Genetic Selection to Improve the Quality and Composition of Pigs

Transcription

1 GENETIC SEL ECTlON Genetic Selection to Improve the Quality and Composition of Pigs David G. McLaren* Collette M. Schultz Introduction The composition of pigs has changed dramatically during the past 100 years, from the lard-type hog prevalent in the late 19th century to the modern meat-type animal developed largely since World War II (Wiseman, 1986). With the substitution of vegetable oils for animal fat, lard production ceased to be a production goal and pigs are now raised primarily as a source of protein for human consumption. Progeny testing to genetically improve quality and composition of pigs began in Denmark in 1907 (Jonsson, 1975). Denmark has, without precedent, used genetic and other technologies to build the world s most advanced system of production of high quality pork products, 85% of which are exported (Christensen, 1986). By the 1960 s, it was realized that, with reasonably accurate ultrasonic backfat probes and selection for moderately to highly heritable growth and carcass traits, performance testing would improve annual rates of genetic improvement by decreasing generation interval. Many pig breeding companies were formed during the 1960 s and 1970 s and now supply pig breeding stock markets throughout the world. Pig Improvement Company (PIC), for example, was formed by a group of pig producers in England in 1962 and has since grown to include whollyowned companies in the U.S., U.K., Germany, Mexico, Spain, Portugal, Denmark and France, and licensed distributors, agents or major contracts in another 23 countries. It is the intent of this paper to review scientific pig breeding as it has been practiced for the past 30 years and the impact this has had on quality and composition of pigs. The genetic basis for meat quality is discussed and the opportunities for pig breeders to select for improved pork quality in the future is considered. Pig Breeding and Genetic Improvement Quantitative genetics technology was developed in the 1940s (Hazel, 1943; Lush, 1945) and considerable genetic improvement of livestock species has been achieved through its applications since that time (Land, 1984; Smith, 1984). Since the 196O s, selection in pigs has focused on growth rate, feed conversion efficiency and indicators of carcass lean, such as probed backfat thickness. Significant improvements have been achieved in these economically important traits over the past three decades (Glodek, 1982; Mitchell et al., 1982; Kennedy et al, 1986; Ollivier, 1986; Tixier and Sellier, 1986). Figures 1 and 2 show best linear unbiased predictions (BLUPs) of breeding value (BV) in PIC pure line nucleus pigs in the U.S. over the past six years. Initially developed for dairy sire evaluation (Henderson, 1973), BLUP has been adopted for genetic evaluation programs in most livestock species, including the pig (Long et al., 1991) Accuracy of genetic evaluations is increased by BLUP using information on all known relatives of an individual. Additionally, animals with differing amounts of information, producing records in different herds and (or) at different times can be compared without bias using BLUP Another advantage of BLUP is that it partitions genetic and non-genetic contributions to phenotypic trends, allowing breeders to assess genetic change over time (Table 1) In our genetic nucleus herds, growth rate and feed conversion efficiency improved at approximately 2% of the mean per year over the past six years (Figure l), while estimated breeding value of backfat has improved at an average rate of 3% of the mean per year (Figure 2). These genetic gains in rate and efficiency of lean growth have been made by performance testing individuallyhoused boars for growth rate, feed intake and backfat thickness and testing group-housed gilts for growth rate and probed backfat thickness. Selection index theory and, beginning in 1990, BLUP breeding value estimation proce- Bv FCR Figure 1 *D. G. McLaren, Pig Improvement Company Inc., P , Franklin, KY Reciprocal Meat Conference Proceedings, Volume 45, W 1 WM03 MM M lsal MO2 I03 U04 ldol Ml BO IIRTHDIY Genetic trends for feed conversion ratio (FCR) and average daily gain (ADG) for pure line pigs born in PIC, Inc nucleus herds between January 1, 1986 and December 31,

2 116 American Meat Science Association BLCKL.mm - Fiaure ~ 2 -,; NMX.I 11 Figure 3 THE PRINCIPLES OF WITHIN LINE IMPROVEMENT ME. OENERATION YEAN PERFORMANCE MEAN PERFORMANCE OF SELECTED PARENTS I Nunbr df4 SELECTED DAMS :...:.:.. : :. :._.: :......:.:-.1 E41 WIl Ua3 UPI lIU llm LMll Wl2 E43 CJM Wl8W W W a01 omt WaJ SIC DlU 9lIU 0101 Bimnom Genetic trends for backfat (CKL=sum of three ultrasonic backfat readings taken on the live pig at the end of the performance test) and economic index (INDEX, $) for pure line pigs born in PIC, Inc. nucleus herds between January 1, 1986 and December 31, dures have been used to select the best 3% to 4% of boars and 30% to 40% of gilts as breeding replacements in PIC nucleus lines (Figure 3). Although the lean composition of market pigs has generally improved over the past thirty years, some correlated responses have been reported between lean tissue growth characteristics and meat quality (ph, muscle, color and water-holding capacity (Froystein et al., 1979; Steane, 1981; Glodek, 1982; Ollivier, 1986). The Danish industry in the 1970 s was the first to include meat quality criteria in the breeding objective (Vestergaard, 1985). Until recently, little interest has been shown by the U.S. pork industry for decreasing the fat content of carcasses. This has been due mainly to the lack of financial incentives and to the relatively low costs of adding energy (as fat) to feed. Moreover, consumers were not overly concerned with the level of fat in their diets. This situation has, of course, changed and continues to change rapidly. Consumers are demanding lean meat products as evidenced by an increase in the demand for fresh pork (Anonymous, 1991) which is leaner than reported by the USDA in This has instigated an increased demand for breeding stock capable of producing the lean pork demanded by packers, processors, retailers and consumers. Substitution of genetically improved animals for tradi- I wnl Response I Average Best m.ebalon -H Rate of response to selection depends on: 1) superiority of selected parents, 2) heritability, Le., the proportion of superiority passed on to offspring and 3) the rate of generation turnover. tional sources of breeding stock replacements will have the most rapid effect on composition of the average market pig in the U.S. Only around 30% of breeding stock replacements are purchased in the U.S., vs 53% in Denmark and 64% in the U.K., for example (Figure 4). Pig breeders must, then, understand and anticipate the market to ensure that their products remain competitive in the future. Demand for pork in the U.S. will probably remain steady at approximately 16 billion pounds per annum; however, demand for better quality raw material will increase from the consumer through the pork chain to hog producers and to their suppliers. The need for the pig breeder to accurately anticipate future market conditions is illustrated in Figure 5 which shows the progression in improvement from the nucleus level to market animals. Setting selection objectives in a pig genetic improvement program is a little like steering a super tanker, the genetic lag involved in disseminating improved genetics generated in nucleus populations is such that a change in selection objective made in 1993 will not impact the market hog population until Now, then, is the time to consider whether or not the breeding objective for the year 2000 should not be to lower the cost of quality lean meat production. Table 1. Estimated Genetic Trends for PIC, Inc. Genetic Nucleus Linesa Annual Six year totala Trend $ value pig Trend $ value pig Average daily gain, gld Feed-to-gain ratio Average backfat, mm Carcass lean, % Total valueb Lean market $ 1.75 $ Backfat market $ 1.22 $ 7.33 Liveweight market $.82 $ born January 1, 1986 to December 31, bassurnptions for economic values: days to market, $.12 per day; feed cost, $.154 per kg; backfat, $.492 per mm; lean percent, $1.50 per pig per 1% change.

3 45th Reciprocal Meat Conference 117 Figure 4 0 Homebred and Purchased 1 U.S.A. U.K. Germany Spain Denmark Mexico Sources of pig breeding stock replacements. Ficrure 5 Improvement Timetable GN GGP GP P MARKET Performance is Known Perfomance can be Predicted Performance can be Changed Genetic lag is the time it takes to raise the commercial parent (P) population to a (pre-existing) genetic nucleus (GN) genetic level. This lag results from the multiplication required to produce required volumes of hybrid parent breeding stock from a finite (3,000 sow) GN population. Great grand parents (GGPs) are pure lines mated cross to produce hybrid grand parents (GPs) that then produce three-or-four-line cross parent boars and gilts. Pork Quality As the importance of the end product increases throughout the pork chain, it is important to define and quantify what is meant by pork quality. Quality may encompass a variety of characteristics, from carcass composition to eating quality, as shown in Table 2. The importance placed upon quality parameters might be driven by a product s reputation or description or, most commonly, by financial incentives to produce a particular type of product. Quality may be determined subjectively as with many eating quality factors, or measured objectively using instruments or laboratory techniques. Ultimately, the consumer defines meat quality through demand, Table 2. Carcass Qualitv. Meat Quality Carcass Composition Muscle Qualitv Fat Qualitv Absence of fat Water holding Flavor Lean content Color Color Lean distribution PH Consistency Muscle thickness Fat distribution Tenderness Juiciness Intramuscular fat and the consumer wants a consistent, lean, palatable product at a reasonable economic value. Table 3 illustrates a number of potential pork quality problems. Undesirable palatability of pork may be experienced when meat is tough, dry, lacks flavor or exhibits some combination of these problems. Tenderness has been found to be the most important factor in eating acceptance of meat (Bratzler, 1971) and may be influenced by a variety of factors including the quantity and state of collagen (Wood, 1990), degree of myofibrilla contraction (Marsh, 1975) and many post-slaughter treatments. Intramuscular fat contributes to palatability by providing some juiciness (Lawrie, 1966) and by exerting a dilution effect on muscle proteins, enhancing tenderness. The ability of the proteins to hold water also contributes to juiciness, tenderness and processing quality. Flavor components associated with meat are released upon heating, and some of those found in pork are in the fat (Smith and Carpenter, 1974) and are distinguishable from other species of meat animals (Gomez- Gonzalez et al., 1991). Genetic Improvement of Pork Quality Figure 6 illustrates that perhaps the most important factors affecting pork eating quality occur after the pig has left the farm, although genetic differences do exist for a number of meat quality traits (Diestre, 1991). Breed differences for PSE pork and meat color in part reflect differences in frequency of the so-called halothane gene in different populations (Webb et al., 1982). These are discussed later in this paper. The Duroc and Meishan are examples of breeds with relatively high intramuscular fat (Sellier, 1988; Wood, 1990), whereas the Hampshire breed carries the so-called acid meat (RN) gene responsible for decreased Napole technological yield (Le Roy et al., 1990). Pietrain pigs tend to have soft fat when compared to Large White pigs (Wood, 1990). Given the existence of breed differences for carcass quality characteristics, it is tempting to speculate that breed or line substitution may be used to achieve desired carcass quality characteristics. Figure 7 illustrates the variation between and within lines for carcass composition and meat quality characteristics. Means were based on entire boars fed ad libitum from 33 to between 90 and 100 kg liveweight in the U.K. The standardized range is the range between line means divided by the pooled within-line standard deviation. As can be seen, there was considerable variation between lines for carcass composition traits (e.g., yield, backfat thickness and carcass lean), moderate variation for some meat quality traits (e.g., drip loss, intramuscular fat Table 3. Carcass Quality Problems. Carcass Meat Quality Composition Muscle Quality Fat Quality Over fatness PSE Soft ness Over leanness DFD Taint Carcass damagec Eating quality Separation apse=pale, Soft and Exudative. bdfd=dark, Firm and Dry. CBlood splash, bruising.

4 118 American Meat Science Association PSE DFD Marbling Boar Taint Residues Carcass Damage Pigment Acid Meat Protein Soft Fat Off-Odors Genetics Figure 6 Environment Breed Sex Feeding Pre-Post Slaughter. t f.. *** t. ***.*I.f*.*I tt. *. The majority of pork quality problems occur after the pig has left the finishing pen (after Diestre, 1991). DH45 Color (EEL unirs) Drip Loss imglg) Lipid in Muscle (mag) Water I Muscle (mag) Nilrogen m Muscle imglg Fat Firmress (penelromelsr unils) Androrlanone 8n Fat lmglg) Cooked Meal Color (scale t7) Abnormal Flavor (scale 0 lo 3) Texlure (scale. -7 IO t7) JYIC~~~SL (5cale 0 Io 4) Overall ACCeplabil8Iy (scale ) I ~..t.. t tt. Figure 7.*t tt. **t Standardized Range 4. Line effects on pork product composition and quality (Bovey et al., 1988). and color), but little opportunity to improve eating quality (e.g., flavor, texture and juiciness) via manipulation of line makeup. Indeed, there appear to be, in general, only small effects of breed on pork quality when carcass fatness is similar (Wood, 1990). Selection for improved meat quality within lines is hampered by the absence of relevant measures that can be taken on live pigs and by the expense and decreased accuracy from having to take carcass measurements on slaughtered sibs. The relatively low heritabilities of meat quality characteristics and the potential for antagonistic correlations between meat quantity and meat quality traits also hinder progress in this direction. Heritability estimates for carcass quality traits are given in Table 4. Unlike carcass composition traits, which are moderately to highly heritable, most meat quality characteristics appear to be between 10% and 30% heritable, with the notable exception of intramuscular fat. In addition, estimates of genetic correlations between intramuscular fat and total carcass fat and total carcass lean are very low (Wood, 1990). This would suggest that selection for high intramuscular fat in lean carcasses might be possible. Apart from poor meat quality associated with the halothane gene, there is no clear association between carcass fatness, intramuscular fat and eating quality, however (Sellier, 1988). Chris Warkup of the U.K. Meat and Livestock Commission (MLC) has represented the relationship between lean tissue growth rate and intramuscular fat as shown in Figure 8 (Personal Communication, 1992). At one extreme, there are genetically unimproved lines with relatively low lean tissue growth rate but high levels of intramuscular fat. At the other extreme are genetically improved lines with fast lean tissue growth rate but low levels of intramuscular fat. The three sexes, barrows, gilts and boars, lie along the same line. Warkup s hypothesis, however, is that meat tenderness is positively related both to lean tissue growth rate and to intramuscular fat. Pigs on restricted feed exhibit lowered lean tissue growth rate and intramuscular fat percent than those on ad libitum feed. Lines characterized at MLC as meat-type fed ad libitum had faster lean tissue growth rate, greater percent intramuscular fat and more tender pork than white-type lines on restricted feed. When the data were adjusted for differences in backfat thickness, line differences in meat tenderness were still evident. However, when the data were adjusted for differences in lean tissue growth rate, no differences in tenderness were detected (Warkup, personal communication, 1992). The mechanisms relating lean tissue growth rate, intramuscular fat and meat tenderness are, to say the least, still obscure. Selecting for improved lean tissue growth rate has been referred to as a biological index in the past (Fowler et al., 1976), as distinct from the economic selection index approach (Hazel, 1943). Even the biological model is, however, still largely a black box in that the consequences of selecting for improved lean tissue growth rate on underlying biological mechanisms are largely unknown. For example, what effects does selection based on growth rate, feed intake and backfat have on fiber type? What is the mechanism of increased rate of muscle growth resulting from selection: increased protein synthesis and (or) decreased Table 4. Heritability Estimates (h2) for Carcass Quality Traitsa Trait # Intra-muscular fat.59 Length.56 Backfat depth.52 Percent lean.48 Loin eye area.47 Flavor.47 Color.34 Water holding capacity.31 Shear force.28 Tenderness.28 Dressing percent.23 PH.20 Drip loss.ll Juiciness.08 afrom literature reviews in Lo (1990) and Stewart and Schinckel (1990).

5 45th Reciprocal Meat Conference 119 LTG R Figure 8 Tend ern e ss? in the short term exclusively by improvements in feeding and management of the pig (Webb, 1982). Aside from eating quality, there is considerable scope for reducing variation in carcass composition traits by a number of management techniques besides genetics (Table 5). One important genetic effect contributing to pork quality, frequency of the halothane gene in slaughter pigs, will be controlled by pig breeders in the future due to development of a DNA test to determine genotype at this locus. The Halothane Gene IMF The relationship between lean tissue growth rate (LTGR), intramuscular fat (IMF) and pork tenderness. (Chris Warkup, 1992, personal communication). rate of protein degradation? That a decrease in protein degradation rate might be accompanied by a down-regulation of enzymes such as the calpains which contribute to meat tenderness has been suggested as a mechanism for decreased eating quality associated with genetic improvement of lean tissue growth rate (Goll, 1991). Danish researchers have consistently concluded that approximately 2% intramuscular fat (total lipid) is required for good eating quality characteristics (Wood, 1990). DeVol et al. (1988) found a significant decrease in tenderness below 2.5% to 3% intramuscular fat in a random sample of pork chops in the U.S. Levels of 2% to 3% total lipid in the longissimus dorsi are not, however, achieved in white European pig breeds at subcutaneous fat thickness levels acceptable to consumers in Europe. Moreover, research in the U.K. has shown pork with highly acceptable juiciness and tenderness can be found at between.8% and 1% extractable lipid, with levels around 1% preferred (Wood, 1990). However, these are at lighter slaughter weights and are probably less mature animals than slaughtered in the U.S. In the future, technologies such as magnetic resonance imaging (Henning et al., 1991) might allow determination of intramuscular fat levels in live pigs, and the heritability of intramuscular fat is such that levels could be improved by selection. However, it is by no means clear that such a selection strategy will serve to improve the eating quality of pork. Furthermore, segments of the industry seeking to formulate products with low fat levels consider marbling to be undesirable. Indeed, it has been suggested by other swine geneticists that eating quality issues should be addressed Table 5. Techniques to Reduce Variation in Carcass Traits. 1. High health status, all-in/all-out 2. Careful environmental control 3. Stocking densities, feeder management 4. Feeding: correct energy-protein ratios; restrict feeding 5. Careful sorting by weight, sex 6. Uniform crossbred type market pig 7. Artificial insemination Selective control of the halothane gene by pig breeders is desirable as the gene conveys both positive and negative attributes. Halothane-positive pigs are homozygous recessive for a mutation of the ryanodine receptor (RYR) gene responsible for malignant hyperthermia or stress syndrome in pigs (Fujii et al., 1991; MacLennan and Phillips, 1992). Halothane-positive pigs have a better feed conversion efficiency and a higher percentage lean in the carcass compared to normal pigs, but have poorer meat quality, smaller litters and greater death loss. Halothane heterozygotes possess approximately half of the advantages of halothanepositive pigs, while not suffering from stress-related mortality and with potentially small effects on meal quality. We are now utilizing a new method, known as the HAL1843TM halothane gene test', which enables the identification of carrier as well as homozygous recessive pigs (Figure 9). Figure 9 Polymerase chain reaction amplification of a section of the ryanodine receptor (RYR) gene, treatment with a restriction endonuclease that cuts normal but not mutant gene DNA, fragment separation by agarose gel electrophoresis, and exposure to photographic fiim of the ultra-violet illuminated gel stained with fluorescent ethidium bromide dye results in the white DNA banding pattern shown. The wells are at the top of the figure. Lighter fragments travel farther through the gel towards the bottom of the figure. The two bands near the bottom represent normal cleaved DNA. The white band of heavier DNA fragments in eight of the fourteen lanes results from the presence of the mutant RYR (halothane) gene. Reading from left to right, then, pigs are normal, carrier, carrier, carrier, normal, stress susceptible, carrier, etc. 'The HAL-1843 trademark is licensed from the Innovations Foundation, Toronto, Ontario, Canada, owner of the trademark.

6 120 American Meat Science Association Table 6. Stress Gene Carrier (Nn) vs Normal (NN) Pias: Exoerimental Evidence. Mean % Advantage (disadvantage) Traita Referenceb Nn-NN Mean Ranae LTGR, g/d Yield, o/o 1,2,3,8,9,10, to 3.9 Carcass lean, o/o 1,2,3,5,6, to 4.7 7,8,9,10,11 Meat quality 4,5,6,7 - (4.5) (1.9) to (10.4) index PSE, Yo 5,7,10, (50.0) 0 to (200.0) altgr = lean tissue growth rate; PSE = pale, soft and exudative pork. 1 = Aalhus et al. (1991); 2 = Carden (1982); 3 = Christian and Rothschild (1981); 4 = Eikelenboom et al. (1980); 5 = Jensen (1981); 6 = Jensen and Andresen (1980); The advantage of the new test is that it will allow pig breeders to exploit the positive values of the mutation, while identifying and avoiding the homozygous recessive animals. By ensuring dam lines are free of the mutant stress gene, it might be advantageous to use terminal sire lines in which the mutation is fixed, thus producing all heterozygous stress gene carrier pigs. Table 6 reviews the experimental evidence for differences between normal and carrier genotypes. Carrier pigs exhibited 15 g/d improved lean tissue growth rate,.9% greater carcass yield and 1.5% higher lean content in the carcass compared to normal pigs. While some penalty may be experienced for meat quality, experimental results have been varied to date. The degree of poor meat quality experienced in halothane gene carrier pigs is dependent both on background genetics and pre-slaughter management. Effects in the Pietrain breed, for example, are more severe than in the Belgium Landrace and carrier vs normal genotype differences vary between different Landraces. Summary and Conclusions The pork industry is moving forward to implement price incentives for producers of quality lean pork. These price incentives are based on responses to consumer requirements for lean meat with good processing and eating qualities. Over the past three decades, pig breeders have focused genetic improvement programs on increasing the efficiency of lean pork production. It seems likely that simply aiming to decrease the cost of production of pork muscle will not be adequate to maintain pork s market share in the future. Quality is likely to be an increasingly important issue, with opportunities for niche marketing of pork products using nutritional labeling or specified intramuscular fat levels, for ex- 7= Jensen and Barton-Gade (1985); 8= Rundgren et al. (1990); 9= Satheret al. (1990); 10= Simpson and Webb (1989); 11= Webb (1981). ample. Nevertheless, in order to justify placing selection pressure on pork quality traits, it must be clear that there will be financial rewards for better meat quality in the future. We will continue to select to improve the efficiency of lean pork production while at the same time eliminating the halothane gene from most, if not all, of our basic lines. Monitoring meat quality as defined by color, ph, shear force, water-holding capacity, chemical composition (water, protein, lipid) and taste panel evaluation of juiciness, tenderness and flavor in our pure line genetic pigs has been a routine practice in recent years and will be continued in all major lines. In addition, research and development projects will be initiated to better understand meat quality characteristics and the relationships between these traits and present and potential selection criteria. There are no simple answers to meat quality. If good pork quality is to be the focus of future production systems, then a// stages of production must be organized with meat quality in mind. This will involve using the right breeding stock with the required health status kept in conditions acceptable both to producer and consumer and fed to produce the best potential meat quality. Loading, transport, lairage and slaughter procedures designed to minimize damage to pork quality will need to be implemented. Finally, the final product must be marketed in an appealing way to the consumer. To produce better quality products, contributions at all stages must be recognized by a premium price that at least covers the additional costs involved. Producers will be challenged to achieve the correct balance between production costs and product value and must take steps to increase their understanding of all sectors of the pork chain. Aalhus, J.L.; Jones, S.D.M.; Robertson, W.M.; Tong, A.K; Sather, A.P Growth characteristics and carcass composition of pigs with known genotypes for stress susceptibility over a weight range of 70 to 120 kg. Anim. Prod. 52, References Anonymous Pork Report lo(3). National Pork Producers Council, Des Moines, IA. Bovey, M.; Bichard, M.; Bruce, E.A Genetic effects on pork product composition and quality. Anim. Prod. 46,503 (Abstr.).

7 45th Reciprocal Meat Conference 121 Bratzler, L.J Palatability factors and evaluation. In Science of Meat and Meat Products (Price, J.F., Schweigert, BS, eds., 2nd ed.), W.H. Freeman and Co., San Francisco, CA. p Carden, A Genetics of halothane susceptibility in pigs. Ph.D. Thesis, Univ. Edinburg, 150pp. Cited by Webb, A.J., Southwood, O.I., Simpson, S.P., Carden, A.E Genetics of porcine stress syndrome. In Stress Susceptibility and Meat Quality in Pigs. (Ludvigsen, J.B., ed.) European Assoc. of Anim. Prod. Publ. No. 33, Rome. pp Christensen, A.; Sorensen, D.A.; Vestergaard, T.; van Kemanade, P The Danish pig breeding program: Current system and future developments. Proc. 3rd World Congr. Genet. Appl. Livest. Prod. X, Christian, L.L.; Rothschild, M.F Performance carcass characteristics of normal, stress carrier and stress-susceptible swine. Iowa State Univ. Anim. Sci. Res. Rep. AS-528-F, Ames. DeVol, D.L.; McKeith, F.K.; Bechtel, P.J.; Novakofski, J.; Shanks, R.D.; Carr, T.R Variation in composition and palatability traits and relationships between muscle characteristics and palatability in a random sample of pork carcasses. J. Anim. Sci. 66, Diestre, A Principales problemas de la calidad de la carne en el porcino. Jornadas Cientificas Sepor 91 Serie Congresos 5, Eikelenboom, G.; Minkema, D.; Van Eldik, P; Sybesma, W Performance of Dutch Landrace pigs with different genotypes for the halothane-induced malignant hyperthermia syndrome. Livest. Prod. Sci. 7, Fowler, V.R.; Bichard, M.; Pease, A Objectives in pig breeding. Anim. Prod. 23, Froystein, T.; Schie, K.A.; Nostvold, S.O Halothane sensitivity, blood CPK-values and meat quality characteristics in pigs selected for rate of gain and backfat thickness. Acta Agric. Scand. Suppl. 21, Fujii, J.; Otsu, K.; Zorzto F.; de Leon, S.; Khanna, V.K.; Weiler, J.E.; O Brien, P.J.; MacLennan, D.H Identification of a mutation in porcine ryanodine receptor associated with malignant hyperthermia. Science. 253, Glodek, P Selection responses in pigs: Results and implications. Proc. 2nd World Congr. Genet. Appl. Livest. Prod. V, Goll, D.E Role of proteinases and protein turnover in muscle growth and meat quality. Reciprocal Meat Conference Proceedings, Volume 44, Gomez-Gonzalez, F.; Rhee, K.S.; Denton, J.H.; Gardner, F.A Selected sensory and chemical attributes of chicken breakfast sausages from different animal fat sources. J. Muscle Foods 2, 93. Hazel, L.N The genetic basis for constructing selection indexes. Genetics 28, Henderson, C.R Sire evaluation and genetic trends. Proceedings of Animal Breeding and Genetics Symposium in Honor of Dr. J.L. Lush, ADSA and ASAS. pp Henning, M.; Huster, E.; Baulain, U.; Kallweit, E Evaluation of porcine body composition during growth by means of Magnetic Resonance Tomography (MRT). Paper presented at the 42nd Ann. Meet. Europ. Assoc. Anim. Prod., Berlin, Germany, mimeo PP. 4. Jensen, P Porcine stress syndrome in pig breeding. In: Proc. Symp. Porcine Stress and Meat Quality (Froystein, T., Slinde, E., Sandal, N., ed.) Agric. Food Res. SOC., As, Norway. pp Jensen, P.; Andresen, E Testing methods for PSE syndrome: Current research in Denmark. Livest. Pro Sci. 7, Jensen, P.; Barton-Gade, P.A Performance and carcass characteristics of pigs with known genotypes for halothane susceptibility. In Stress Susceptibility and Meat Quality in Pigs. (Ludvigsen, J.B., ed.) European Assoc. of Anim. Publ. No. 33, Rome. pp Jonsson, P Methods of pig improvement through breeding in the European countries. Livest. Prod. Sci. 2, Kennedy, B.W.; Hudson, G.F.S.; Schaeffer, L.R Evaluation of genetic change in performance tested pigs in Canada. Proc. 3rd World Congr. Genet. Appl. Livest. Prod. X, Land, R.B Knowledge for animal breeding. Phil. Trans. R. SOC. Lond. 6310, Le Roy, P.; Naveau, J.; Elsen, J.M.; Sellier, P Evidence for a new major gene influencing meat quality in pigs. Genet. Res., Camb. 55, Lo, L.L A genetic analysis of growth, real-time ultrasound, carcass and pork quality data in Duroc and Landrace swine. M.S. Thesis, University of Illinois, Urbana. Long, T.; Brandt, H.; Hammond, K Application of best linear unbiased prediction to genetic evaluation in pigs. Pig News and Information 12:2, Lush, J.L Animal Breeding Plans. 3rd Ed. The Collegiate Press, Inc., Ames. MacLennan, D.H; Phillips, M.S Malignant hyperthermia. Science. 256, Marsh, B.B Tenderness. In Meat (Cole, D.J.A., Lawrie, R.A., eds). AVI Publishing Co., Inc. Westport, CT. p Mitchell, G.; Smith, C.; Makower, M.; Bird, P.J.W.N An economic appraisal of pig improvement in Great Britain 1. Genetic and production aspects. Anim. Prod. 35, Ollivier, L Results of a long-term selection experiment for lean tissue growth in the pig. Proc. 3rd World Congr. Genet. Appl. Livest. Prod. XII, Rundgren, M.; Lundstrom, K.; Edfors-Lifja, I A within-litter comparison of the three halothane genotypes. 2. Performance, carcass quality, organ development and long-term effects of transportation and amperozide. Livest. Prod. Sci. 26, Sather, A.P.; Murray, A.C.; Zawadski, S.M.; Johnson, P Meat quality from halothane-negative heterozygous and homozygous pigs raised in a commercial environment. Lacombe Research Highlights. pp Sellier, P Aspects genetiques des qualites technologiques et organoleptiques de la viande chez le porc. Journees Rech. Porcine en France. 20, Simpson, S.P.; Webb, A.J Growth and carcass performance of British Landrace pigs heterozygous at the halothane locus. Anim. Prod. 49, Smith, C Rates of genetic change in farm livestock. J. Agric. Res. Dev. 1, Smith, G.C.; Carpenter, Z.L Eating quality of meat animal products and their fat content. In Fat Content and Composition of Animal Products. NAS, Washington, D.C. Steane, DE Antagonistic traits in pig breeding. Livest. Prod.Sci. 8, Stewart, T.S.; Schinckel, A.P Genetic parameters for swine growth and carcass traits. In Genetics of Swine (L.D. Young, ed.) USDA-ARS, Clay Center, NE. pp 77-79, Tixier, M.; Sellier, P Estimated genetic trends for growth and carcass traits in two French pig breeds. Genet. Sel. Evol. 18, USDA Composition of foods. Pork products: raw, processed, prepared. United States Dept. of Agriculture. Human Nutrition Service. Agric. Handbook No. 8-10, US. Government Printing Office, Washington, D.C. Vestergaard, J Inclusion of meat quality criteria in the aggregate genotype to prevent deterioration of meat quality in the Danish pig breeding program. In Evaluation and Control of Meat Quality in Pigs (Tarrant, P.V., Eikelenboom, G., Monin, G. ed.). Martinus Nijhoff Publ., Dublin, Ireland. p Webb, A.J The halothane sensitivity test. In: Proc. Symp. Porcine Stress and Meat Quality (Froystein, T.; Slinde, E.; Standal, N., eds.) Agric. Food Res. SOC., As, Norway. pp Webb, A.J.; Carden, A.E.; Smith, C.; Imlah, P Porcine stress syndrome in pig breeding. Proc. 2nd World Congr. Genet. Appl. Lives. V, Wiseman, J A History of the British Pig. Gerald Duckworth & Co., Ltd. London, England. Wood, J.D Consequences for meat quality of reducing carcass fatness. In Reducing Fat in Meat Animals (Wood, J.D., Fisher, A.V., eds.) Elsevier Science Publishers Ltd, Essex, England. pp