Intramuscular fat levels in sheep muscle during growth

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1 Australian Journal of Experimental Agriculture, 2008, 48, CSIRO PUBLISHING Intramuscular fat levels in sheep muscle during growth M. J. McPhee A,B, D. L. Hopkins A,C and D. W. Pethick A,D,E A Australian Sheep Industry Cooperative Research Centre, Armidale, NSW 2350, Australia. B NSW Department of Primary Industries, Beef Industry Centre of Excellence, JSF Barker Building, NSW 2351, Australia. C NSW Department of Primary Industries, Centre for Sheep Meat Development, Cowra, NSW 2794, Australia. D Department of Veterinary Biology and Biomedical Science, Murdoch University, South Street, Murdoch, WA 6150, Australia. E Corresponding author. D.Pethick@murdoch.edu.au Abstract. A5 4 factorial experiment was designed in which lambs representing five genotypes were slaughtered at four ages (110, 236, 412 and 662 days of age). The genotypes represented were Poll Dorset growth Border Leicester Merino, Poll Dorset growth Merino, Poll Dorset muscling Merino, Merino Merino and Border Leicester Merino. Both sexes (ewes and wethers) were represented for each genotype and slaughter age combination. In total, 595 animals were slaughtered and the carcass composition and intramuscular fat were measured. Carcass composition [fat, ash and protein (lean)] was determined by dual energy X-ray absorptiometry, with the intramuscular fat percentage determined using nearinfrared spectroscopy following removal and weighing of the entire longissimus thoracis et lumborum (LL) muscle. Analysis revealed that the proportion of intramusular fat in the loin relative to total carcass fat decreases as animals mature, thus indicating that intramusular fat deposition occurs early in the maturation of sheep. Furthermore, as animals became heavier and older the accretion rate of intramuscular fat in the LL muscle slowed down. Both genotype (P < 0.05) and sex (P < 0.001) were found to impact on this pattern, with Border Leicester Merino animals exhibiting the largest increase in intramuscular fat proportion in the LL muscle (4.92 and 5.50% at 22 months of age for ewes and wethers, respectively). The Poll Dorset growth Border Leicester Merino animals were found to have the greatest absolute levels of intramuscular fat in the whole LL muscle (80.95 and g at maturity for ewes and wethers, respectively). The amount of intramuscular fat significantly increased as the sheep became older and fatter; however, these differences were quantitatively small. As such, finishing prime lambs to high levels of total carcass fatness would have little effect on any eating quality benefits associated with increased intramuscular fat proportion. Introduction Many carcass grading systems, particularly for beef, use marbling score (intramuscular fat) as a major factor in predicting eating quality. Despite this, the literature suggests that marbling is only one component of eating quality. For example, Dikeman (1987) concluded that marbling accounted for only % of the variance in palatability. The research undertaken by Meat Standards Australia would agree (Thompson 2002). However, Thompson (2004) concluded that as variations in tenderness are controlled by schemes implemented by Meat Standards Australia, marbling will become a more important determinant of palatability due to its specific contribution to juiciness and flavour. A concern raised by the Australian lamb industry was that very low proportions of intramuscular fat (IMF) could lead to meat that is perceived as dry and less tasty. Such a situation has been found in young, highly muscled cattle such as double-muscled cattle genotypes, young bulls from Belgian Blue and Blonde d Aquitaine, and in many cuts from modern pig genotypes (Channon et al. 2001). The minimum requirement for ether extractable fat in order to achieve acceptable consumer satisfaction for grilling red meat cuts, such as beef and lamb, is quoted at 3--4% by Savell and Cross (1986) and at 5% for sheepmeat (Hopkins et al. 2006). For pork, a value of % is cited (Bejerholm and Barton-Gade 1986) on a fresh uncooked basis while a maximum level of 3.5% is thought to achieve optimal consumer acceptability (Fernandez et al. 1999). To evaluate the IMF levels in sheep muscle (M. longissimus thoracis et lumborum; LL) during growth, a serial experiment was conducted. Animals for sheep meat production, representing the major genotypes in Australia, were serially slaughtered at 4, 8, 14, and 22 months of age. Production and meat quality aspects have been detailed by Hopkins et al. (2007a, 2007b) and carcass aspects by Ponnampalam et al. (2007a, 2007b). Some of the preliminary results have also been reported (Pethick et al. 2007). The aims of this study were to examine the weight and concentration of IMF in sheep LL muscle during growth and investigate the differences between sex and genotype in relationship to the proportion of maturity of total carcass fat (PropMat). Materials and methods Full experimental details have been provided by Hopkins et al. (2007a). Briefly, a 5 4 factorial experiment was designed in Ó CSIRO /EA /08/070904

2 Intramuscular fat levels in sheep muscle Australian Journal of Experimental Agriculture 905 which lambs representing five genotypes were slaughtered at four ages (110, 236, 412 and 662 days of age or 4, 8, 14 and 22 months of age). The genotypes represented were Poll Dorset growth Border Leicester Merino (PD g BLM), Poll Dorset growth Merino (PD g M), Poll Dorset muscling Merino (PD m M), Merino Merino (M M) and Border Leicester Merino (BL M). Two genders (ewes and wethers) per genotype and slaughter weight were considered. After weaning, the lambs grazed as one flock on a combination of lucerne and pasture grasses supplemented with oats, lupins, wheat and lucerne hay as detailed by Hopkins et al. (2007a). Slaughter allocation All lambs were weighed before weaning in September 2003 and, based on this weight, were randomly allocated in a stratified way to the four slaughter age groups, balanced for rearing type, gender and sire. The first age group of progeny were slaughtered in November 2003 as suckers (unweaned; mean 110 days of age), with the other age groups scheduled for March 2004 (weaned lambs; mean 236 days of age), September 2004 (mean 412 days of age) and May 2005 (mean 662 days of age). Slaughter protocol Animals to be slaughtered in each age group were allocated to two slaughter days (Tuesday and Thursday) and two slaughter sessions within slaughter days based on stratified weight and balanced for sire. Lambs to be slaughtered on a particular day were yarded the day before, held for 3 h and then weighed. They were then allowed to drink for 2--3 h before being transported 180 km to a commercial abattoir (3 h trip) where they were held in lairage overnight and slaughtered the following day. The slaughter sessions within slaughter days were within 1--2h of each other and the oldest group of animals was slaughtered at a different abattoir to the three younger groups, due to their heavy liveweights. All lambs were electrically stunned (head only) with the time from mustering to stunning being on average 22.5 and 25.5 h for the two slaughter days respectively. Carcasses were trimmed according to the specifications of AUS-MEAT (Anon. 1992), hot carcass weights were recorded and the GR (total tissue depth over the 12th rib, 110 mm from the midline) measured using a GR probe. Carcasses were chilled at a mean temperature of 4--5 C. Sampling and meat quality measurements For the first three slaughters (4-, 8- and 14-month-old animals), the left side of each whole carcass was partially boned on the rail at 1 day post mortem leaving the long bones in the hindleg (femur and tibia) and the scapula and long bones in the forequarter (radius--ulna and humerus). The carcasses of the oldest animals (22 months) were split in half along the spine with a bandsaw. The entire right sides of carcasses were transported chilled by road to Werribee (Department of Primary Industries, Victoria). The bone mineral content, as well as the fat, lean and total tissue mass, was subsequently determined by dual energy X-ray absorptiometry (DXA) using a Hologic QDR 4500A fan beam X-ray bone densitometer (Hologic, Waltham, MA, USA) with the half carcasses positioned cut surface down on the DXA table. The whole LL was subsequently isolated and the muscle weighed after all subcutaneous fat had been removed. Predictions of the fat, lean and bone mineral weights were made using the DXA, as previously reported by Suster et al. (2004) based on models which relate DXA estimates to chemically derived values, as outlined by Dunshea et al. (2007). A section of the LL (~50 g) from the caudal end was collected for determination of IMF and frozen at 20 C. The caudal site has been extensively used in previous research where IMF has been measured (e.g. Hopkins et al. 2005a). The percentage of IMF was determined by nearinfrared spectroscopy as described by Perry et al. (2001) and the weight of fat in the whole LL calculated. Statistical analysis The mean total carcass fat at 22 months for each genotype and sex was used as an index of maturity to calculate the PropMat (proportion of maturity of total carcass fat) (Butterfield 1988) for each individual animal. The calculation was as follows: PropMat ¼ individual carcass fat=mean carcass fat at 22 months Non-linear regression was applied to the amount of IMF (g) in the LL, with the independent variables PropMat, genotype, sex, and their two-way interactions. One outlier for the weight of IMF (more than 3 standard deviations from the mean) was removed. Both the weight of IMF in the LL and PropMat were subjected to a log transformation. The models were developed using linear regression (S-Plus version 7.0 for Windows; Insightful Corporation, Seattle, WA). Results In total, 595 animals were slaughtered over the four age groups with the sample size, hot carcass weight (kg), total body fat (kg), LL (kg), and IMF proportion (%) in the LL reported for each genotype and sex (Tables 1 and 2). The general trend (non-linear curve; Fig. 1a) of the proportion of IMF in the loin relative to total carcass fat decreases as animals mature, thus indicating that IMF is early maturing. In terms of the weight of IMF in the loin the rate of accretion declined as maturity was reached (Fig. 1b). From the regression analysis the following terms had a significant effect on IMF levels PropMat (P < 0.001), genotype (P < 0.05), sex (P < 0.001), and their interactions: PropMat genotype (P < 0.01) and PropMat sex (P < 0.01). The model terms and their coefficients are as follows: Log½ðweight of IMF in the LLðkgÞŠ ¼ 2:12ð0:06Þþ0:57ð0:05Þ logðpropmatþ 0:02ð0:01Þ genotype 0:19ð0:03Þ sex þ 0:03ð0:01Þ logðpropmatþ genotype 0:08ð0:03Þ logðpropmatþ sexðs:e: ¼ 0:24; R 2 ¼ 0:73Þ The differences between genotypes and sexes for the weight of IMF (g) in the LL across the range in PropMat from 0.2 to 1.2 are shown in Table 3. The predicted means (Table 3) show that the amount of IMF (g) in the LL is higher for wethers than ewes and among genotypes for PD g BLM than BL M sheep. However,

3 906 Australian Journal of Experimental Agriculture M. J. McPhee et al. Table 1. Hot carcass weight, total body fat, longissimus muscle (LL) weight and intramuscular fat proportion in the LL muscle by ewe age and genotype Means within slaughter age followed by different letters are significantly different at P = PD g BLM, Poll Dorset growth Border Leicester Merino; PD m M, Poll Dorset muscling Merino; PD g M, Poll Dorset growth Merino; M M, Merino Merino; BL M, Border Leicester Merino. Values are means s.e. Genotype 4 months 8 months 14 months 22 months n n n n Hot carcass weight (kg) PD g BLM c c c c 1.18 PD m M b b b b 0.80 PD g M b b b b 1.26 M M a a a a 0.84 BL M b b b b 1.07 Total body fat (kg) PD g BLM b c c b 0.68 PD m M c b b c 0.34 PD g M c b b c 0.42 M M ac a a a 0.34 BL M bc b b b 0.57 LL muscle weight (kg) PD g BLM c d b c 0.04 PD m M b c b c 0.06 PD g M b cd b c 0.09 M M a a a a 0.05 BL M b b b b 0.08 Intramuscular fat proportion in the LL (%) PD g BLM ac PD m M a PD g M a a 0.19 M M a BL M bc b 0.24 Table 2. Hot carcass weight, total body fat, longissimus muscle (LL) weights and intramuscular fat proportion in the LL by wether age and genotype Means within slaughter age followed by different letters are significantly different at P = PD g BLM, Poll Dorset growth Border Leicester Merino; PD m M, Poll Dorset muscling Merino; PD g M, Poll Dorset growth Merino; M M, Merino Merino; BL M, Border Leicester Merino. Values are means s.e. Genotype 4 months 8 months 14 months 22 months n n n n Hot carcass weight (kg) PD g BLM c d d d 0.90 PD m M b bc b bc 1.24 PD g M bc c c c 0.83 M M a a a a 1.36 BL M ab b bc b 1.04 Total body fat (kg) PD g BLM c c d c 0.60 PD m M b b c b 0.51 PD g M bc b b b 0.50 M M a a a a 0.78 BL M b b bd b 0.62 LL muscle weight (kg) PD g BLM c d c c 0.05 PD m M b c b b 0.05 PD g M bc c c bc 0.06 M M a a a a 0.05 BL M a b b a 0.05 Intramuscular fat proportion in the LL (%) PD g BLM b b a 0.19 PD m M ab a a 0.18 PD g M a ab a 0.25 M M ab ab ac 0.29 BL M ab b bc 0.26

4 Intramuscular fat levels in sheep muscle Australian Journal of Experimental Agriculture 907 Proportion of intramuscular fat in the loin/total carcass fat (%) Intramuscular fat in the loin (g) (a) (b) PD BLM PD(muscling) M PD(growth) M M M BL M PD BLM PD(muscling) M PD(growth) M M M BL M Proportion of maturity of total carcass fat Fig. 1. Relationship of the proportion of maturity of total carcass fat for: (a) the proportion of the amount of intramuscular fat in the LL relative to total carcass fat (%); and (b) the accretion of intramuscular fat in the LL (g) according to genotype and sex (wether: not filled or not bold; ewe: filled or bold). the percentage of IMF in the LL is generally higher in the BL M genotype (Tables 1 and 2). Discussion An increase in IMF proportion as animals age is expected and consistent with previous reports (Martínez-Cerezo et al. 2005; Pethick et al. 2005; Hopkins et al. 2007b; Okeudoand Moss 2007), although there is a large degree of overlap between animals of differing ages (Hopkins et al. 2005b). Data, from fatty acid composition of lambs presented by Díaz et al. (2005), showed variation in IMF between breeds. Even though the comparison Table 3. Predicted means of the intramuscular fat content (g) in the whole longissimus muscle (LL) according to genotype and sex at increments of proportion of total carcass fat maturity calculated by non-linear regression PD g BLM, Poll Dorset growth Border Leicester Merino; PD m M, Poll Dorset muscling Merino; PD g M, Poll Dorset growth Merino; M M, Merino Merino; BL M, Border Leicester Merino. Values are least square means with significant differences between genotype (P < 0.05) and between sex (P < 0.001) Genotype Wether (g) by Díaz et al. (2005) was confounded by differences in production systems, the differences between breeds were of similar magnitude to those presented in this study. The clear impact of the Border Leicester on IMF percentage (BL M; Tables 1 and 2) is consistent with overall carcass fat levels which have been reported (Hopkins and Fogarty 1998; Hopkins et al. 2007b; Ponnampalam et al. 2007b) for this breed. In this study, the comparison of sex and genotype effects on IMF levels in sheep muscle duringgrowth is uniquegiventhat theanimals inthecurrent study were from the same lambing. The animals were run together over the life of the experiment under a regime designed to ensure positive growth over the entire length of the study. Sex Ewe (g) Proportion of maturity of total carcass fat (0.20) PD g BLM PD m M PD g M M M BL M Proportion of maturity of total carcass fat (0.40) PD g BLM PD m M PD g M M M BL M Proportion of maturity of total carcass fat (0.60) PD g BLM PD m M PD g M M M BL M Proportion of maturity of total carcass fat (0.80) PD g BLM PD m M PD g M M M BL M Proportion of maturity of total carcass fat (1.0) PD g BLM PD m M PD g M M M BL M Proportion of maturity of total carcass fat (1.2) PD g BLM PD m M PD g M M M BL M

5 908 Australian Journal of Experimental Agriculture M. J. McPhee et al. Several studies (Thompsonet al. 1987; Butterfield 1988) have investigated the maturity patterns of fat partitions in Merino sheep; for example, the partitioning of muscle fat (chemical fat) for rams and ewes. This study confirms that IMF deposition occurs early in maturation of the sheep as reported by Thompson et al. (1987). Maturity patterns of IMF to test the effects of sex and genotype simultaneously have not yet been elucidated. Effect of maturity on IMF deposition Several studies in cattle (Hood and Allen 1973; Cianzio et al. 1985) have suggested that IMF deposition occurs late in maturation. Indeed, as animals mature, fat is deposited at a greater rate than lean tissue. In a related study in sheep, Ponnampalam et al. (2008) showed that total fat depots increased relative to lean as protein accretion declined. The concentration of fat in muscle will inevitably increase later in an animal s life. This can be interpreted to mean that IMF percentage is late maturing, but it does not mean that the rate of fat accretion in intramuscular adipocytes in the muscle is also late maturing relative to other fat depots. The study of Johnson et al. (1972) over a wide range of slaughter ages (210 days gestation, followed by slaughters at 28, 355 and 1056 mean days of age) showed that the proportional distribution of fat between carcass pools in Angus cattle was constant as total fat increased. However, further studies investigating fat deposition in beef cattle are required to determine if the major fat depots grow in the same proportion as animals fatten across breeds. The data presented here suggest that IMF is indeed early maturing in sheep in relation to total carcass fat. The study of Johnson et al. (1972) also showed that the relative contribution of IMF in relationship to the total side of fat was early maturing. This suggests that prolonged feeding will be of little benefit for increasing IMF levels, which is an important production outcome that has particular ramifications for producers. The carcasses from the older animals in the current study had very high levels of subcutaneous fat (Ponnampalam et al. 2007a), and the maturity pattern was such that the greatest proportional increase in IMF occurred during early life. Therefore, in terms of energy efficiency, holding animals for long periods will be unproductive and would also produce retail cuts of meat that would require extensive trimming of salvage fat. As discussed by Pethick et al. (2007), the link between carcass fatness and IMF development is not always clear. In their review, Pethick et al. (2007) showed that across a number of species, Angus and Japanese Holstein beef cattle, lambs, sheep and modern pig, there was evidence to suggest that the percentage of IMF in relation to the percentage of carcass fat was relatively constant as animals fatten. Exceptions were observed, however, such as those observed in the Japanese Black Holstein cross cattle where the percentage of IMF was higher at lower percentages of carcass fat. Interestingly, this breed has undergone prolonged genetic selection for increased IMF. This result emphasises the power of genetic manipulation for shifting the emphasis from one depot to another. Effect of sex on IMF deposition In this study, we found a significant difference in IMF percentage due to sex (Table 3). In direct contrast, Okeudo and Moss (2007) did not report any differences between sexes, but the maximum slaughter age of the animals in their study was 13 months and the relationship between the percentage of intramuscular lipid and slaughter weight was reported as a linear trend. Our study clearly shows a non-linear relationship between maturity and IMF (Fig. 1). Effect of genotype on IMF deposition Differences between all genotypes within sex for the weight of intramuscular in the LL (grams) were found when compared with increments of maturity based on total carcass fat (Table 3). This new knowledge suggests that the distribution of fat is different between depots across genotypes (Table 3). However, in terms of percentage of IMF (based on LL), when compared with slaughter age, only some differences were detected (Tables 1 and 2). For ewes slaughtered at 22 months of age, a difference was found between PD g M and BL M genotypes (Table 1), whereas for wethers at the same age a difference was found between PD g BLM, PD m M, PD g M, and BL M genotypes (Table 2). The BL M sheep had the highest IMF percentage across both sexes and the highest increases in IMF percentage between 14 and 22 months (Tables 1 and 2). These findings concur with those of Hopkins et al. (2007b). Eating quality To achieve better than every day eating quality in sheep meat, Hopkins et al. (2006) recently showed that an IMF level of 5% was required, but this only explained a very small amount of the variation in eating quality. Previously, Savell and Cross (1986) suggested that the minimum ether-extractable fat (IMF) required for grilling cuts to achieve acceptable consumer satisfaction was 3% on a fresh, uncooked basis. This suggests that, on average, the meat from animals slaughtered over the wide age range in the current study would be acceptable. Furthermore, there could be some marginal advantage in meat from Border Leicester Merinos, but the significantly greater subcutaneous fat levels (Ponnampalam et al. 2007a) would counteract this, because of greater waste during cut preparation. Conclusion In conclusion, it has been shown that Border Leicester genes impact on IMF, but what might be considered a beneficial advantage for eating quality is diminished by increased carcass fatness. This study indicates that (1) differences between genotype and sex exist, and (2) IMF is an early maturing depot in relation to total carcass fat and that IMF increases only marginally as sheep mature. As a consequence the sheep industry will not benefit from prolonged feeding of animals if the objective is to significantly increase IMF levels. The genotype and sex differences in the IMF (g) of the LL would suggest that genetic manipulation is a more worthwhile strategy. Acknowledgements Technical support for this study was provided by David Stanley, Leonie Martin, Edwina Toohey, Tony Markham, Jayce Morgan, Sally Martin, Brent McLeod, Steve Sinclair, Joe Brunner, Stuart McClelland and Amanda Lang (NSW Department of Primary Industries), Kirstie Martin and Kirsty Thomson (formerly University of New England), Peter Allingham (CSIRO) and Tracy

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