Review
A review of carcass conformation in sheep:
assessment, genetic control and development
S.J. Nsoso
a,b,*, M.J. Young
b, P.R. Beatson
b aBotswana College of Agriculture, Private Bag 0027, Gaborone, Botswana bAnimal and Veterinary Sciences Group, Lincoln University, Canterbury, New ZealandAccepted 29 June 1999
Abstract
Stud breeders, farmers and meat traders have considered carcass conformation or shape in sheep an important trait. This trait can be assessed on either carcasses or live animals for slaughter and breeding. Nevertheless, there is no single universally accepted de®nition of the term carcass conformation across the sheep industry. The use of the word carcass conformation in the sheep industry is further confused by the fact that different indices are used to describe a complex 3-dimensional shape, which causes variation in the interpretation of results. This review of current knowledge on carcass conformation in sheep will identify areas, which offer opportunities for research. Visual carcass conformation appears to be poorly related to meat yield and is also probably predominantly under non-genetic control and as such has little commercial relevance. Visual carcass conformation assessments appeal to farmers because they are cheap and easy to apply. In contrast, objective conformation and muscularity (another measure of conformation) require measurements, which may be complicated, dif®cult or costly depending on the system under consideration. However, precisely de®ned objective conformation and muscularity, which can be standardised and automated are desirable in breeding and carcass classi®cation schemes. Though, muscularity is highly heritable and is positively related to meat yield, information on objective conformation, muscularity and, their relationships to meat yield and other production traits is not adequate. Furthermore, there is little information on whether objective conformation is under genetic or non-genetic control. Provision of such information would lead to the design of ef®cient sheep production systems.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Carcass conformation; Sheep and meat quality
1. Introduction
Visual carcass conformation or shape in sheep is traditionally an important trait to stud sheep breeders and meat traders (Meat and Livestock Commission, 1987). Historically, there has been confusion as to
whether visual carcass conformation referred to the proportional size of body parts, or the relationship of the thickness of soft tissues to the skeletal size, or both. Therefore, resulting in many different meanings of this trait. Even today animal breeders, meat traders and scientists usually have a personal impression of visual carcass conformation, but rarely can they clearly de®ne this to others (Butter®eld, 1988). This lack of a common de®nition has led to dif®culties in *Corresponding author. Tel.:267-328831; fax:267-328753.
E-mail address: [email protected] (S.J. Nsoso).
interpreting and drawing conclusions from the innu-merable visual carcass conformation studies and pub-lications available (Kempster et al., 1982). To have a common de®nition some countries de®ne visual car-cass conformation as ``the visual assessment of the thickness of muscle and fat in relation to the size of the skeleton'' (Butter®eld, 1988). Skeletal size in carcass conformation is presumed to mean length or width or other linear dimensions of the skeletal system. In addition to lack of a common de®nition, the other shortcoming of the use of the concept of visual carcass conformation in assessment of both live animals and carcasses is the lack of a precise description of com-plex 3-dimensional shape by a simple index. This can result in variations of indices used in assessments (Kempster et al., 1982) depending on such factors as skill of assessor and part(s) of animal assessed. The values of visually assessed carcass conformation in sheep are considered as useful indicators of individual animal carcass composition and various aspects of measures of productivity (e.g. milk production, easy of birth and easy movement). Visually assessed con-formation as a useful indicator of carcass composition has been the subject of many studies in the literature whereas as a useful indicator of productivity it has received little attention despite its paramount impor-tance (Kempster et al., 1982) as means of selecting breeding stock by stud sheep breeders. Therefore, the aim is to review the current state of knowledge on carcass conformation in sheep with the view of iden-tifying areas, which offer opportunities for research.
2. Assessment of visual carcass conformation in live sheep
Butter®eld (1988) arbitrarily divided visual carcass conformation of live animals into that for breeding stock and that for slaughter animals. This arbitrary division was prompted by the differences in de®ni-tions that the people involved in meat production attach to these subdivisions of animal production. The author then pointed out that visual carcass con-formation and that of the live animal are related, yet it is usual for a breeder to talk about an entirely different concept from that of the butcher using the word visual conformation. The author then de®ned visual carcass conformation of the live animal in breeding stock as
``the manner in which the total animal conforms to the preconceived ideal animal by the beholder. Beauty in the eye of the beholder concept''. In contrast the visual carcass conformation of the live animal in slaughter stock, is de®ned as ``thickness of muscle and fat in relation to skeletal size''. In conclusion, there is a feasibility of using a common de®nition for both or choice of a de®nition for the carcass to be a component of the live animal's visual conformation.
2.1. Assessment of visual carcass conformation in carcasses of sheep
con-formation has merit for the prediction of lean to bone ratio andM. longissimusdepth.
The above ®ndings in general agree with those from other sheep studies examining the value of carcass conformation as an indicator of carcass composition within breed, but without adjustment to equal fatness. Findings from these studies have varied, but on bal-ance have indicated that carcass conformation is positively related to meat yield but the degree of correlation is very low, so that predictions are subject to considerable error (Kempster et al., 1982). A num-ber of scientists consider carcass conformation as a
trait of less signi®cance. In contrast meat traders, tend to agree that carcass conformation is an indicator of retail yield and value (Kempster et al., 1987). Visual conformation is included in the carcass classi®cation scheme of sheep in the UK, not so much for its relationship with the saleable meat yield but for its visual appeal to buyers in local and export markets (A. Cuthbertson, personal communication). To have con-sistency of carcass classi®cation and grading for visual conformation, photographic scales are used (Kempster et al., 1982). Animals of good conformation are alleged to have carcasses with more lean meat, a higher proportion of joints and lean meat in the expensive cuts (loin/leg) than animals of poor con-formation. These relationships are said to exist not only within sheep breeds, but also in other species (Meat and Livestock Commission, 1987).
2.2. Relationship between carcass conformation and classification
In the UK meat market, lambs with good visual conformation are valued more highly and receive better prices than those with average or poor confor-mation. A typical price difference between successive visually assessed conformation classes on a 5-class scale would be about 3 p/kg (Meat and Livestock Commission, 1987). These premiums are signi®cant, however, there is little evidence that carcasses of good conformation yield higher returns to the retailer than those of poor conformation. Indeed, at any given level of fatness, there is a tendency for carcasses of good visual conformation to have lower yield of saleable meat than those with poor visual conformation (Table 1). It can be concluded that, although the UK retailers
Fig. 1. Sire breed means for carcass lean proportion against conformation score assessed at equal fatness. Sire breeds are identified as follows: Border LeicesterBL; Dorset DownDD; Hampshire DownHD; Ile de FranceIF; North Country CheviotNCC; Oxford DownOD; SouthdownSD; Suf-folkSF; TexelTX; WensleydaleW; early flocks1; late flocks2; results are averaged over the three dam breeds (adapted from Kempster et al., 1987).
Table 1
The relationship between saleable meat yield and carcass classificationa
Conformation Saleable meat yield in different fat classes (%)
1 (very lean) 2 3L 3H 4 5 (very fat)
E (best) 91.0 90.7 89.5 88.8 87.6
U
R (intermediate) 92.4 91.1 90.1 89.1 87.5 84.7 O
P (worst) 92.5
aSaleable meat yield is expressed as a percentage of carcass weight. Statistical significance between classes are not reported in the original
do not get higher returns in terms of meat yield, they get higher return through the premiums for good than for poor carcass conformation. Based on these pre-miums charged by meat traders, which indicate what consumers are willing to pay, it seems shape per se is important rather than as an indicator of the content of saleable meat in carcass. If this is the case, under what conditions or circumstances is shape per se commer-cially valuable? Meat and Livestock Commission (1987) reported a trial comparing two carcass cutting techniques, i.e. steaking and conventional for car-casses of the same weight and fat class but differing in visually assessed carcass conformation. The steak-ing method clearly demonstrated the superiority of good conformation carcasses in terms of higher sale-able meat yield than poor carcass conformation. The difference in total saleable meat (steaks, breast, scrag, ®llet, lean, trim and mince) was 3% higher in good than poor conformation carcasses. The differences were evident in different parts of the carcass. The good conformation carcasses had signi®cantly higher yield in leg (6% in weight of steaks and 5% in area of steaks) and loin (13% in weight of steaks and 17% in area of steaks) than carcasses of poor conformation. However, the conventional method only demonstrated a 3% signi®cant advantage of good conformation carcasses in weight of leg compared to poor carcasses (Table 2). These differences mean better ®nancial returns from carcasses of good than those of poor visual conformation (Meat and Livestock Commis-sion, 1987). To demonstrate the higher meat yield of good than poor visual conformation carcasses, new cutting techniques like steaking may be desirable.
2.3. Assessment of objective conformation in sheep carcasses
Since the meat trading and farming sectors attach a lot of importance to carcass conformation, when results from visually assessed conformation studies indicate that it is poorly related to meat yield, a few studies have been undertaken to evaluate the relation-ship between objectively assessed conformation and meat yield (Spurlock et al., 1966; Cunningham et al., 1967; Kempster et al., 1982; Hopkins et al., 1997; Abdullah et al., 1998; Tatum et al., 1998). Like subjective conformation, objective conformation is an attempt to describe a complex 3-dimensional shape
with a simple index. However, unlike visual confor-mation it is based on precisely de®ned and standar-dised measurements (De Boer et al., 1974), which leads to easier comparison and interpretation of results from experiments and other sources. Research on
Table 2
Comparison of yield of poor (class O) and good (class U) conformation carcasses from conventional cutting and steaking methodsa
Poor Good Number of carcasses 32 32
Eye muscle depth (mm) 25 28b Carcass length (mm) 599 568 Conventional cuttingc
Weight of leg (kg) 2.05 2.11b
Weight of chump (kg) 0.54 0.55 Weight of loin (kg) 0.80 0.83 Total weight of saleable meat
(includes mince, stew, kidney) (kg)
7.44 7.45
Steaking methodc
Topside
Number of steaks 5.2 5.4 Total weight of steaks (kg) 0.40 0.44b Average area (cm2) 61.4 66.4b
Leg
Number of steaks 9.4 9.8 Total weight of steaks (kg) 0.98 1.04b
Average area (cm2) 63.7 66.8 Loin
Number of steaks 8.1 7.8 Total weight of steaks (kg) 0.71 0.80c
Average area (cm2) 54.1 63.1c
Shoulder
Number of steaks 2.5 2.4 Total weight of steaks (kg) 0.35 0.33 Average area (cm2) 84.9 81.3
All steaks
Number of steaks 25.1 25.4 Total weight of steaks (kg) 2.43 2.60b
Average area (cm2) 63.6 68.0
Total weight of saleable meat by steaking method (Including lean trim, fillet, breast, scrag) (kg)
5.14 5.30b
aCarcasses compared at standard carcass weight of 16.8 kg,
and fat class 3L. Results adapted from Meat and Livestock Commission, 1987.
bA statistically significant difference (P< 0.05).
cSaleable meat from steaking method is without bone but is
objective conformation has characterised conforma-tion in terms of the ratio of carcass weight with length (Kempster et al., 1982; Hopkins et al., 1997; Abdullah et al., 1998; Tatum et al., 1998). Generally, the results have indicated that this is less well related to meat yield than visual conformation scores. However, in carcass assessment objective measurements are easier to standardise and are more easily adapted to auto-matic recording than visual conformation scores, hence there has been a persistent interest in their research and use (Kempster et al., 1982; Harrington and Kempster, 1989; Kirton, 1989; Abdullah et al., 1993; Kirton et al., 1993). Attempts should be made to extend the research to other objective conformation assessments in different parts of the body, which directly measure actual tissue contents.
2.4. Genetic control and development of conformation
There is not much information available on genetic control of either subjectively or objectively assessed conformation in live animals. The studies of Lopez-de-Torre et al. (1991) and Conington et al. (1998) reported low heritability (<0.10) of visual carcass conformation in sheep. Visual carcass conformation in Conington et al. (1998) was based on Meat and Livestock Commission scoring system (Meat and Livestock Commision, 1983), while the basis of car-cass conformation assessment was not reported in Lopez-de-Torre et al. (1991). Nevertheless, such low heritability estimates indicate that response to selection for this trait will be low (Dalton, 1986). Circumstances under which visual carcass conforma-tion has to be included in breeding schemes has to be investigated further since farmers and meat traders attach a lot of importance to it. There is also scanty information on development of either subjectively or objectively assessed conformation in live animals. With regard to visual carcass conformation, Kempster et al. (1981, 1987) concluded that there are minor differences found between British sheep breeds in carcass conformation at equal fatness i.e. same matur-ity stage (Taylor, 1985, 1987), despite selection over long periods of time for different conformation types. In addition the poor relationships between visually assessed carcass conformation and carcass composi-tion found within breeds in their studies, suggested
that selection for carcass composition based on the visual assessment of conformation is unlikely to be effective in sheep (Kempster et al., 1981, 1987). This is a note of concern since farmers use subjectively assessed conformation when selecting their breeding stock.
3. Assessment, development and genetic control of muscularity
Zealand Meat Producer, 1992). Therefore, new tech-niques such as the steaking method shown to produce signi®cantly more meat yield from carcasses of high visual conformation than the traditional method (Meat and Livestock Commission, 1987) need to be developed.
Young (1990) reported increases in muscularity with increase in age in a single sheep genotype (Suf-folk). More recently, Abdullah et al. (1998) and Sailer et al. (1995) reported muscularity differences between Southdown rams selected for high and low backfat depth and foetuses of ®ve sheep genotypes (Merino, Romney, MerinoRomney, Drysdale and Wiltshire) respectively. The ®ndings of Abdullah et al. (1998) were that muscularity (characterised as muscle depth relative to bone length) was signi®cantly greater for the high line than the low backfat line in anatomical areas around and besides the femur, tibia, total pelvic area and scapula but not in the radius and humerus areas. Also the level of muscularity increased with carcass weight which implies that muscularity increases with age hence being consistent with the study of Young (1990), that reports this phenomenon. Salier et al. (1995) concluded that the Merino group had lower scores than the other four breeds. Further-more, females had lower scores than males. The study of Salier et al. (1995) only covered foetal development from 69 to 146 days, while that of Abdullah et al. (1998) characterised muscularity in selected lines, so it is not clear whether these differences will be man-ifested throughout growth and what the differences are at slaughter weights in normal sheep genotypes, which is the most commercially relevant end point for meat producing sheep genotypes. The study of Hopkins (1996) assessed muscularity in 57 lamb carcasses (averaging 22.5 kg) representing two sexes (cryp-torchids and ewes) of one sheep genotype (Poll Dor-setBorder LeicesterMerino). The conclusions from this study were that muscularity increased with age, which supports the ®nding of Young (1990) who reported a similar phenomenon. There was no effect of sex on muscularity, which was consistent with the results of Purchas and Wilkin (1995) and the overall yield from hindquarters was the same for the sexes. However, cryptorchids carcasses produced signi®-cantly heavier round and midlion cuts but signi®signi®-cantly lighter chump and ribloin cuts. Since different cuts receive different premiums, there is need to further
characterise muscularity in more sheep genotypes during development because sheep meat is commer-cially important worldwide. A study comparing mus-cularity calculated as average muscle depth divided by bone length (Purchas et al., 1991) in six sheep geno-types (Poll DorsetBorder Leicester, Texel Bor-der Leicester, Poll DorsetMerino, TexelMerino, Border LeicesterMerino and MerinoMerino) was reported by Hopkins et al. (1997). An important ®nding from this study was that Texel sired progeny had signi®cantly higher muscularity values than those sired by Poll Dorset at the same adjusted mean carcass weight within sex. Furthermore, carcasses of Texel sired progeny had signi®cantly more muscle in the hindleg than those sired by Poll Dorset, with the differences being greater in the heavier cryptorchids (1.8%) than ewes (1.1%). The study of Kirton et al. (1997), which compared crossbred progeny from 39 sire breeds and strains crossed with Romney ewes has also shown that Texel and Poll Dorset crossbred progeny had the highest proportion of muscle in leg and shoulder cuts. These ®ndings are commercially important and there is need to carry out more of this research in more diverse sheep genotypes. Waldron et al. (1992) reported heritability estimates of muscular-ity based on linear dimensions of muscle and carcass length, which ranged from 0.43 to 0.68. These esti-mates are high, therefore, genetic response to selection can be achieved.
4. Opportunities for research
Considerable research has been carried out on visual assessed carcass conformation. The bulk of the results indicated that this is poorly related to meat yield and as such it is not useful to include in breeding schemes and trading terms. In contrast objectively assessed carcass conformation and muscularity, are related to meat yield and can be standardised and as such have commercial relevance. However, to date there has been scanty research on these two aspects of carcass conformation. Additional work that need to be done include:
developmental growth and by comparing diverse sheep genotypes;
2. determine the extent of genetic control (herit-ability) for objective carcass conformation and muscularity, and their genetic correlations with other body composition traits in diverse sheep genotypes;
3. determine the sources of error and bias in live animal assessment of objective carcass conforma-tion and muscularity in live and slaughter sheep; 4. assess ways in which this information could be
used in breeding programmes and commercial carcass classi®cation schemes at sheep meat plants and abattoirs, with the aim of enhancing the production of higher value sheep meat products than at present.
5. Conclusions
Despite the fact that there is no universally accepted de®nition of visual carcass conformation, sheep bree-ders, farmers and meat traders are continuously using this term. Therefore, there is need to adopt such a de®nition to enable easy communication of carcass data, research results and trading terms in the sheep industry. For scientists and sheep breeders, both visual and objective carcass conformation probably have little commercial application because of their poor relationship to meat yield. Furthermore, visual con-formation is probably predominantly under non-genetic control hence its use in breeding schemes is of little value. Therefore, efforts would be well expended if they would be concentrated on muscu-larity, which is another measure of conformation. There is a lot of interest in objective conformation and muscularity because these are precisely de®ned, can be standardised and automated, which is desirable in breeding and carcass classi®cation schemes. Mus-cularity has been shown to be highly heritable, which means fast responses to selection are possible. Com-bining muscularity and modern carcass steaking tech-niques would provide a tremendous boost to farmers incomes if they trade in animals of high muscularity. It is therefore, important that there should be more research than at present focussing on objective carcass conformation and muscularity with the view to utilise results to increase meat production from sheep.
Acknowledgements
The authors would like to thank Dr. A.A. Aganga and Dr. R.G. Chabo for their comments.
References
Abdullah, A.Y., Purchas, R.W., Davies, A.S., 1998. Patterns of change with growth for muscularity and other composition characteristics of Southdown rams selected for high and low backfat depth. N.Z. J. Agric. Res. 41, 367±376.
Abdullah, A.Y., Purchas, R.W., Davies, A.S., Kirton, A.H., 1993. Relationships between objective and subjective measurements of the carcasses muscularity. Proc. N.Z. Soc. Anim. Prod. 53, 397±402.
Butterfield, R.M., 1988. New Concepts of Sheep Growth. University of Sydney Press, Sydney, 168 pp.
Conington, J., Bishop, S.C., Waterhouse, A., Simm, G., 1998. A comparison of growth and carcass traits in Scottish Blackface lambs sired by genetically lean or fat rams. Anim. Sci. 67, 299± 309.
Cunningham, N.L., Carpenter, Z.L., King, G.T., Butler, O.D., Shelton, J.M., 1967. Relationship of linear measurements and certain carcass quality and tenderness of ewe, whether and rams lambs. Anim. Sci. 26, 683±687.
Dalton, D.C., 1986. An Introduction to Practical Animal Breeding, 2nd ed. BSP Professional Books, Oxford, 182 pp.
De Boer, H., Dumont, B.L., Pomeroy, R.W., Weniger, J.H., 1974. Manual on E.A.A.P. reference methods for the assessment of carcass characteristics in cattle. Livest. Prod. Sci. 1, 151± 164.
Harrington, G., Kempster, A.J., 1989. Improving lamb carcass composition to meet modern consumer demand. In: Dyrmunds-son, O.R., ThorgeirsDyrmunds-son, S. (Eds.), Reproduction, Growth and Nutrition in sheep. Dr. Halldor Palsson Memorial Publication, Agricultural Research Institute and Agricultural Society, Reykjavik, Iceland, pp. 79±90.
Hopkins, D.L., 1996. The relationship between muscularity, muscle:bone ratio, muscle:bone ratio and cut dimensions in male and female lamb carcasses and the measurement of muscularity using image analysis. Meat Sci. 44, 307±317. Hopkins, D.L., Fogarty, N.M., Menzies, D.J., 1997. Differences in
composition, muscularity, muscle:bone ratio, muscle:bone ratio and cut dimensions between six lamb genotypes. Meat Sci. 45, 439±450.
Kempster, A.J., Croston, D., Jones, D.W., 1981. Value of conformation as an indicator of sheep carcass composition within and between breeds. Anim. Prod. 33, 39±49.
Kempster, A.J., Croston, D., Guy, D.R., Jones, D.W., 1987. Growth and carcass characteristics of crossbred lambs by ten sire breeds, compared at the same estimated carcass subcutaneous fat proportion. Anim. Prod. 44, 83±98.
Kirton, A.H., 1989. Current methods of on-line carcass evaluation. Anim. Sci. 67, 2155±2163.
Kirton, A.H., Duaganzich, D.M., Mercer, G.J.K., Clarke, J.N., Dobbie, J.L., Wilson, J.A., 1997. Muscle distribution in lamb progeny from several breeds. Proc. N.Z. Soc. Anim. Prod. 57, 267.
Kirton, A.H., Mercer, G.J.K., Duaganzich, D.M., Ulijee, A.E., 1993. The use of grading probes for classifying lamb carcasses. Proc. N.Z. Soc. Anim. Prod. 53, 393±396.
Lopez-de-Torre, G., Albardonezo-Rico, R., Espejo-Diaz, M., Mateos-Mateos, I., 1991. Genetic parameters of daily gain and conformation score in early maturing sheep breeds. ITEA-Production-Animal 87A; 2±3, 242±247.
Meat and Livestock Commission, 1983. Sheep Yearbook, Meat and Livestock Commission. Milton Keynes, Bletchley, pp 61±71. Meat and Livestock Commission, 1987. Sheep Yearbook, Meat and
Livestock Commission. Milton Keynes, Bletchley, pp 39±42. Purchas, R.W., Davies, A., Abdullah, A.Y., 1991. An objective
measure of muscularity: changes with animal growth and differences between genetic lines of Southdown sheep. Meat Sci. 30, 81±94.
Purchas, R.W., Wilkin, G.H., 1995. Characteristics of lamb carcasses of contrasting subjective muscularity. Meat Sci. 41, 357±368.
Sailer, S., Cottam, Y.H., Purchas, R.W., McCutcheon, S.N., 1995. Developmental patterns of muscle, bone, and organ growth in fetal lambs of 5 genotypes. N.Z. J. Agric. Res. 38, 225±236. Spurlock, G.M., Bradford, G.E., Wheat, J.D., 1966. Live animal
and carcass measurements for the prediction of carcass traits in lambs. Anim. Sci. 25, 454±459.
Tatum, J.D., Samber, J.A., Gillmore, B.R., LeValley, S.B., Williams, F.L., 1998. Relationship of visual assessments of feeder lamb muscularity to differences in carcass yield traits. Anim. Sci. 76, 774±780.
Taylor, St.C.S., 1985. Use of genetic size scaling in evaluation of animal growth. Anim. Sci. 61(2), 118±143.
Taylor, St.C.S., 1987. An evaluation of genetic size scaling in breed and sex comparisons of growth, food and body composition. Proc. Aust. Assoc. Anim. Breed. Genet. 6, 1±12.
The New Zealand Meat Producer, 1992. Quality at a premium. The New Zealand Meat Producers Board 20 (3), 18.
Waldron, D.F., Clarke, J.N., Rae, A.L., Woods, E.G., 1992. Selected responses in carcass composition to selection for muscularity in sheep. Proc. N.Z. Soc. Anim. Prod. 52, 29± 31.
