Effects of dietary energy density on feed intake, body weight gain
and carcass chemical composition of Omani growing lambs
O. Mahgoub
*, C.D. Lu, R.J. Early
1Department of Animal and Veterinary Sciences, College of Agriculture, Sultan Qaboos University, PO Box 34, Al Khod 123, Oman
Accepted 5 October 1999
Abstract
Forty male Omani lambs were used in a feeding trial to study the effects of feeding diets containing various levels of metabolizable energy (ME) on growth and carcass composition. Ten lambs were selected randomly and slaughtered at the start of the trial to provide information on initial carcass composition. Thirty lambs were divided randomly into three groups and fed three diets varying in ME concentration (low, medium and high) from weaning (at average 76 days) until slaughter at the mean weight of 30 kg. Digestibility of dry matter (DM) was 66.9, 68.7 and 73.9% for low, medium and high energy diets, respectively. Apparent gross energy digestibility was 66.8, 67.2 and 73.3% corresponding to dietary concentrations of 12.2, 12.6 and 13.9 MJ of DE/kg for low, medium and high energy diets, respectively. Daily DM intake ranged between 3.12 and 3.73 % of body weight (BW) which was equivalent to 76.5±97.5 g/kg0.75 or 0.738±1.142 MJ ME/kg0.75. Daily BW gain increased (P< 0.001) with increasing ME density with a maximum of 154 g/day observed in lambs on high energy diet during the last 4 weeks of the experiment. Feed conversion ratio (FCR), i.e., kg feed/kg BW, improved with increasing ME density (P< 0.001). Sheep fed high energy diet had heavier BW (P< 0.01), empty BW weight (P< 0.001), carcass weight (P< 0.01) higher dressing percentage (P< 0.05) but lower gut content (P< 0.001) than lambs fed medium and low energy diets. Sheep slaughtered at the end had lower water, protein but higher carcass and non-carcass chemical fat than sheep slaughtered at the start of the experiment. This study indicated that meat production from sheep in Oman will be improved in form of higher BW gains and better carcass composition by increasing energy levels in the diet.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Omani lambs; Carcass composition; Energy density; Feed conversion ratio
1. Introduction
Sheep are important meat-producing animals in Oman as well as in many parts of the tropic and sub-tropic regions. Through intensive management, the performance of Omani sheep is improved; with higher growth rates and more desirable carcass com-position as compared to those raised under traditional systems (Mahgoub and Lodge, 1994a,b; Al-Nakib
*Corresponding author. Tel.:968-515252;
fax:968-513418.
E-mail address: osmahgob@squ.edu.om (O. Mahgoub)
1Present address: University of Hawaii Manoa, Department of
Animal Science, 1800 East-West Road, Honolulu, Hawaii 96822, USA.
et al., 1996). Omani sheep are raised under harsh environmental conditions characterized by low rain-fall and high temperatures, and energy is the major limiting factor of growth. Furthermore, the feed avail-able from natural grazing is limited and of low quality. Traditionally, animals are grazed in the morning and brought back to home early in the afternoon where they may be fed some additional hay, fresh alfalfa, palm date fruits or house leftovers. Most of the energy they acquire from grazing is likely to be spent on walking from and back to home. Energy supplemen-tation may increase the ef®ciency of growth. The objective of this study was to investigate the effects of feeding diets containing various energy levels on growth rate and carcass composition of growing Omani sheep.
2. Materials and methods
The trial was carried out during the hot summer months, July±October 1996. Maximum ambient tem-perature during this period reach up to 488C and relative humidity ranged between 60±100%.
A feeding trial using 40 Omani intact male lambs was carried out at the Agricultural Experiment Sta-tion, Sultan Qaboos University, Oman. Lambs were randomly assigned after birth to one of the four groups with 10 animals per group. Lambs on Group 1 were slaughtered at the start of the experiment to determine initial carcass composition. Lambs in the other three groups were assigned randomly to one of three dietary treatments which were initially formulated to provide 8.67, 9.95 or 11.22 MJ ME/kg DM. The composition and chemical analysis of the diets offered as a total mixed ration are in Table 1. Lambs stayed with their dams and had creep feed ad libitum from birth to weaning at an average age of 76 days. From weaning to slaughter, lambs were individually housed in
(12 m) pens with concrete ¯oors and fed the
respective experimental diets until slaughter when the mean BW of all lambs reached 30 kg.
Animals were weighed on a weekly basis before offering fresh feed allowance. Individual feed intakes were recorded daily by weighing the feed offered and orts. The quantity of feed offered each day was adjusted to ensure that the feed troughs contained feed throughout the day. Samples of feed were
col-lected biweekly for proximate chemical analysis (A.O.A.C., 1990). The gross energy (GE) content of the diets were determined by a Gallenkamp adiabatic bomb calorimeter. Total nitrogen was determined by the macro-kjeldahl method. Acid detergent ®bre (ADF) and neutral detergent ®bre (NDF) were deter-mined by the methods of Goering and Van Soest (1975). A digestibility trial was conducted on four animals from each treatment towards the end of the growing phase of the experiment. Feed samples were collected at each feeding and all faecal output was collected and stored at ÿ108C until the end of the collection period. At the end of the collection period, the total faeces collected were mixed and sub-sampled. Feed and faecal samples were dried at 558C (DM content determined at this time) and ground before chemical analysis. Urine output was measured daily and collected in a container that contained 25 ml of 10% H2SO4to prevent loss of urinary ammonia. Urine
aliquots (10% of daily output) were kept for analysis. The apparent digestible energy (DE) was calculated from the gross energy of the feed and faeces by the following equation: DE, %(GE feedÿGE faeces)/
Table 1
Ingredients and chemical composition of the experimental diets (DM basis)
Item Dietary energy concentration
Low Medium High
Ingredients, g/kg
Rhodesgrass hay 600 400 200
Barley grain 190 400 400
Corn grain ÿ 17 226
Soybean meal (49% CP) 170 143 134
Corn oil 12 10 8
Limestone 8 10 12
Vitamin/Mineral Premixa 10 10 10
Sodium bicarbonate 10 10 10
Chemical Composition, g/kg
Crude protein 162 160 160
Neutral detergent fibre 443 341 237
Acid detergent fibre 258 186 124
Ash 79 69 62
Ca 8.9 8.9 8.8
P 1.9 2.3 2.8
aVitamin-trace mineral pre-mix provides per kg of mixed
ration: 18,750 IU Vitamin A; 3750 IU Vitamin D3; 7.5 IU Vitamin
GEfeed100. Metabolizable energy was calculated
from DE0.82 (Johnson, 1972).
Lambs were slaughtered according to the Islamic
tradition, Halal. Non-carcass components were
weighed at slaughter and stored in sealed polyethylene bags at -208C. Digestive tract contents were calculated as the difference between the full and empty digestive tract. Empty body weight (EBW) was computed as the difference between slaughter weight and digestive tract contents. Shrinkage was calculated as the weight lost after chilling at 48C for 24 h. The carcass was wrapped in a polyethylene bag and stored atÿ208C. Carcasses were split along the midline with a bandsaw. Both the frozen left half carcass and non-carcass portion were ground in a whole carcass grinder. The product was further ground in a meat grinder ®tted with a ®ner screen and mixed before samples were collected for chemical analysis. Proximate analysis was carried out on the minced samples for DM, crude protein (CP), ether extract (EE) and ash according to the methods of A.O.A.C. (1990).
Experimental data were analysed using the General Linear Model procedures (SAS Institute, 1991). Effect of age was investigated using orthogonal polynomial contrasts between initial group versus low me-diumhigh energy groups. Analysis of variance between low, medium and high energy groups was conducted to investigate the effect of diet on body weight growth, feed intake, feed conversion ratio, body composition and carcass chemical composition. Body components were expressed as percentages of empty body weight. Carcass chemical components were expressed as percentages of carcass and non-carcass DM as well as percentages of non-carcass and
non-carcass weights. Least signi®cant difference method was used to compare means.
3. Results and discussion
3.1. Digestibility
Digestibility of DM and energy is presented in Table 2. Digestibility coef®cient values for DM and GE were signi®cantly higher in high than in low energy density diet (P< 0.001). Coef®cients for med-ium energy density diet were somewhere in between. The digestibility of DM was 66.9, 68.7 and 73.9% for low, medium and high energy diets, respectively. As expected, digestibility of DM was inversely related with the ®bre content of the diet. Low, medium and high energy diets had 258, 186 and 124 g ADF/kg and 443, 341 and 237 g NDF/kg, respectively. A similar ranking was observed with digestibility of GE (66.8, 67.2 and 73.3%).
3.2. Feed intake
Feed intake in Omani sheep is presented in Table 3. The DM intake as percentage of BW in Omani sheep ranged between 3.12 and 3.68 and between 3.55 and 3.73 at 80 and 194 days of age, respectively. This intake is comparable to that of 4.00% BW obtained from studies using this breed under similar conditions (El Hag and Al-Shargi, 1998). An exceptionally high intake of 6.4±7.4% was reported for Omani and OmaniChios (Cypriot) sheep fed diets based on dried ®sh sardines (El Hag and Al-Shargi, 1995).
Table 2
Dry matter and energy digestibility of three iso-nitrogenous diets containing low, medium and high energy concentrationsa
Diet energy concentration
Low Medium High Pooled SE Significance of difference
Digestible dry matter, % 66.9b 68.7a,b 73.9a 0.591 ***
Gross energy, MJ/kg DM 18.2 18.7 18.9 0.134 *
Digestible energy
Digestion coefficient, % 66.8b 67.2a,b 73.3a 0.429 ***
Diet concentration, MJ/kg 12.2c 12.6b 13.9a 0.079 ***
Although feeds in the latter study were pelleted and contained a high proportion of palatable sardines, this level of intake is unlikely to be achieved by animals under local management systems except in special conditions such as compensatory growth. Even for temperate breeds, a 4.3±5% BW intake was suggested for early weaned lambs with growth rates as high as 200±250 g/day (NRC, 1985). Gatenby (1986) sug-gested that an intake of 3.5% of BW is expected for sheep on moderate quality diet whereas an intake of 4±5% BW is expected for those on high-quality diet. When expressed as per unit of metabolic BW, feed intake of Omani sheep was 76.5±82.1 and 88.6± 97.5 g/kg BW0.75at 80 and 194 days of age, respec-tively. The intake of sheep fed high energy diet was
similar to that of 97.8 of Omani sheep raised under similar conditions (El Hag and Al-Shargi, 1998). This intake is also comparable to that of tropical sheep breeds and that of temperate breeds of lighter weights fed ®nely-prepared diets (Gatenby, 1986).
Metabolizable energy intake per kilogram of meta-bolic BW in Omani sheep was 0.738±0.981 and 0.959±1.142 MJ/kg BW0.75 at 80 and 194 days of age, respectively. At 80 and 194 days of age there was a signi®cant (P< 0.001) increase of ME intake with increasing energy density (Table 3). This is not in line with studies suggesting that energy density adversely affect DM intake in sheep (Gatenby, 1986; Kusina et al., 1991) and goats (Lu and Potch-oiba, 1990). The reason may be that the difference Table 3
Intake, body weight gain, and feed ef®ciency of male Omani sheep fed low, medium and high ME energy dietsa
Item Dietary energy level Pooled SE Effect of diet
Low Medium High
DMI (% BW)
At 80 days 3.12b 3.65a 3.68a 0.12 **
At 194 days 3.73 3.55 3.71 0.11 NS
DMI (g/kgBW0.75)b
At 80 days 76.45b 80.21a 82.10a 2.63 **
At 194 days 90.36b 88.58b 97.49a 2.74 **
ME (MJ/kgBW0.75)
At 80 days 0.738c 0.914b 0.981a 0.029 ***
At 194 days 0.959b 0.986b 1.142a 0.031 ***
Body weight (kg)
At 80 days 17.3 17.28 17.95 0.86 NS
At 194 days 27.40b 30.16b 34.41a 1.31 **
Daily gain (g/day)
Week 1±4 84b 123a 143a 10 **
Week 4±8 91b 95b 141a 11 **
Week 8±12 95b 120a,b 153a 13 *
Week 12±16 91b 124a,b 154a 13 *
Overall daily gain 90c 115b 147a 8 ***
FCR (kg feed/kg weight gain)
Weeks 1±4 9.88a 6.90b 5.98b 1.01 *
Weeks 4±8 10.16 9.76 7.34 1.06 NS
Weeks 8±12 11.94a 9.00a,b 7.41b 1.28 *
Weeks 12±16 11.94 8.84 8.67 0.8 NS
Overall 1±16 10.05a 8.47b 7.31c 0.4 ***
aMeans in rows with the same superscripts (a,b,c) are not different (P> 0.05).
between the energy densities used in the current study was not large. Metabolizable energy intake between low and medium energy diets was different, yet, the ME contents of the two diets were similar. This might be due to the fact that the low energy/high ®bre diet generates a greater heat increment than medium energy diet and consequently limits feed intake.
Feed intake in sheep is affected by BW, age, type of feed, and dietary energy density (Agricultural Research Council, 1980; Lu and Potchoiba, 1990). The effect is greater in sheep of small body size (20 kg) than those of large size (40 kg). Older sheep are better able to cope with coarse diets presumably due to the well developed rumen. The particle size of feeds affects their intake by ruminants. For coarse feeds, an increase in energy density is associated with higher intake whereas for ®ne feeds an increased density decreased feed intake (Gatenby, 1986). Intake of ®ne feeds such as concentrates or ground roughage is considerably greater than that of coarse foods with or without concentrates (Gatenby, 1986). Intake in goats seems to be controlled by gut ®ll when dietary energy density was below 2.46 Mcal ME/kg (8.66 MJ/ kg) (Lu and Potchoiba, 1990).
Feed intake studies on ruminants fed high concen-trate diet have shown that feed intake generally decreases when animals are fed high concentrate diets, presumably because dietary roughage no longer limits feed intake and the animal is eating to meet its energy needs (Dinius and Baumgardt, 1969). Animals in hot climates, however, may behave differently in this respect. Battachharya and Uwayjan (1975) demon-strated that feed intake progressively increased as the proportion of grain in the diet increased (highest grain level fed was 75% of diet). Similarly, the present study observed progressive feed intake with increasing grain fed (highest grain level was 80% of diet) (Table 1). Overall, feed intake tends to be lower in hot versus cooler environments. Grains, which generate a lower heat increment than roughage, tend to improve feed intake by lowering the overall heat increment of the diet.
In the current study, DMI expressed in all three forms was higher at 194 than 80 days of age which is in line with previous ®ndings (Gatenby, 1986) that older sheep have higher intake compared to younger ones. This may be attributed to their better developed digestive system including physical structure of the
rumen, papillae, and rumen micro-organisms coloni-zation.
3.3. Growth rate and feed conversion
Initial weights of animals were not signi®cantly different but ®nal weights of animals, and conse-quently average daily gains, were greater on the higher energy diets, re¯ecting the progressive increase in energy density of the diet. The rate of growth was linear throughout the trial on all three diets. Faster growth rates may be chie¯y attributed to an increased DM and ME intake throughout the experimental period. Lambs growth rate, at a given age, is a function of food intake rather than time (Butter®eld, 1988). Growth rates recorded in the current study, although were comparable to those of improved tropical breeds (Gatenby, 1986; Kusina et al., 1991), they were lower than those reported for the breed under similar con-ditions. Mahgoub and Lodge (1994a,b) reported a daily BW gain of 184 g for intact Omani sheep raised on concentrate diet up to 38 kg BW and Chesworth et al. (1996) reported a gain of 172 g/day for Omani sheep of weights and ages similar to the current study. These differences may be attributed to differences in preparation of animal feeds. The previous two experi-ments used pelleted sheep concentrate in contrast to the current experiment where ground concentrate was mixed with chopped Rhodesgrass hay.
respectively. El Hag and Al-Shargi (1998) reported a poorer value of 17.8 FCR in Omani sheep of older ages (12±15 month). However, values obtained from the current study were lower than those obtained with Omani sheep in other studies. Chesworth et al. (1996) reported a 5.48 and 6.02 FCR for Omani sheep raised on diets based on Rhodesgrass hay and palm fronds, respectively. The poor FCR may explain the lower growth rates achieved in the current study compared to those (174 g/day) of Chesworth et al. (1996).
3.4. Body composition
Sheep fed various energy densities and slaughtered at the end of the experiment (ages 192±196 day) had signi®cantly higher weights of body, hot and cold carcass and empty body weight {EBW} as well as lower proportions of full and empty digestive tracts, blood and gut contents in the EBW than those slaugh-tered at the start of the trial (average age of 104 day) (Table 4). These effects of age have been documented in sheep (Butter®eld, 1988). However, there was no
effect of age on shrinkage or dressing percentage apparently because of the extremely low values for the low energy density group.
Sheep fed high energy diet had heavier BW at slaughter, EBW, carcass weight, dressing percentage and shrinkage than sheep fed medium and low energy feeds (Table 4). Expressed as % of the EBW, there were no differences in hot carcass weight between sheep raised on different energy levels (Table 4). Animals fed low energy diet had higher digestive tract content than those fed medium and high energy diets (Table 4). Larger gut ®lls were expected when low-energy diet were ingested (Lu and Potchoiba, 1990). This is because of the higher ®bre content in the low energy diet.
Age decreased carcass water and protein but increased levels of fat and had no effects on ash in both carcass and non-carcass parts of Omani sheep. This is in line with reports on sheep as fat is a late maturing body tissue (Butter®eld, 1988). Effects of diet on carcass chemical composition, on the other hand, were small (Table 5). Low energy density animals had higher carcass protein (as % DM) than
Table 4
Weights of body components in Omani male sheep fed low, medium and high ME energy dietsa
Item Dietary energy concentration Pooled SE Effects
Initialb Low Medium High Agec Dietd
Age (d) 104 192 196 195 3 *** NS
Slaughter W (kg) 16.18 27.40b 30.16b 34.41a 1.31 *** **
Empty BW (kg) 14.47 23.20c 27.03b 30.72a 1.2 *** ***
Hot carcass W (kg) 7.71 12.24c 14.29b 16.44a 0.7 *** **
Cold carcass W (kg) 7.48 12.02c 14.06b 16.17a 0.69 *** **
Digestive tract content (kg) 2.5 4.20a 3.13b 3.70a,b 0.21 *** ***
Shrinkage (kg) 0.22 0.22b 0.24a,b 0.27a 0.01 NS *
Dressing (%) 47.5 44.5b 47.3a 47.7a 0.8 NS *
% of Empty BW
Hot carcass 53.28 52.76 52.87 53.53 0.62 NS NS
Full digestive tract 27.64 25.76a 18.24b 18.27b 1.08 *** ***
Empty digestive tract 10.34 7.36a 6.59b 6.18b 0.33 *** *
Blood 4.75 4.08 3.89 3.7 0.15 *** NS
Digestive tract contente 15.45 15.33a 10.38b 10.75b 0.67 *** ***
aMeans in rows with the same superscripts (a, b, c) are not different (P< 0.05). bAnimals slaughtered at the start of the trial.
both medium and high energy density animals. There was a trend of increasing fat deposition in the carcass and non-carcass with increasing energy density with the results being signi®cant in the non-carcass. This is in line with reports in cattle (Ferrell et al., 1978) and sheep (Ferrell et al., 1979) where high energy diets resulted in slightly higher fat content of the empty body tissue compared to low energy diets. The results are also similar to those of Kusina et al. (1991) with native Sabi sheep in Zimbabwe. As for Sabi, Omani sheep in the current study has been slaughtered at approximately two-thirds of their mature weight. This suggests that the most variable and late maturing carcass component, fat, is still at early stages of maturity which must have resulted in little diffe-rences in carcass composition. Non-carcass fat matures earlier (Butter®eld, 1988) and therefore, effects of dietary energy density are likely to be more profound in non-carcass fat than the carcass fat which is late maturing.
Acknowledgements
The authors like to thank Mr A.R. Richie, Mr N.M. Al Saqry, Mr R.M. Al-Busaidi, Mr A.S. Al Halhali, Mr R.S. Al Muqbali, Mrs K. Anamalai of the Dept. of Animal and Veterinary Sciences, Sultan Qaboos Uni-versity for rendering the technical assistance in animal experiments and chemical analysis. This paper is published with the approval of the College of Agri-culture, Sultan Qaboos University, as paper number 041098.
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