Growth performance and carcass characteristics of Dorset lambs fed different concentrates: Forage ratios or fresh grass

Small Ruminant Research 95 (2011) 113–119

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Growth performance and carcass characteristics of Dorset lambs fed different concentrates: Forage ratios or fresh grass Joannie Jacques a , Robert Berthiaume b , Dany Cinq-Mars a,∗ a b

Département des sciences animales, Université Laval, Quebec City, Quebec, Canada Dairy and Swine Research and Development Centre, Agriculture and Agri-Food Canada, Lennoxville, Quebec, Canada

a r t i c l e

i n f o

Article history: Received 22 April 2010 Received in revised form 1 October 2010 Accepted 1 October 2010 Available online 3 November 2010 Keywords: Lamb Growth Carcass Concentrate Forage Grazing

a b s t r a c t Forty male Dorset lambs were divided at weaning into four dietary treatment groups: ad libitum concentrates (C), restricted concentrates (RC), zero grazing (ZGR) and grazing (GR). All the lambs were weaned and slaughtered at similar weights, 24 kg for weaning and 47 kg for slaughter. The average daily gain (ADG) of the RC-fed lambs (347 g/d) was lower than that of the C-fed lambs (449 g/d) but higher than that of the lambs in the ZGR (267 g/d) and GR (295 g/d) treatments (P < 0.0001). There was no significant difference between the ZGR and GR lambs for ADG. To reach slaughter weight, the RC and ZGR-GR lambs required 20 and 40 additional days, respectively (P < 0.0001), compared to the C-fed lambs. The lambs fed C had better feed efficiency than the lambs on mixed (RC) or forage-based (ZGR, GR) diets (P < 0.0001). Values for body score, in vivo (P < 0.05) back fat thickness (P < 0.0001), and back fat thickness after slaughter (P < 0.05) were higher in the carcasses of the C-fed lambs compared to the values obtained with the other dietary treatments. No difference was observed among the treatments for leg and shoulder muscle classification (P > 0.05). However, the loins of the C-fed lambs obtained a higher classification score than those of the lambs raised under ZGR or GR (P < 0.05). Carcass yield was greater (P < 0.0001) for the C-fed lambs compared to the RC and ZGR lambs, mostly because of a lighter full digestive tract (P = 0.0007). The carcasses of the grazing lambs obtained a lower global rating classification (P < 0.05), mainly because of a lack of back fat thickness. Feeding system had a significant effect on subcutaneous fat lightness (L*) (P = 0.004) and yellowness (b*) (P < 0.0001) but did not affect redness (a*). Overall, forage-based diets may prevent excessive carcass fat in heavy lambs while producing similar muscle development, resulting in a leaner product for consumers. © 2010 Elsevier B.V. All rights reserved.

1. Introduction In North America, traditional lamb meat production promotes rapid growth and is based on diets with high levels of concentrates. However, ad libitum consumption of concentrates results in fatter lambs compared to those fed more forage, when the lambs are slaughtered at a constant

∗ Corresponding author. Tel.: +1 418 656 2131x11362; fax: +1 418 656 3766. E-mail address: [email protected] (D. Cinq-Mars). 0921-4488/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.smallrumres.2010.10.002

final weight (Fisher et al., 2000; Archimède et al., 2008; Resconi et al., 2009). The increasing demand for healthy and safe meat products is stimulating market interest in more extensive systems. Forage-based growing systems can thus be a good alternative to indoor lamb production systems in order to use natural resources and provide the high-quality meat required by consumers (Grunert et al., 2004). Moreover, Blackburn et al. (1991) concluded that meat from lambs raised on forage diets versus ad libitum concentrates contains less fat, a characteristic that is more appealing to consumers. Forage-based diets may also offer the option of reduced daily production costs in compari-

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son to indoor systems (Notter et al., 1991; Woodward and Fernández, 1999). A great deal of research shows that lambs grow faster on concentrate-based diets than on foragebased ones (McClure et al., 1994, 2000; Murphy et al., 1994; Fimbres et al., 2002; Turner et al., 2002; Borton et al., 2005; Demirel et al., 2006; Archimède et al., 2008). However, other studies have shown that high-quality pastures and forages may competitively produce high-quality lamb carcasses with similar ADG to what is achieved in the drylot (McClure et al., 1994; Aurousseau et al., 2007). Most of the studies mentioned above compared grazing animals with animals fed indoors. As a result, environmental differences such as space allowance and physical activity could confound the interpretation of the results (Priolo et al., 2001). The present study evaluated lambs fed fresh herbage indoors and outdoors on pasture compared to lambs fed concentrates indoors. Furthermore, little information is reported in the literature with respect to the chosen end point for slaughter. The objectives of the present study were to compare the growth performance and carcass characteristics at slaughter market weight of Dorset lambs that were fed ad libitum concentrates, restricted concentrates, ad libitum fresh herbage or ad libitum grazing. 2. Materials and methods 2.1. Experimental site This study was conducted at the Centre d’expertise en production ovine du Québec (CEPOQ), in Quebec, Canada, 47◦ 21 00 N latitude and ◦  70 2 00 W longitude. The project began in May 2008 and ended in August 2008. During the experiment, the average temperature was 16 ◦ C (max. 30 ◦ C, min. 2 ◦ C) and the average weekly rainfall was 20 mm. 2.2. Animals and diets Forty Dorset male lambs were used in a completely randomized design experiment to determine the effects of growing-finishing system on rate of gain, time required to finish, carcass characteristics and classification. The lambs were reared with their dams until weaning, with free access to a commercial starter concentrate and hay. At weaning, the lambs were divided into four equal groups according to live weight (average of 24 kg) and genetic potential for growth (Tosh and Wilton, 2002). Each group of 10 lambs was allotted to one of the four feeding management systems: ad libitum concentrates (C), restricted concentrates (RC), zero grazing (ZGR) or grazing (GR). Over seven days, the lambs were adapted to their experimental diets. The lambs in the C, RC and ZGR treatments were kept indoors in individual pens (1.2 m2 ), whereas the GR lambs were rotationally grazed as a single group. Protocols for animal care followed the guidelines set out in the Guide to the Care and Use of Experimental Animals from the Canadian Council on Animal Care (1993). The lambs in the C treatment had ad libitum access to good quality hay (37% acid detergent fiber [ADF], 63% neutral detergent fiber [NDF] and 15% crude protein [CP], dry matter [DM] basis) and a commercial pelleted concentrate (9% ADF, 27% NDF, 17% CP and 2.96 Mcal metabolizable energy [ME]/kg DM). The lambs in the RC treatment had access to the same feed, but the hay:concentrate ratio was maintained at 60:40. The lambs in the ZGR treatment were fed ad libitum fresh grass cut twice a day from the same field grazed by the lambs in the GR treatment. Pasture management for the GR-fed lambs consisted of intensive rotational grazing, with the lambs moved to a new paddock every 24 h in order to maintain herbage quality over the experimental period. The grass was approximately 15–25 cm high when the lambs entered each day’s paddock and 5–8 cm high when they exited. The size of the pasture was adjusted as the lambs grew, in order to maximize dry matter intake (DMI) and minimize waste. The average stocking density was 15 m2 /lamb at pasture. The herbage used to feed the lambs in the ZGR and GR treatments was a mixture of Dactylis glomerata and Alfalfa early in the season followed

Table 1 Herbage mass (kg DM/ha) and chemical composition (% DM basis) of the pasture.

Herbage mass DM CP ADF NDF Ash

Average

Range

1525 19.7 17.8 30.8 47.6 8.8

495–3315 13.8–25.5 12.0–22.5 22.7–36.0 39.1–58.9 6.2–11.8

DM, dry matter; CP, crude protein; ADF, acid detergent fiber; NDF, neutral detergent fiber.

by a mixture of Phleum pratense and Trifolium repens (31% ADF, 48% NDF, 18% CP and 9% ash, DM basis; Table 1). Average forage availability for the growing period was 2.3 kg DM/head/day. The lambs in the ZGR and GR treatments had no access to concentrates but were offered mineral and vitamin supplementation daily. The GR lambs had access to shelter. Water and salt blocks were always available to all the animals. Twice during the experimental period (at the end of June and the end of July), fecal samples were collected for an egg count for internal parasites, but because of the low incidence, no treatment was done. One lamb in the C treatment died and one in the RC treatment performed poorly because of physical problems not related to the project; these lambs were consequently removed from the data set. 2.3. Intake and feed analysis Uneaten hay, concentrates and grass were weighed and removed every day at 8:00, before fresh feed was supplied. The amount of feed offered was adjusted daily on the basis of the previous day’s intake, allowing refusals of 15%. To determine individual DMI for the GR lambs, they were fitted with total fecal collection bags for a five-day period when the animals weighed between 35 and 40 kg. The indigestible NDF (NDFi ) was used as a marker to determine the DMI for GR lambs. The NDFi was obtained after a 10 days incubation of feed and feces in a forage-fed fistulated cow. To determine the recovery rate of NDFi in the feces, ZGR lambs were also fitted with total fecal collection bag. The average recovery rate of NDFi was 64.5%. Samples of feed and orts were collected once a week for analysis. For pasture, samples were taken before turning the lambs in and after turnout. Ten randomly selected samples (1 ft2 , or approximately 0.09 m2 ) of grass were taken by cutting the grass 5 cm above the ground. Grass samples were then pooled and frozen for subsequent analysis. Feed samples were analyzed to determine their chemical composition. Values for DM were determined by oven drying at 65 ◦ C until constant weight, ADF and NDF were determined with the filter bag technique (ANKOM Technology, Methods and 6, 08-6-06), CP was determined according to the AOAC official method 2001–11 (AOAC, 2005), and ash content was measured after combustion in a muffle furnace at 550 ◦ C overnight. All analyses performed on air-dried samples were corrected to a DM basis using a 100 ◦ C DM value. 2.4. Growth and slaughter All lambs were weighed weekly, on the same day, with an electronic scale. A subjective body condition score was estimated (on a scale of 0, emaciated, to 5, obese) by palpation over the ribs, withers, loin and dock. Loin thickness and back fat thickness were measured between the third and fourth lumbar vertebrae (halfway between the last rib and the hip bone) with an ultrasound scanner (Ultrascan 50, Linear 3.5 MHz scan, Alliance Medical Inc., 2000). Average daily gain (ADG) and days required to finish were calculated for all the lambs. Feed efficiency (grams of gain per gram of feed intake) was calculated for all the lambs when they weighed between 35 and 40 kg. Upon reaching their target end weight (47 ± 1 kg), the lambs were slaughtered according to the standard commercial procedure. After 24 h of fasting, they were electronically stunned and slaughtered by exsanguination. 2.5. Carcass measurements Immediately after bleeding, the full digestive tract was removed and weighed. The hot carcass was weighed and hung in a refrigerated room at

J. Jacques et al. / Small Ruminant Research 95 (2011) 113–119 4 ◦ C for 24 h. Professional scoring of muscular conformation and fatness was carried out in accordance with the regulations on carcass classification (Agriculture and Agri-Food Canada, 1992). The conformations of the shoulder, loin and leg were scored with grade values from 0 (poor) to 5 (excellent). Back fat thickness was measured at the 12th rib. Subcutaneous fat color was determined 48 h after slaughter using the L*, a*, b* system with a colorimeter (CR–300 Chroma Meter and DP–301 Data Processor, Minolta Co. Ltd., Japan). 2.6. Statistical analysis Data were analyzed as a completely randomized design using the MIXED procedure of SAS (SAS Institute, Inc., Cary, NC). Individual lambs were considered as the experimental units. The following model was fitted for all variables: Yij =  + Ti + Eij , where Yij is the dependent variable,  is the overall mean, Ti is the mean effect of the ith treatment, and Eij is the random residual variation. The statistical model included treatment as a fixed effect. Least squares means were compared using the PDIFF option of SAS with a Tukey–Kramer adjustment for multiple comparisons. Data concerning body score, conformation of the shoulder, loin, leg and global rating, were analyzed by applying the pairwise Mann–Whitney U test. Significance was declared at P < 0.05.

3. Results and discussion 3.1. Lamb behavior and performance Lambs adapted quickly to pasture. They were playful and remained actively grazing most of the time for 4–6 h after being given a new pasture. Then they rested while ruminating. By the end of the afternoon and at night they went back grazing the remaining herbage. Lambs appeared to graze legumes first followed by grasses, although no specific protocol was followed to study lamb behavior at pasture. Pasture was mostly grazed uniformly to 5 cm height after 24 h. The growth performances of Dorset lambs reared under ad libitum concentrate (C), restricted concentrate (RC), zero-grazing (ZGR) or grazing (GR) feeding systems are shown in Table 2. As planned in the experimental design, the initial and final body weights (BWs) of the lambs did not differ among treatments (P > 0.05). Dietary treatment had an effect on ADG that led to an increase in days required to reach slaughter weight for the lambs fed restricted or no concentrates (P < 0.0001). Differences were, in order, C > RC > ZGR-GR for ADG and ZGR-GR > RC > C for slaughter age. These data are in agreement with those obtained by several authors who reported lower ADG and longer finishing periods under pasture management (Ely et al., 1979;

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Notter et al., 1991; McClure et al., 1994, 1995; Murphy et al., 1994; Zervas et al., 1999; Turner et al., 2002; Borton et al., 2005) or forage-based diets (Zygoyiannis et al., 1999; Archimède et al., 2008) when compared to concentratebased diets for lambs slaughtered at heavy weights. In those studies, however, the lambs were raised on different types of pasture or hay, which may have determined the degree of gain reduction. These differences in ADG are presumably due to the differences in the energy density of the feed consumed and, therefore, to total energy intake (Murphy et al., 1994). In fact, many factors, including breed, botanical species and corresponding nutritive value, years, and management system, exert an important influence on lamb ADG. As suggested by Priolo et al. (2001), the comparison of data is difficult between pasture-raised animals allowed to move freely and animals restricted in feedlots. The effect of feeding may be confounded with the different levels of physical activity. In the present study, ADG and age at slaughter of the ZGR or GR lambs were not significantly different. The lambs in those two treatments were offered the same feed (fresh herbage without concentrates). These results suggest that environmental conditions such as exercise, insects, rain and outside temperature were not responsible for the observed changes in growth rate between the concentrate- and grass-fed lambs. In this study parasitism was monitored and remained very low for GR lambs. However, for commercial lambs grazing systems, parasitism should be monitored and veterinary intervention applied when needed. 3.2. Dry matter intake and feed efficiency Table 2 reports the performance of the lambs when their BW was between 35 and 40 kg. Consistent with the findings for overall ADG, significant differences (P < 0.0001) among dietary treatments were observed for ADG between 35 and 40 kg BW. The DMI of the lambs in the C treatment was less (P = 0.03) than that of the lambs in the ZGR treatment, with intermediate values for the RC and GR treatment groups. In their study, Fimbres et al. (2002) also reported an increase in DMI as the level of hay in the finishing ration increased. Similar observations were made with goats, as well (Lu and Potchoiba, 1990). The difference in DMI could be attributed to variations in fiber content between rations. It is gener-

Table 2 Growth performance and feed efficiency of lambs reared under different feeding systems, namely ad libitum concentrates (C), restricted concentrates (RC), zero grazing (ZGR) or grazing (GR). C

RC

ZGR

GR

n=9

n = 10

n = 10

n = 10

23.7 46.9 122b 347b

23.6 47.0 146c 267a

397b 1655ab 25b

315a 1805b 18a

Initial BW (kg) 23.6 Final BW (kg) 47.2 Slaughter age (d) 105a ADG (g/d) 449c Performance between 35 and 40 kg BW ADG (g/d) 486c DMI (g/d) 1548a Feed efficiency (%) 32c a,b,c

SEM

P-value

23.6 47.1 145c 295a

0.8 0.3 3.0 10.2

ns ns <0.0001 <0.0001

353ab 1775ab 20ab

15.2 66.6 1.3

<0.0001 0.03 <0.0001

Means within the same row with different letters differ significantly. SEM, standard error of the mean; BW, body weight; ns, non-significant; ADG, average daily gain; DMI, dry matter intake.

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Table 3 In vivo body score, loin thickness and back fat thickness at the end of the fattening period (43 kg) for lambs reared under different feeding systems, namely ad libitum concentrates (C), restricted concentrates (RC), zero grazing (ZGR) or grazing (GR).

Body scoreA Loin thicknessB (mm) Back fat thicknessB (mm)

C

RC

ZGR

GR

n=9

n = 10

n = 10

n = 10

4.1b 28.9 5.1b

3.8a 27.0 3.2a

3.6a 26.8 3.3a

3.6a 27.2 2.5a

SEM

P-value

0.1 0.6 0.3

<0.05 ns <0.0001

a,b,c

Means within the same row with different letters differ significantly. SEM, standard error of the mean; ns, non-significant. A Subjective body score estimated on a 0–5 scale (0: emaciated; 5: obese). B Loin thickness and back fact thickness measured between the third and fourth lumbar vertebrae with an ultrasound scan.

ally accepted that more fiber could result in physical fill of the rumen and thus regulate intake. Moreover, McClure et al. (1994) pointed out that lower fiber content in forages results in greater DMI. The average ADF and NDF contents in the complete rations for C, RC and ZGR-GR were, respectively, 13%, 26% and 31% for ADF and 33%, 49% and 48% for NDF. It therefore does not appear that physical fill was responsible for the difference in DMI observed in this study, given that DMI increased as the ADF and NDF levels in the ration increased. As Fimbres et al. (2002) suggested, the increase in DMI as a result of feeding higher levels of forage could more likely be attributed to the regulatory effect of dietary energy. Indeed, lambs on forage-based diets need to eat more feed to meet their requirements for growth, because of the lower energy density of the ration. It is possible that the lambs in the C and ZGR treatments underwent two different patterns that regulate intake. First, the lambs fed C ate an average of 13% hay. Under these circumstances, physical fill of the rumen was likely not an issue. Yet intake might have been regulated by metabolic signals in order to prevent an excess of energy and acidosis. Second, the lambs that did not graze had a much lower energy density in their feed. Since these lambs needed more feed to meet their energy requirements for growth, physical fill might have been the determining factor that regulated their intake. The fact that there was a difference in DMI between the ZGR and C lambs but not between the GR and C lambs could be due to the higher fiber content and lower energy density of the grass ingested by the ZGR lambs even though they had access to the same herbage. The GR lambs were free to be selective and could eat from the top leafy layer downwards (Molle et al., 2008), whereas the ZGR lambs ate the whole plant that had previously been cut for them. It is well known that leaves contain less fiber and more energy than stems. Moreover, as soon as the plant is cut off, a portion of the sugar (a major source of energy in forage) is lost via plant respiration. These reasons could explain the lack of difference between the GR- and C-fed lambs. The lambs fed C had better feed efficiency than the lambs in the other dietary treatments (P < 0.0001). This finding is consistent with those in a previous study, in which level of forage in the ration was inversely related to feed efficiency (Fimbres et al., 2002). In the present study, the highest feed efficiency was achieved with C, followed by RC, GR and ZGR, in order of decreasing value. The feed efficiency of the GR lambs was intermediate between those of the RC and ZGR lambs although not significantly different. Fur-

thermore, feed efficiency was relatively high compared to other studies (Fimbres et al., 2002; Tripathi et al., 2007; Archimède et al., 2008), probably because of the lambs’ genetic potential for growth. Indeed, the growth performance of the flock averaged 320 g/d in this experiment, whereas the lambs in previous studies achieved between 150 and 250 g/d (Fimbres et al., 2002; Tripathi et al., 2007; Archimède et al., 2008). 3.3. Carcass measurements before slaughter As shown by the body score and back fat thickness values (Table 3), the lambs fed C had more external fat cover than the lambs in the other groups (P < 0.0001). No significant difference was observed among the dietary treatments (P > 0.05) for in vivo loin thickness (Table 3). These results agree with those of Murphy et al. (1994), who concluded that, even if fat accretion is altered, the quantity of lean tissue from lambs fed forages or concentrates is not affected by feeding treatment. These observations are probably due to the partition of energy for tissue gain, as tissue maturation follows the order of bone, lean and fat (Rouse et al., 1970; Borton et al., 2005). Therefore, in a situation of slightly lower energy intake, lean tissue accretion is usually not affected, whereas fat accretion might slow down, stop or even regress. 3.4. Carcass yield The results for carcass characteristics are presented in Table 4. At the same slaughter weights (Table 2), the lambs fed C had heavier carcass weights (P = 0.0007) and higher carcass yields (P < 0.0001) than the lambs in the RC and ZGR dietary treatments. The GR lambs had intermediate carcass weights that were statistically heavier than those of the lambs in the ZGR treatment (P = 0.03). The full digestive tracts of the lambs in the RC and ZGR treatments were heavier than those of the lambs in the C and GR treatments (P < 0.0001). These results are consistent with those obtained in other comparative trials between lambs fed concentrates and those fed forage-based diets with slaughter at similar heavy weights (Fluharty et al., 1999; Borton et al., 2005; Archimède et al., 2008). It was found that concentrate-fed lambs gave heavier carcasses, mostly because the digestive tract weighed less. Furthermore, Priolo et al. (2002) suggested that animals raised on forage-based diets have a more developed digestive tract because of their higher DMI, as observed in the present

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Table 4 Carcass characteristics and classification of lambs reared under different feeding systems, namely ad libitum concentrates (C), restricted concentrates (RC), zero grazing (ZGR) or grazing (GR).

Carcass weight (kg) Digestive tract weight (kg) Reticulorumen weight (g) Carcass yieldsA (%) Back fat thickness (mm) Subcutaneous fat color L* a* b* Carcass classification ShoulderB LoinB LegB Global ratingC

C

RC

ZGR

GR

SEM

P-value

n=9

n = 10

n = 10

n = 10

21.2b 8.1a 669a 45.0b 11.2c

19.5a,c 9.2b 676a 41.6a 7.3b

19.4a 9.5b 778b 41.2a 6.6b

20.4b,c 8.1a 810b 43.2ab 4.6a

0.3 0.3 24.0 0.6 0.8

0.0007 <0.0001 <0.0001 <0.0001 <0.05

74.5a 2.1 9.0a

77.2b 2 10.8ab

77.9b 1.7 14.4c

77.4b 1.9 13.6bc

0.7 0.4 0.8

0.005 ns <0.0001

3.4 3.8b 3.1 102.4b

3.3 3.6ab 3.0 102.5b

3.2 3.2a 3.0 102.4b

3.1 3.2a 3.0 98.6a

0.2 0.2 0.5 1.0

ns <0.05 ns <0.05

a,b,c

Means within the same row with different letters differ significantly. SEM, standard error of the mean; ns, non-significant. A Carcass yield = carcass weight/slaughter weight × 100. B Conformation score determined on a 0–5 scale (0: poor; 5: excellent), according to Agriculture and Agri-Food Canada (1992). C Global rating based on 100 (100 = normal) obtained after professional classification according to Agriculture and Agri-Food Canada (1992).

study. The differences in carcass and full digestive tract weights between the ZGR and GR lambs could be explained by the fact that lambs usually graze from the top leafy layer downwards (Molle et al., 2008). Therefore, the quality of the forage ingested by the GR lambs was likely higher than that offered to the ZGR lambs, which did not have the liberty to choose and thus ate the whole plant cut 5 cm above the ground. The empty reticulorumens of the lambs that received no concentrates (ZGR and GR) were 15% heavier (P < 0.0001) than those of the lambs given concentrates (C or RC). This study is the first to report such a difference in the reticulorumen weights of lambs fed with or without concentrates. These results do not agree with previous research by Fluharty et al. (1999) and Joy et al. (2008), who reported no difference in reticulorumen weight between grazing lambs and lambs fed only concentrates for heavy lambs (Fluharty et al., 1999) and weaned lambs (Joy et al., 2008). However, Fluharty and McClure (1997) reported heavier reticulorumens for lambs fed all-concentrate diets with a 15% increase in DMI. In the present study, the DMI of the lambs fed fresh herbage (ZGR and GR) was 10% higher than that of the lambs fed concentrates (C and RC). Hence, it is possible that the differences in reticulorumen weight between the lambs on concentrates and those on grass diets were due to the difference in DMI that could result in a heavier reticulorumen complex or thicker external layers of the rumen. Since determining the exact causes for these differences was beyond the scope of this study, more research is needed in this area. 3.5. Carcass fatness Back fat thickness, measured by ultrasound (Table 3) or on the carcass after slaughter (Table 4), was greater for the lambs fed C compared to the lambs in the other dietary treatments (P < 0.0001). These results support observations by other researchers that forage feeding results in less fat

accumulation in lambs (McClure et al., 1995; Díaz et al., 2002; Borton et al., 2005; Joy et al., 2008; Carrasco et al., 2009). Lambs fed concentrates generally display significantly greater fatness than lambs raised on forage-based diets, as indicated by back fat thickness (Díaz et al., 2002). In contrast, Fimbres et al. (2002) found no effect of levels of hay on back fat thickness. Those researchers fed their lambs with hay levels between 0% and 30%, however, whereas the lambs in the present study were fed up to 60% hay. That variation could explain the significant difference shown in Table 4 for back fat thickness between lambs fed different levels of hay (C versus RC; P < 0.05). Several factors may account for the reduction in carcass fat observed in the lambs fed forage-based diets. Some authors reported that the lower fatness of grazing lambs could be linked to their greater physical activity compared to the activity level of stall-fed lambs (Priolo et al., 2002). With physical activity, the mobilization of body lipid reserves is increased to form muscle tissue, with the subsequent reduction of carcass fatness at the expense of subcutaneous fat (Díaz et al., 2002). The results of the present study support the results of Priolo et al. (2002), given that there was a significant difference (P < 0.05) for fat thickness between GR and ZGR (Table 4). Lambs that grazed likely walked and exercised more than their ZGR counterparts, which were kept indoors in individual pens. As Murphy et al. (1994) concluded, the reduced carcass fatness of the forage-fed lambs may also be attributable to lower energy intake. The amount of carcass fat is positively correlated with energy intake (Field et al., 1990). Daily energy intake may have been sufficient to meet the energy requirements for lean tissue but provided less energy for fat accretion, especially subcutaneous fat. This finding is consistent with the results reported by other researchers (McClure et al., 1994; Borton et al., 2005; Joy et al., 2008; Carrasco et al., 2009), who found that lambs fed forage diets stored less energy as fat than did lambs fed concentrates.

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Moreover, concentrates contain more starch, thus producing higher levels of ruminal propionate and leading to an increase in insulin secretion, which stimulates fat synthesis (Bines and Hart, 1982). 3.6. Fat color Feeding system had a significant effect on subcutaneous fat lightness (L*) (P = 0.004) and yellowness (b*) (P < 0.0001) but did not affect redness (a*) (Table 4). The subcutaneous fat lightness value was greater, corresponding to a lighter coloration, for the lambs fed forage-based diets (RC, ZGR and GR). The subcutaneous fat of the grass-fed lambs was yellower (higher b*) than the subcutaneous fat of the lambs fed C (P < 0.0001). Similar results were reported by Díaz et al. (2002) for fat lightness and yellowness in lambs raised on pasture or drylot. The lambs fed RC had an intermediate b* value for subcutaneous fat that was only different when compared to that of the ZGR lambs (P = 0.009). Other studies also reported yellower subcutaneous fat for lambs raised on forage-based diets (Priolo et al., 2002; Joy et al., 2008). The fat color variation found in grazing treatments is a consequence of the amount of green forage carotenoids stored in the fat deposits (Priolo et al., 2002; Carrasco et al., 2009). Yellow fat is generally not appreciated by consumers worldwide. However, Kirton et al. (1975) reported that, in New Zealand, a country where the grass feeding system is typical, only one carcass in a thousand is rejected for its yellow fat. Moreover, in the paper written by Prache et al. (1990) on lambs with carcass fat defaults, carcasses that were singled out for yellowness had b* values higher than 25. Given that the highest b* value in the present study was 14.4, the slightly yellowish fat color is likely of no significance from the consumer’s point of view. 3.7. Carcass classification Carcass classification results are shown in Table 4. No significant differences for shoulder and leg classification were observed among the dietary treatments, whereas the lambs fed C had a better loin classification (P < 0.05) than the lambs in the ZGR and GR treatments. Earlier studies by Borton et al. (2005) and McClure et al. (1994) also reported greater longissimus muscle development (16% and 27%, respectively) for concentrate-fed lambs compared to grazing lambs. The loins of the lambs fed RC had intermediate values. These results are consistent with those of Fimbres et al. (2002), who reported no effect of hay levels on rib eye area. The GR lambs obtained lower global ratings (P < 0.05) compared to the lambs in the other dietary treatments, owing to a lack of back fat thickness. In fact, 20% of the GR lambs had back fat thickness less than 4 mm, not enough to meet the minimum classification standard. In previous studies, the carcasses of lambs that had been raised in a grazing system without any supplementation and slaughtered at heavy weights usually presented a slightly inferior conformation compared to lambs raised on concentratebased diets (Ely et al., 1979; Murphy et al., 1994; McClure et al., 1995; Priolo et al., 2002; Borton et al., 2005).

4. Conclusion In this study, the forage-fed lambs had lower daily gains and longer finishing periods. However, using forage finishing systems may improve production efficiency and processing by preventing excessively fat carcasses for lambs slaughtered at 47 kg of live weight. Forage-based diets have the potential to produce similar muscle development and a leaner product for consumers. The relatively small differences between the zerograzing and grazing lambs suggest that, under the conditions of this study, diet composition had a greater incidence on lamb performance and carcass characteristics than did the environmental conditions encountered by the lambs raised on pasture. Grass feeding under good pasture management has the potential to provide high-quality carcasses at low cost and should be considered by producers when it is available. However, zero grazing requires a considerable amount of work to provide enough fresh grass every day to support lamb growth. Considering the recent increases in fuel costs, this option should be evaluated on an economic basis before it is used by lamb producers.

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