and pST treatment has been reported previously by Kanis et al. (1990). As was observed in ow study, internal fat deposits of lean genotypes are not greatly influenced by pST (Kanis et al., 1988% B m e a u et al., 1989), whereas important reductions are generally obtained in fat genotypes (Grebner et al., 1987; McLaughlin et al., 1989). Similarly, lean cut percentage and muscle content were dramatically improved in MS and only slightly increased in PI pigs. The present results ate in good agreement with most previous studies reporting that pST has no large effect on physical measurements (pH, reflectance, water-holding capacity) of meat quality (Beemann et al., 1988; Kanis et al., 1988b; Bonneau et al., 1989). However, a significant favorable effect was observed on water-holding capacity of. CR and PI pigs. Meat color was also improved in PI pigs, as previously reported by Demeyer et al. (1988) and Skaggs et al., (1989b), whereas the results of Kanis et al. (1988b) indicated a deterioration in meat color. Intramuscular fat content was reduced by pST treatment in MS and CR pigs, as observed in previous studies (Beermann et al., 1988; Bonneau et al., 1989). However, the lack of effect in PI pigs is in agreement with the results of Demeyer et al. (1988) and Kanis et al. (1988b), indicating a noticeable influence of genotype on intramuscular fat response to pST treatment. Little has been published on the effects of pST on skeletal metabolism. In Yorkshire growing pigs treated with pST from 25 to 55 kg, Capema et al. (1989) did not observe any change in weight, calcium, or ash contents of femurs. Femur weight or length were unchanged in Yorkshire pigs treated from 30 to 60 kg, and cartilage plate width was slightly increased (Chung et al., 1985). M e r et al. (1990) described heavier and stronger metatarsal bones in pST-treated MS pigs slaughtered at 40 kg. In pigs slaughtered at 95 kg, Komegay and Wood (1989) reported slightly thicker and heavier metacarpal and metatarsal bones; the ash content was less, and there was no difference in shear force. Osteochondrosis appeared and was aggravated by increasing pST doses in crossbred pigs slaughtered at approximately 100 kg (Evock et aL, 1988). Thus, it seems likely that the effects of pST on


EFFECT OF pST IN DEFERENT PIG GENOTYPES

bone metabolism might depend on the age of the pig. The decreased minerahation and bone strength in CR and PI pigs in the present study and the results of Komegay and Woad (1989) and of Evock et al. (1988) in Yorkshire or crossbred genotypes suggest that pST treatment might alter bone development in finishing pigs. However, some difference might exist between genotypes, because the mineral content and breaking strength of MS bones were almost unaffected, in contrast to those of the two other groups. Nevertheless, water content of MS bones was elevated, as was that of CR,by pST administration. This was also described in younger crossbred barrows (Yorkshire x Duroc) by Capema et al. (1989). In conclusion, the effects of pST on bone composition are not yet clearly understood. Whether or not they are associated with mobility problems is also not well established. Thus, further investigations on the effects of pST on bone metabolism are required. lmpilcatlons

The observed interactions between the magnitude of the response to porcine somatotropin and the ability of a genotype to deposit fat may have important genetic consequences. The relative economic value of crossbreeding plans might be altered. The administration of porcine somatotropin tends to lower differences in feed efficiency and carcass compsition, thus reducing the relative merit of lean genotypes and raising that of fat genotypes. Within breed, it may lead to changes in selection objectives toward a lower emphasis on carcass composition and a higher emphasis on appetite and reproductive traits. This problem and, more generally, the influence of porcine somatotropin on selection schemes (objectives, genetic parameters, and performance testing) remains to be investigated. Literature Cited

Bark, L. J., T. S. Stahly and G. L. Cromwell. 1989. Influence of genetic capacity for lean tissue growth on responses of pigs to recombinant porcine somatotropin.J. Anim. Sci. 67 (Suppl. 1):212 (Abstr.). Beermann, D. H., G. Ambraster, R D. Boyd, K. Roneker and K.D. Fagin 1988. Comparison of the effects of two recombinant forms of porcine somatotropin@ST) on pork composition and palatability. J. Anim. Sci. 66 (Suppl. 1):281 (Abstr.). Bidanel, J.-P. 1989. Etude de stmt6gies de valorisation en croisement de la race Meishan. >Evaluation compar&-de diffkents s y s t b n s de croisement. Joum.

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Rech. Porcine Fr. 21561. Bidanel, J.-P., J. C. Caritez, J. Flewy, J. Gruand and C. Legault. 1989. Etude de strat6gies de valorisation en croisement de la race porcine Meishan. 2-Estimation des p m & m du croisemeat pour les caracttxes de production. Joum. Rech. Porcine Fr. 21:353. BOMM 1990. Regulation of swjne growth by somatotropin hormones: II-The effect of exogenous GRF or pST adminisiration on performawe and meat quality. Pig News Info. 12:39. B o ~ e a M., y L. Lefaucheux and J. Mourot. 1989. Influence de I’administration de somatotropineporcine @ST) SUI les performances de croissaace, la composition corporelle et la qualie des viaudes. Joum. Rech. Porcine R. 21:31. Boyd, R. D. and D. E. Bauman. 1988. Mechanisms of action for somatotropin in growth. In: D. R. Campion, G. J. Hausman and R. J. Maain (Ed.) Current Concepts of Animal GrowthRegulation. Plenum Press,New York. Campbell, R G.,R. J. Johnsoo, R H.King and M.R. Taverner. 1990. Effects of gender and genotype on the response of growingpigs to exogenous administration of porcine growth hormone. J. Anim. Sci 68:2674. Campbell, R G. and M R Taverner. 1988. Genotype and sex effects on the responsiveness of growing pigs to exogenous porcine growth hormone @GH)administration. J. Anim. Sci. 66 (Suppl. 1):257 (Abstr.). Caperna, T. J., R G. Campbell and N. C. Steele. 1989. Interrelationships of exogenous porcine growth hormone administration and feed intake level affecting various tissue levels of iron, copper, zinc and bone calcium of growing pigs. J. Anim. Sci, 67:654. Charpentier,J., G. Monh and L.Ollivier. 1971. Correlations between carcass characteristics and meat quality in Large White pigs. In: Roc. 2nd Int. Symp. on Condition and Meat Quality of Pigs. pp 255-260. Pudoc, Wageningen, Netherlands. Chug, C. S., T. D. Etherton and J. P. Wiggins. 1985. Stimulation of swine growth by porcine growth hormone. J. Anim. Sci. 60:118. Demeyer, D., R. Verbeke, G.van de Woorde, J. Pabry. E. Deroover and R H. Dalrymple. 1988. Effect of recombinant porcine somatotropinon carcass and meat quality of Belgian Landrace and Dutch pigs. In: P. van der Wal, G. J. Nieuwhof and R. D. Politiek (Ed.) Biotechuology for Control of Growth and Product Quality in Swine. Implications and Acceptability. Pudoc, Wageningen, Netherlands. Etheatoq T. D., J. P. Wiggins, C. M.Evock, C. S. Chung,J. F. Rebhun, P. E. Walton and N. C. Steele. 1987. Stimulation of pig performance by porcine growth hormone: Determination ofthe dose-responserelationship. J. Anim. Sci. W433. Evock, C. M.,T. D. Elherton, C. S. Chung and R. E. Ivy. 1988. Pituitary porcine growth hormone @GH)and a recombiaant pGH analogue stimulate pig growth performance in a similar manner. J. Anim. Sci. 66: 1928. Polch, J.. M Lees and G. H. Sloaue-Stanley.1957. A simple method for the isolation and purification of total lipids from animal tissues. J. Biol. Chem. 226:497. Grebner, G. L.,E K. McKeith, J. Novakofski, R A. Easter, D. G.McLaren, K. B = M ~and P. J. Bechtel. 1987. Carcass characteristics of pigs injected with different levels of ~ t ~ ~porcine r a l somatotropin (PST) from 57 to 103 kg weight. J. Anim. Sci. 65 (Suppl. 1):245 (Abstr.). Harvey, W. R 1975. Least-squares analysis of data with


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unequal subclass numbers. USDA, A R S H-4, Washington,

Dc.

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