Abstract: In this study, two levels of Metabolizable Energy (ME) (2.3 and 2.5 Mcal kg-1 DM) and three levels of Crude Protein (CP) (12, 14 and 16%) and their interactions were studied to identify the optimum levels of dietary energy and protein for lambs of the Taleshi breed. The growth performance variables measured included Average Daily Gain (ADG), final weight, Daily Dry Matter Intake (DDMI, kg day-1) and Feed Conversion Ratio (FCR) whilst other commercially important factors were assessed during specific periods. The urea dilution method was used for predicting the body composition of live lambs. At the end of the experiment, all animals were slaughtered, the carcasses were cut and the chemical composition of the area around the 9th, 10th and 11th rib was measured. The study comprised a completely randomized design with a 2x3 factorial arrangement with four replications per diet. The results showed that the energy and protein levels had a significant effect (p<0.05) on growth. The diet with 2.5 Mcal ME kg-1 and 14% CP was associated with the best final weight, ADG, FCR, feed cost kg-1 gain, un-variable profit kg-1 live weight, un-variable profit/total gain, carcass weight, shoulder weight, thigh weight and ribeye area (REA). A high dietary energy level helps to produce more ME and fermentable products for paunch microorganisms resulting in an increase in the synthesis of microbial protein and therefore the amount of protein available to the animal. Increasing the dietary protein level causes a change in the process of fermentation in the paunch whilst increasing fatty acid production and the ratio of propionate to fatty acids. These changes in the paunch improve the lamb`s energy balance allowing more nitrogen to be stored and increasing the body weight.
INTRODUCTION
A range of sheep products are used around the world and nearly 2% of the global milk volume consumed is produced by sheep. In addition, an annual yield of 900,000 tons of wool, 800,000 tons of mutton and 20 million lamb carcasses contribute to world food marketing (FAO, 2006).
A number of factors affect growth performance, the quality and quantity of the carcass, as well as productivity in sheep marketing, of which the dietary energy and protein levels and their interaction are probably the most important (Manso et al., 1998; Muwalla et al., 1998; Haddad et al., 2001; Bellof and Pallauf, 2004).
Thomas et al. (2007) suggested that, by enhancing the dietary protein level in Merino lambs, the amount of dry matter intake, growth rate, wool growth and nitrogen storage can be increased in a linear manner; a proposition supported by the findings of Dove and Milne (2006) and Milis et al. (2005) showed that the amount of dry matter intake in lambs that were given a diet high in energy was increased, but this also resulted in a reduction in the amount of protein and ash on the carcass and an increase in kidney size and pelvic fat content.
Muwalla et al. (1998) showed that, when the level of dietary protein is increased from 12 to 14%, the daily weight gain rises whilst dry matter intake and food conversation ratio remain unchanged. Increasing the dietary protein level was found by Muwalla et al. (1998) to cause an elevation in growth rate, feed intake and blood nitrogen concentration. This conforms with the findings of Manso et al. (1998) and Titi et al. (2000) also showed that the optimum level of dietary protein is 16% in some breeds.
Yurtman et al. (2002) assessed the interactions of two crude protein levels (12.5 and 13.5%) and two ME levels (2.5 and 2.7 Mcal ME kg-1) and showed that the combination of 2.7 ME (Mcal ME kg-1) and 13.5% CP had the greatest beneficial effect on growth performance.
Other researchers have reported that an optimum carcass quality and yield can be obtained by a dietary energy level in the range 2.3 to 2.5 Mcal ME kg-1 (Dove and Milne, 2006) whilst others have suggested that the optimum range for crude protein is between 12-16%.
Taleshi is one of the most important sheep breeds in Iran, numbering some 400,000 animals in the north of the country (ASRI, 2007). This breed is important mainly due to its small size, high meat quality (palatability) and ability to live in mountainous areas. This study measured the effect of different levels of dietary energy and protein and their interaction in lambs of the Taleshi breed.
MATERIALS AND METHODS
Lambs and Experimental Conditions
This study was conducted during 2005-2006 in Guilan Natural Resources
and Agricultural Research Center at Iran. Twenty four male Taleshi lambs
were used with an age average of 5-6 months and an initial live weight
of 19.7±1.84 kg. The animals were allowed a one month period to
adjust to the new feeding and housing conditions prior to the start of
the experiment. They were sheared, given an anti-parasite drug (albendazole)
and a subcutaneous vaccination against diseases known to be present in
the herd (enterotoxaemia- anthrax and pox). The lambs were housed in experimental
pens and fed two times a day for a period of 105 days.
Experimental Diets
Chemical analysis of the feeds was performed according to AOAC (1990).
Subsequently, with reference to the average age and weight of the lambs,
six food rations were prepared with two levels of energy (2.3 and 2.5
Mcal ME kg-1) and one of three levels of protein (12, 14 and
16% CP). Food was offered in the morning and evening and that refused
from the previous day was removed before the new meal was given on the
following morning.
Prediction of Carcass Chemical Composition Using the Urea Dilution
Technique
The determination of carcass composition in the live animal has increased
in economic importance due to the emphasis placed on lean carcass production,
reduced resources for research activities and the desire not to slaughter
animals purely to determine their body composition (Bartle et al.,
1987, 1988; Urbaniak et al., 2001). In the current study, the Urea
Dilution (UD) procedure was used two to three days before slaughter to
estimate the chemical composition. Feed was withheld for 16 h before infusion
and a blood sample was collected from the jugular veins. All blood samples
were stored on ice in a sterile Vacutainer that contained EDTA as an anti-coagulant.
Following this, 0.65 mL of 20% urea solution was injected and blood was
extracted between 9 and 12 min post-injection. In the lab, the blood samples
were centrifuged at 3,000 x G for 20 min. Plasma urea nitrogen was estimated
and the percentage of urea with respect to live lamb weight was determined.
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Carcass Characteristics and Chemical Composition of the 9th, 10th
and 11th Rib Area
At the end of the test, feed was withheld for 16 h and the lambs were
slaughtered. The weight of the edible and inedible parts of the carcass
were determined separately. The carcasses were then butchered and each
part was separately recorded using the method of Nik-Khah and Assadi-Moghaddam
(1977). The area of the 9th, 10th and 11th ribs, together with the attached
section of spine, was used to estimate the amount of bone-free meat, fat
and bone in the carcass. The weight of these components was measured after
separation and the bone-free meat was used for chemical analysis.
Statistical Analysis
A completely randomized design of 6 diets in a 2x3 factorial design
was replicated four times. The model was:
|
Xijk = M+Bj+Bk+Bjk+Eijk |
| Where: |
| Xijk | = Observes |
| M | = Treatment average |
| Bj | = Effect of dietary protein level |
|
Bk |
= Effect of dietary energy level |
| Bjk | = Effect of the interaction between dietary energy and protein levels |
| Eijk | = Experimental error |
Data were statistically analyzed using the General Linear Model procedures of SAS with the Duncan test.
RESULTS AND DISCUSSION
Growth performance means are provided in Table 1. The energy and protein levels were found to have a significant effect on growth (p<0.05). The ration containing 2.5 Mcal ME kg-1 and 14 % CP resulted in the most favorable final weight, Average Daily Gain (ADG), Feed Conversion Ratio (FCR), feed intake cost kg-1 gain, un-variable profit kg-1 live weight and un-variable profit/total gain (p<0.05). The main aim in sheep fattening is to maximise muscle tissue growth at the lowest feed cost whilst avoiding additional fat storage in the carcass (Table 1).
The ration containing 2.5 Mcal ME kg-1 and 14% CP had the greatest effect on carcass weight, cold carcass weight and neck, shoulder, thigh, full stomach, spleen and empty intestinal weights (p<0.05) (Table 2).
The levels of energy and protein had a significant effect
on the weight of spine, ribs and fat and on the fat thickness and percentage
of moisture and crude fat (Table 3).
| Table 1: | Effects of protein and energy levels on lamb performancea |
| aIn each line, means with different superscripts are significantly different (p<0.05). (1$US = 9200 Rials), bBUN: Blood Urea Nitrogen | |
| Table 2: | Effects of protein and energy levels on lamb carcass traitsa |
| aIn each line, means with different superscripts are significantly different (p<0.05). (1$US = 9200 Rials) | |
Dietary levels of energy and protein were found to have a significant effect on lamb growth performance in agreement with the results of Khorshidi (1996) and Villalba and Provenza (2005). Increasing the energy level allows the production of more fermentable ME for paunch microorganisms resulting in a rise in the synthesis of microbial protein and in the amount of protein available to the animal (Khorshidi, 1996; Cheema et al., 1991; Early et al., 2001). This is likely to be the reason why daily weight gain was seen to rise when increasing the energy level from 2.3 to 2.5.
Conversely, increasing the level of dietary protein will
change the fermentation process in the paunch causing an increase in total
fatty acid production and a rise in the ratio of propionate to total fatty
acids. These changes in the paunch will improve the lamb`s energy balance
allowing the storage
| Table 3: | Effects of protein and energy levels on tissue properties in the area of the 9th, 10th and 11th ribsa |
| aIn each line, means with different superscripts are significantly different (p<0.05). (1$US = 9200 Rials) | |
of more nitrogen and an increase in body weight (Kabir et al., 2002). Enhancing the level of protein in the diet will change the pattern of fermentation in the rumen and will be followed by a rise in feed intake in a process regulated by light levels and peptides. Moreover, the rise in the level of protein in the diet will decrease the level of growth hormone in the blood, following which, ad libitum intake will rise in response to the increased digestibility of the diet. In addition the dietary protein content is related to the stability of the food in the rumen (Khorshidi, 1996; Ebrahimi et al., 2007). Increasing the protein content elevates the amino acid density and their absorption in the small intestine that subsequently improves animal growth (Church, 1998; Pittroff et al., 2006).
The significant effect of energy and protein levels on carcass characteristics, carcass composition and the chemical characteristics of the meat in the area of the 9th, 10th and 11th ribs agrees with the findings of Dziba et al. (2007) and Kawashima et al. (2000).
The parallel enhancement of dietary energy and protein levels in ruminants is associated with a rise in amino acids that are used as preglicolitic components in the production of acetate that can be used for acetyl-CoA production and, in turn, the biosynthesis of fatty acids. Liu et al. (2005) showed that, by increasing the dietary protein level, the amount of fat on the carcass will also rise. The results of the current study with respect to growth performance and efficiency are in agreement with those of Milis and Liamadis (2008).
In the present experiment, there was a significant correlation between urea dilution and carcass characteristics. Moreover, the relation between the amount of urea and the percentage of protein and ash in the carcass was significant and negative, in agreement with findings of Karim et al. (2001), Silva et al. (2003) and Yurtman and Coskuntuna (2006).
Eugène et al. (2004) showed that urea dilution is a reliable indicator for determining carcass composition in lambs. Bartle et al. (1987), Jetana et al. (2000) and Tebot et al. (2004) also showed that urea dilution is correlated with the chemical composition in lamb carcasses.
Thus, the current study has shown that dietary energy and protein levels have significant impacts on growth performance, carcass characteristics and commercial productivity of Taleshi lambs and that the ration with 2.5 Mcal ME kg-1 and 14% CP was superior. Furthermore, it is possible to use the urea dilution test as a predictor of carcass chemical components in lambs of this species.
ACKNOWLEDGMENTS
The authors wish to thank Dr. E. Halajiyan, Dean of Islamic Azad University, Ghaemshahr Branch, Iran and Dr. A. Qotbi for their valuable assistance. We would also like to thank the Guilan Natural Resources and Agricultural Research Center at Iran and Animal Science Research Organization of Iran for their cooperation.