J. Anim. Sci. 2005. 83:1345-1352
© 2005 American Society of Animal Science
Characteristics of lambs fed concentrates or grazed on ryegrass to traditional or heavy slaughter weights. II. Wholesale cuts and tissue accretion1
R. J. Borton*,2,
S. C. Loerch
,
K. E. McClure and
D. M. Wulf
* Agricultural Technical Institute and
and
Department of Animal Sciences, Ohio Agricultural Research and Development Center, The Ohio State University, Wooster 44691; and
and
Department of Animal and Range Science, South Dakota State University, Brookings 57007
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Abstract
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Targhee x Hampshire lambs (average BW 24 ± 1 kg) were used to determine the effect of finishing on concentrate or by grazing ryegrass forage on slaughter weights of 52 kg (N) or 77 kg (H) on tissue accretion and lamb wholesale cutout. When fed to similar slaughter weights, the wholesale cuts of concentrate-fed lambs were heavier (P < 0.05) than the same cuts from forage-fed lambs; however, when expressed as a percentage of side weight, carcasses of forage-fed lambs had a higher (P < 0.001) percentage of leg than concentrate-fed lambs. Increasing slaughter weight from 52 to 77 kg resulted in a 1-kg increase in loin weight for lambs finished on concentrate and a 0.60-kg increase for lambs finished on forage (diet x slaughter weight, P < 0.03); however, the increased loin weight for lambs finished on concentrate was due largely to increased fat deposition. For lambs slaughtered at 77 kg, those finished on forage had more lean mass in the leg, loin, rack, and shoulder than those finished on concentrate, but lean mass in these cuts did not differ between diets for lambs slaughtered at 52 kg (diet x slaughter weight, P < 0.01). At the normal slaughter weight (52 kg), concentrate-fed lambs had 50% more dissectible fat than forage-fed lambs, whereas at the heavy slaughter weight, a 79% greater amount of dissectible fat was observed for concentrate- vs. forage-fed lambs (diet x slaughter weight, P < 0.001). Lean and fat accretion rates were higher (P < 0.001) for concentrate-fed lambs than for forage-fed lambs. The lean-to-fat ratio of forage-fed lambs was higher (P < 0.001) than that of concentrate-fed lambs; however, forage finishing decreased accretion rates of all tissues compared with concentrate feeding, and these differences between forage and concentrate feeding were magnified at heavier slaughter weights.
Key Words: Carcass • Concentrate • Fat • Forage • Lambs • Lean
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Introduction
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Traditional slaughter weight of lambs in the United States is approximately 60% of the physiological mature size of the dam, and excessive fat content of larger carcasses decreases the incentive to increase slaughter weight. However, nutrition and management strategies may allow for the production of lambs with heavier lean carcasses. Previous research at The Ohio State University has shown that lambs finished to typical slaughter weights on concentrate were fatter than lambs grazed on perennial ryegrass forage (McClure et al., 1994
, 1995; Murphy et al., 1994a) or alfalfa (McClure et al., 2000). Lambs grown on pasture have greater lean-to-fat accretion ratios than those fed high-grain diets. Therefore, pasture-based finishing systems may allow lambs to be slaughtered at heavier weights without excessive fat composition. Earlier studies (Rouse et al., 1970; Ely et al., 1979; Arnold and Meyer, 1988) focused on tissue accretion rates and characteristics of wholesale cuts for lambs weighing less than 55 kg, and there are no published carcass composition results available on potential interactions between diet and slaughter weight for lambs heavier than 55 kg BW. Thus, this study was undertaken to determine the effects of grazing ryegrass vs. feeding high-concentrate diets on accretion of lean, fat, and bone, and on the weights and percentages of wholesale cuts from lambs fed to 52 or 77 kg.
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Experimental Procedures
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Lamb allotment and production procedures are detailed in Borton et al. (2005), and protocols relating to animal care adhered to recommended guidelines (FASS, 1999). Briefly, Targhee x Hampshire lambs (n = 70) were randomized by sire (six sires) and rearing-type (single- and twin-reared). Six contemporaries (three ewes and three wethers) were slaughtered at the initiation of the experiment to determine initial body composition. The remaining 64 lambs were assigned randomly within gender (32 ewes and 32 wethers) to two dietary treatments (a high-concentrate diet fed in dry lot vs. rotationally grazed on perennial ryegrass until October, fed medium-quality grass hay until April, and returned to ryegrass pastures) and two slaughter end weights (52 vs. 77 kg). Thus, there were eight lambs (four ewes and four wethers) per treatment combination (two genders x two dietary treatments x two slaughter end weights). Composition of the concentrate diet is shown in Table 1. Lambs were weighed every 2 wk until they reached their slaughter end weight. When the mean BW for each group reached the targeted end weight, lambs in that group were slaughtered at The Ohio State University Meat Laboratory. For the concentrate-fed lambs, 82 and 161 d on test were required to reach the normal and heavy slaughter weights, respectively. For the forage-fed lambs, 161 and 442 d on test were required to reach the respective targeted slaughter weights. One normal-slaughter-weight, concentrate-fed lamb performed poorly due to physical problems, three normal-slaughter-weight, forage-fed lambs also performed poorly due to health problems, and one heavy-slaughter-weight, forage-fed lamb died; therefore, these lambs were removed from the data set. Six heavy-slaughter-weight, forage-fed lambs exhibited no or very slow growth rates late in the trial and did not reach the specified slaughter weight by the end of the trial; thus, these lambs weighed at least 10 kg less than the target end weight, and were not slaughtered for this experiment to avoid bias in the slaughter weight treatment. Hot carcass weights were recorded, and, after chilling at 0°C for 48 h, each carcass was split in half. The left side was merchandised, whereas the right side was fabricated into wholesale cuts, without trimming, according to North American Meat Processors (NAMP) Association guidelines (NAMP, 1997), and weights of each cut were subsequently recorded. The leg (NAMP #233), loin (NAMP #231), rack (NAMP #204), and shoulder (NAMP #206) from each lamb were then dissected into lean, fat, and bone. Dissected fat was further separated into s.c. and intermuscular fat. The semimembranous, semitendinosus, and biceps femoris muscles were excised from the leg and weighed separately, and then combined with other lean from the leg to determine total weight of lean. The LM was excised from the loin, weighed, and the rack and shoulder were dissected into lean, fat, and bone without isolating any particular muscle. Lean, fat, and bone weights from each of these four primal cuts were combined to provide a total weight of lean, fat, and bone. Tissue accretion was calculated by taking the weight of these tissues and subtracting the average weight of these tissues from that of the six counterpart lambs slaughtered at the initiation of the experiment.
Data were analyzed as a completely randomized design using the GLM procedures of SAS (Cary, NC), with individual lamb as the experimental unit. The 2 x 2 x 2 factorial model for percentage of wholesale cuts, amount of wholesale cuts, and dissectible tissues included the main effects of diet, end BW, and gender of lamb, as well as all interactions. Gender of the lamb was in the model as a main effect, but interactions with gender of lamb and other main effects were not significant (P > 0.10); therefore, only diet and end BW treatments, along with their interactions, are discussed in the text. Residual mean squares were used as the error term, and treatment least squares means were compared using the PDIFF option of SAS protected by a significant (P < 0.05) F-test.
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Results and Discussion
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Wholesale Cuts
Details regarding growth performance, carcass characteristics, and organoleptic properties of these lambs are presented in a companion paper (Borton et al., 2005). Briefly, feeding lambs to heavier-than-traditional slaughter weights using high-concentrate diets resulted in excessively fat carcasses. Leaner lamb carcasses were produced through forage finishing; however, much lower growth rates, much longer feeding periods, and less palatable lamb meat resulted. Despite the long feeding period required to achieve a 77-kg slaughter weight, Borton et al. (2005) reported only one carcass had a spool joint present, indicating a yearling mutton carcass.
There were significant (P < 0.05) interactions between diet and slaughter weight for many of the weights and percentages of wholesale lamb cuts (Table 2). Lambs fed concentrate and slaughtered at the heavy weight had 9.5 kg heavier (P < 0.05) sides than those slaughtered at the normal weights, whereas the increase was only 8.2 kg for lambs fed forage. This interaction was caused primarily by treatment effects on transportation shrink and dressing percent (Borton et al., 2005). The diet x slaughter weight interactions for slaughter weight and hot carcass weight were not significant (P = 0.10). Increasing slaughter weight increased (P < 0.03) loin weight by 1 kg when lambs were finished on concentrate and by 0.6 kg when lambs were finished on forage. Diet x slaughter weight interactions were detected (P < 0.01) for rack weight, breast weight, flank weight, rack percent, shoulder percent, breast percent, and flank percent. These interactions resulted from differences in the magnitude of response between forage-and concentrate-fed lambs taken to the heavy slaughter weight.
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Table 2. Effects of diet and slaughter weight on weight and percentage (carcass weight basis) of wholesale lamb cuts
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When interactions were not detected, both diet and slaughter weight influenced weight and percentage of wholesale lamb cuts. Legs of lambs fed concentrate were 12% heavier (P < 0.001) than those of lambs finished on forage. Weights of shoulder and foreshank also were greater (P < 0.05) for lambs fed concentrate than those fed forage. These weight differences were due in part to unintended differences in carcass weight between lambs fed forage vs. those fed concentrate. As previously discussed, differences in transportation shrink and dressing percent resulted in 12% lighter carcass weights for lambs fed forage than those fed concentrate; however, leg and foreshank comprised a larger (P < 0.001) percentage of total side weight for lambs fed forage. Loins from carcasses of concentrate-fed lambs represented a higher (P < 0.001) percentage of the side weight than loins from carcasses of forage-fed lambs, likely due to the greater amount of fat in loins from concentrate-fed lambs (see Table 4). The forage-finishing system resulted in a consistent increase in the proportions of wholesale cuts from the carcass that would be associated with motor function (leg and foreshank). Increasing slaughter weight did not change the percentage of leg or shoulder in carcass from lambs fed in confinement to gain 380 vs. 300 g/d (Lambuth et al., 1970) or from lambs all fed the same diet in confinement (Kemp et al., 1970). As would be expected, in the present trial, wholesale cuts from lambs slaughtered at the heavy weight were heavier (P < 0.001) than those slaughtered at the normal weight; however, leg and foreshank weight comprised a smaller percentage of side weight for lambs slaughtered at 77 vs. 52 kg.
Lean Tissue
The total lean mass in the four wholesale lamb cuts followed the same pattern as that observed individually for the shoulder, leg, and loin (Table 3). For lambs slaughtered at 77 kg, those finished on forage had 25% more (P < 0.01) lean mass in these cuts than those finished on concentrate; however, total lean mass did not differ between diets for lambs slaughtered at 52 kg (diet x slaughter weight; P < 0.01). This result suggests forage finishing delayed physiological maturity and allowed continued lean growth beyond that observed in lambs fed concentrate. Comparisons of forage vs. concentrate finishing on carcass lean tissue content have been previously reported (McClure et al., 1994; Murphy et al., 1994a); however, these studies did not investigate growth processes beyond typical lamb slaughter weights.
An interaction (P < 0.001) between diet and slaughter weight was observed for total percentage of total lean in the carcass. Increasing the slaughter weight increased the percent lean for the forage-fed lambs, whereas increasing slaughter weight decreased the percent lean for the concentrate-fed lambs. This same general pattern of lean percent was evident for the leg, shoulder, and loin. McClure et al. (1994) reported that, at similar carcass weights, lambs finished on alfalfa pasture were leaner than those finished on concentrate. These results support our hypothesis that forage finishing allows lambs to be fed to heavier weights without sacrificing the lean composition of the carcass. The major negative consequence of this system is the prolonged feeding period required to achieve these heavier weights (Borton et al., 2005).
In contrast to most other tissues, the LM from concentrate-fed lambs was 10% heavier (P < 0.001) than that of the forage-fed lambs, and no (P = 0.650) interaction between diet and slaughter weight was observed. Lean weight of the rack and weights of all four muscles excised were greater (P < 0.001) for lambs slaughtered at 77 kg than those slaughtered at 52 kg.
Fat Tissue
Interactions (P < 0.001) were observed between diet and slaughter weight for most measures of dissectible fat in wholesale cuts (Table 4
). Increasing the slaughter weight of forage-fed lambs resulted in 50 to 80% increases in dissectible fat in the leg, loin, rack, and shoulder. For concentrate-fed lambs, the amount of these cuts was approximately doubled at the heavier slaughter weight. When slaughtered at 52 kg, concentrate-fed lambs had 50% more total fat (0.86 kg) in the wholesale cuts than forage-fed lambs, and when slaughtered at 77 kg, a 79% increase in total fat (2.2 kg) was observed for concentrate- vs. forage-fed lambs (diet x slaughter weight interaction; P < 0.001). When expressed as a percentage of wholesale cut weight, finishing lambs to heavier weights on forage had little to no effect on the fat percent, whereas finishing lambs to heavier weights on concentrate increased fat percent (diet x slaughter weight interaction; P < 0.001). Most of the increased fat percent found in carcasses of heavy-weight, concentrate-fed lambs was a result of increased s.c. fat, whereas intermuscular fat percentage was not affected (P = 0.16) by slaughter weight. However, feeding a concentrate diet resulted in a greater (P < 0.001) proportion of intermuscular fat than finishing lambs on forages. Forage finishing results from this study are similar to those of McClure et al. (1994) and Murphy et al. (1994b), who found that lambs finished on pasture had similar lean contents but lower fat contents than those finished on grain-based diets. Moreover, present results revealed that these differences were magnified as slaughter weights approach physiological maturity. Ely et al. (1979) reported that lambs finished on forage and slaughtered at 41 kg had 20% carcass fat, whereas those slaughtered at 50 kg had 24% carcass fat. Additionally, concentrate-fed lambs slaughtered at 50 kg had 7% more carcass fat than those slaughtered at 41 kg (Ely et al., 1979). For lambs slaughtered at 48 kg BW, feeding a concentrate diet increased carcass fat by nearly 50% compared with lambs finished on pastures, but carcass lean content did not differ between the two finishing systems (Murphy et al., 1994b). Some of the effects on carcass composition for pasture- vs. concentrate-finishing systems were likely due to effects of growth rate on the partitioning of energy for tissue gain. When lambs were limit-fed a concentrate diet to achieve gains from 253 to 420 g/d, increasing ADG resulted in a 14% increase in carcass fat and an 8% decrease in carcass lean (Murphy et al., 1994a).
Bone Tissue
Total weight of dissectible bone increased more for forage-fed lambs than for concentrate-fed lambs with increasing slaughter weight (diet x slaughter weight interaction; P < 0.03; Table 5). Similarly, effects of slaughter weight on backbone and femur length were greater for forage- vs. concentrate-fed lambs (diet x slaughter weight interaction; P < 0.02). Lambs slaughtered at 77 kg had more (P < 0.01) dissectible bone mass in leg, loin, and rack, and a higher percentage of bone, than those slaughtered at 52 kg. Similarly, forage-fed lambs had more (P < 0.04) bone mass in these cuts than concentrate-fed lambs. When bone weight was expressed as a percentage of carcass weight, carcasses of forage-fed lambs had a higher (P < 0.001) percentage of bone than did those of concentrate-fed lambs, which may be a dilution effect caused by the greater fat content of carcasses from concentrate-fed lambs.
Rouse et al. (1970) found that lambs slaughtered at 50 kg had achieved 75% of their mature bone weight, but only 50% of their mature lean weight and 33% of mature fat weight. In the present study, lambs slaughtered at 52 kg had 77, 64, and 55% of bone, lean, and fat mass, respectively, compared with lambs slaughtered at 77 kg. These results provide evidence that potential exists to increase lean mass in lambs slaughtered at weights greater than 52 kg. Moreover, the interactions (P < 0.02) between diet and slaughter weight for total mass of these tissues, suggest forage finishing has a greater potential to increase lean mass. Lambs fed concentrate and slaughtered at 52 kg had 74% of the lean mass as those slaughtered at 77 kg BW, whereas lambs fed forage and slaughtered at 52 kg had 44% of the lean mass as those slaughtered at 77 kg BW. Unfortunately, an even greater potential exists to increase fat mass at heavier slaughter weights.
Lean, Fat, and Bone Accretion
The six initial slaughter lambs used for calculation of lean, fat, and bone accretion rates had an average carcass weight of 11.3 ± 0.67 kg and an average fat thickness of 0.8 ± 0.1 mm. Weights of leg, loin, rack, and shoulder from the right side of the carcass were 1.91 ± 0.11, 0.41 ± 0.02, 0.37 ± 0.02, and 1.00 ± 0.06 kg, respectively. Total weight of dissectible lean, fat, and bone from these cuts was 2.38 ± 0.13, 0.32 ± 0.02, and 1.00 ± 0.06 kg, respectively.
Lean accretion rate for the concentrate-fed lambs was greater (P < 0.001) than that for the forage-fed lambs (Table 6). This occurred, in part, because the targeted slaughter end weights were the same and forage-fed lambs took more days to achieve their final weights. Each of the four individual cuts (leg, loin, rack, and shoulder) followed the same pattern, with concentrate-fed lambs exhibiting greater (P < 0.001) lean accretion rates than the forage-fed lambs. Fat accretion rates for the four wholesale cuts also were greater (P < 0.001) for concentrate-fed vs. forage-fed lambs, which reflects the greater fat content of carcasses from concentrate-fed lambs. Decreases in carcass fat content with forage finishing have been reported previously (Ely et al., 1979). Murphy et al. (1994a) reported that, when slaughtered at 48 kg, lambs finished on concentrate required 33 fewer days on feed, grew 50% faster, and had 50% more dissectible carcass fat than lambs finished on alfalfa pastures. McClure et al. (2000) reported that concentrate-fed lambs had greater subcutaneous fat depth but similar LM area to those finished on alfalfa pastures and slaughtered at 37 kg. Grain fermentation increases propionate production in the rumen (Luther and Trenkle, 1967), and propionate increases blood glucose and insulin secretion (Bines and Hart, 1982), which stimulates synthesis of fat and protein in the body (Weekes, 1986). Thus, energy source may play a role in nutrient partitioning in the carcass. Moreover, daily energy intake also was likely involved in the partitioning of nutrients to fat vs. lean in the present study. Lambs finished on pasture routinely have slower growth rates than those finished on grain-based diets (McClure et al., 1994). When grain-based diets were limit-fed to match energy intake of lambs grazing forage, carcass composition was similar (Murphy et al. 1994b).
Bone accretion rate of the four wholesale cuts was greater (P < 0.001) for the concentrate-fed vs. forage-fed lambs. This finding reflects the faster growth rate of concentrate-fed lambs. In addition, lambs slaughtered at 52 kg had a greater (P < 0.01) bone accretion rate than did those slaughtered at 77 kg, which indicates that younger, growing lambs deposit bone at a greater rate than do older lambs.
Increasing the slaughter weight of forage-fed lambs resulted in a 70% increase in lean-to-fat accretion ratio, whereas increasing slaughter weight of concentrate-fed lambs resulted in an 11% decrease in lean-to-fat accretion ratio (diet x slaughter weight interaction; P < 0.001). This result indicates that lean accretion rate was faster than that of fat in forage-fed lambs, whereas fat accretion rate was faster than that of lean in concentrate-fed lambs.
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Implications
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Although forage-fed lambs have carcasses with higher lean-to-fat ratio, they have slower growth, particularly if a heavier slaughter weight is desired. Concentrate-fed lambs can achieve a heavier slaughter weight more quickly, but they have fatter and lower-yielding carcasses than forage-fed lambs. Increasing slaughter weight of lambs finished on forage increases lean accretion to a greater extent than fat accretion, whereas increasing slaughter weight increases fat accretion to a greater extent than lean accretion in lambs finished on concentrate.
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Footnotes
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1 Salaries and research support provided by state and federal funds appropriated to the Ohio Agric. Res. and Dev. Center, The Ohio State Univ., Wooster 44691-4096. The authors acknowledge the assistance of D. D. Clevenger for animal and forage management, G. D. Lowe for technical assistance, data tabulation and statistical analysis of results, and G. D. Dunlap for lamb slaughter and carcass measurements. 
2 Correspondence: 1328 Dover Rd. (phone: 330-287-1310, fax: 330-287-1333; e-mail: borton.1{at}osu.edu).
Received for publication February 3, 2004.
Accepted for publication February 25, 2005.
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Literature Cited
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Abstract
Introduction
