A review of carcass conformation in sheep: assessment, genetic control and development

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1 Small Ruminant Research 35 (2000) 89±96 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 a Botswana College of Agriculture, Private Bag 0027, Gaborone, Botswana b Animal and Veterinary Sciences Group, Lincoln University, Canterbury, New Zealand Accepted 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 * Corresponding author. Tel.: ; fax: address: snsoso@temo.bca.bw (S.J. Nsoso). 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 /00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S (99)

2 90 S.J. Nsoso et al. / Small Ruminant Research 35 (2000) 89±96 interpreting and drawing conclusions from the innumerable visual carcass conformation studies and publications available (Kempster et al., 1982). To have a common de nition some countries de ne visual carcass 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 complex 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 conformation 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 importance (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 identifying 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 nitions that the people involved in meat production attach to these subdivisions of animal production. The author then pointed out that visual carcass conformation 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 Assessment of visual carcass conformation in carcasses of sheep After examining the literature on visually assessed carcass conformation as an indicator of carcass composition in sheep, Kempster et al. (1982) suggested that studies on this value of conformation should be aimed at addressing the question: ``What additional information is provided about carcass composition by an assessment of visual conformation among carcasses of the same weight and fatness?'' Assessment of visual conformation at equal fatness is the most appropriate because animals are of the same maturity (Taylor, 1985, 1987) hence they are at the same stage of growth and development (Kempster et al., 1987). Few workers have examined their data in this way. The few studies that have assessed visual carcass conformation of crossbred progeny in between sire-breed comparison studies at equal fatness and at the same weight in some instances. The ndings were the same. There was relatively little difference between sirebreed means for visually assessed conformation, all means were approximately within two points on a 15- point scale (Fig. 1). Furthermore, there was very little relationship between sire-breed means for visually assessed carcass conformation and lean proportion in carcass (Fig. 1) or proportion of lean in the higher than lower priced cuts both within and between breeds (Kempster et al., 1981, 1987). The prediction was sensitive to the way fatness was controlled. Poor control of fatness led to visual conformation identifying fatter rather than leaner carcasses. This is an undesirable outcome but inevitable in commercial classi cation, when visually assessed conformation scores are used to further classify carcasses in one of the limited number of fat classes (Kempster et al., 1981). However, Kempster et al. (1981) and Purchas and Wilkin (1995) found that visually assessed con-

3 S.J. Nsoso et al. / Small Ruminant Research 35 (2000) 89±96 91 Fig. 1. Sire breed means for carcass lean proportion against conformation score assessed at equal fatness. Sire breeds are identified as follows: Border Leicester ˆ BL; Dorset Down ˆ DD; Hampshire Down ˆ HD; Ile de France ˆ IF; North Country Cheviot ˆ NCC; Oxford Down ˆ OD; Southdown ˆ SD; Suffolk ˆ SF; Texel ˆ TX; Wensleydale ˆ W; early flocks ˆ 1; late flocks ˆ 2; results are averaged over the three dam breeds (adapted from Kempster et al., 1987). formation has merit for the prediction of lean to bone ratio and M. longissimus depth. 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 balance 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 number 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 consistency 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 conformation. These relationships are said to exist not only within sheep breeds, but also in other species (Meat and Livestock Commission, 1987) 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 conformation. 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 Table 1 The relationship between saleable meat yield and carcass classification a Conformation Saleable meat yield in different fat classes (%) 1 (very lean) 2 3L 3H 4 5 (very fat) E (best) U R (intermediate) O P (worst) 92.5 a Saleable meat yield is expressed as a percentage of carcass weight. Statistical significance between classes are not reported in the original document. Results adapted from Meat and Livestock Commission (1987).

4 92 S.J. Nsoso et al. / Small Ruminant Research 35 (2000) 89±96 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 premiums 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 commercially valuable? Meat and Livestock Commission (1987) reported a trial comparing two carcass cutting techniques, i.e. steaking and conventional for carcasses of the same weight and fat class but differing in visually assessed carcass conformation. The steaking method clearly demonstrated the superiority of good conformation carcasses in terms of higher saleable 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 Commission, 1987). To demonstrate the higher meat yield of good than poor visual conformation carcasses, new cutting techniques like steaking may be desirable 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 relationship 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 Table 2 Comparison of yield of poor (class O) and good (class U) conformation carcasses from conventional cutting and steaking methods a Poor Good Number of carcasses Eye muscle depth (mm) b Carcass length (mm) Conventional cutting c Weight of leg (kg) b Weight of chump (kg) Weight of loin (kg) Total weight of saleable meat (includes mince, stew, kidney) (kg) Steaking method c Topside Number of steaks Total weight of steaks (kg) b Average area (cm 2 ) b Leg Number of steaks Total weight of steaks (kg) b Average area (cm 2 ) Loin Number of steaks Total weight of steaks (kg) c Average area (cm 2 ) c Shoulder Number of steaks Total weight of steaks (kg) Average area (cm 2 ) All steaks Number of steaks Total weight of steaks (kg) b Average area (cm 2 ) Total weight of saleable meat by steaking method (Including lean trim, fillet, breast, scrag) (kg) b a Carcasses compared at standard carcass weight of 16.8 kg, and fat class 3L. Results adapted from Meat and Livestock Commission, b A statistically significant difference (P < 0.05). c Saleable meat from steaking method is without bone but is bone-in from conventional method. with a simple index. However, unlike visual conformation it is based on precisely de ned and standardised measurements (De Boer et al., 1974), which leads to easier comparison and interpretation of results from experiments and other sources. Research on

5 S.J. Nsoso et al. / Small Ruminant Research 35 (2000) 89±96 93 objective conformation has characterised conformation 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 automatic 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 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 Lopezde-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 carcass 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 conformation 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 maturity 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 composition 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 Control of fatness in assessment of objective conformation leads to assessment of muscularity. Muscularity is another measure of conformation. A working de nition of muscularity proposed by De Boer et al. (1974) is ``the thickness of muscle in relationship to skeletal size''. However, there have been dif culties using this de nition under practical situations due to dif culties in measuring average muscle depth in intact carcasses (Abdullah et al., 1993). All the information on muscularity in the literature reported here is based on objective measures of muscularity. Like good conformation carcasses, carcasses of good muscularity are alleged to have better yielding characteristics, which, provide the cuts the markets wants (The New Zealand Meat Producer, 1992; Purchas and Wilkin, 1995). This has prompted The New Zealand meat producers board to introduce a muscling class ``E'' to further identify well muscled carcasses that are 13.5 kg or greater with 12 mm or less soft tissue over the rib at the GR site (Waldron et al., 1992). Purchas and Wilkin (1995) reported a study where 114 E and 114 non-e lamb carcasses were individually matched for weight and fatness and then were compared for yield of the leg and saddle cuts and muscularity. The mean lamb carcass weight for both groups was 17 kg. The nding from this study was that the E group relative to the non-e group had 4.3% higher yield of leg and saddle cuts and 14.2% higher leg muscularity index (calculated from the weights of trimmed boneless cuts around the femur and femur length). The conclusion from this study was that higher muscularity led to only a slightly higher meat yield. This is consistent with the feeling within some sectors of the meat industry in New Zealand, that the present carcass measuring techniques are not sensitive enough to detect commercially signi cant differences between different muscularity classes (The New

6 94 S.J. Nsoso et al. / Small Ruminant Research 35 (2000) 89±96 Zealand Meat Producer, 1992). Therefore, new techniques 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 (Suffolk). 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, Merino Romney, 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. Furthermore, 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 manifested 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 (cryptorchids and ewes) of one sheep genotype (Poll Dorset Border Leicester Merino). 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 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 commercially important worldwide. A study comparing muscularity calculated as average muscle depth divided by bone length (Purchas et al., 1991) in six sheep genotypes (Poll Dorset Border Leicester, Texel Border Leicester, Poll Dorset Merino, Texel Merino, Border Leicester Merino and Merino Merino) 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 muscularity based on linear dimensions of muscle and carcass length, which ranged from 0.43 to These estimates 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: 1. determining more clearly the relationships between objective carcass conformation, muscularity, body composition, lean meat yield, retail cuts and prices consumers are willing to pay during

7 S.J. Nsoso et al. / Small Ruminant Research 35 (2000) 89±96 95 developmental growth and by comparing diverse sheep genotypes; 2. determine the extent of genetic control (heritability) 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 conformation 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 breeders, 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 conformation is probably predominantly under nongenetic control hence its use in breeding schemes is of little value. Therefore, efforts would be well expended if they would be concentrated on muscularity, 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. Muscularity has been shown to be highly heritable, which means fast responses to selection are possible. Combining muscularity and modern carcass steaking techniques 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., 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., Relationships between objective and subjective measurements of the carcasses muscularity. Proc. N.Z. Soc. Anim. Prod. 53, 397±402. Butterfield, R.M., New Concepts of Sheep Growth. University of Sydney Press, Sydney, 168 pp. Conington, J., Bishop, S.C., Waterhouse, A., Simm, G., 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., Relationship of linear measurements and certain carcass quality and tenderness of ewe, whether and rams lambs. Anim. Sci. 26, 683±687. Dalton, D.C., 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., 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., Improving lamb carcass composition to meet modern consumer demand. In: Dyrmundsson, O.R., Thorgeirsson, 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., 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., 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., 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., 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. Kempster, A.J., Cuthbertson, A., Harrington, G., Carcass Evaluation in Livestock Breeding, Production and Marketing. Granada Publishing Limited, London, 306 pp.

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