The definition of energy utilisation efficiency is the total energy requirement to deposit 1 kJ energy in body mass. For example, the total energy requirement for protein deposition (kp) was 1.66 kJ/kJ and 1.25 kJ/kJ for fat deposition (kf) in pigs (Van Milgen and Noblet, 1999). For broilers it was 2.1 kJ/kJ for protein deposition and 1.4 kJ/kJ for fat deposition (Emmans, 1994; Emmans and Fisher, 1986). The energetic efficiencies can estimated experimentally or theoretically based on biochemical and biological transformations.
2.1.4.6.1 Experimental and theoretical energetic efficiency for protein and fat deposition
Observed efficiency of energy utilisation for protein and fat deposition varies, due to factors such as the nature of feed, dietary energy to protein ratio, strain, sex, age and environmental factors (Boekholt et al., 1994; Lopez et al., 2007; Sakomura et al., 2005).
Petersen (1970), using the White Plymouth Rock breed, estimated the efficiencies of energy utilisation to be 0.51 and 0.78 for protein and fat deposition, respectively. Sakomura et al. (2005), using the Ross breed, found the efficiency of energy utilisation for protein ranged from 0.36 to 0.58, and for fat it ranged from 0.55 to 0.92 (Table 2.2). The diet composition can have a major impact on energy efficiency (Lopez and Leeson, 2008a; De Groote, 1974). Experimental evidence of the energetic efficiency of nutrients (carbohydrate, fat and protein) in broilers is scarce.
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Table 2.2. Experimental efficiencies of energy utilisation for protein (kp) and fat deposition (kf) in poultry
The theoretical efficiency of fat and protein deposition can be estimated from biochemical considerations. The theoretical efficiency of fat and protein deposition in the animal body, from different energy sources, are summarised in Table 2.3
Table 2.3. The theoretical efficiency for fat and protein deposition from different energy sources Energy sources Product Blaxter, (1989) Baldwin, (1995) Quiniou, (1996) Van Milgen, (2006)
Carbohydrate Body fat 0.80 0.78 0.81 0.81-0.84
Fat Body fat 0.96 0.97 0.97 0.97
Protein Body fat 0.66 0.67 0.81 0.52
Body protein 0.86 0.83 - 0.85-0.90
The theoretical (biochemical) efficiency of using carbohydrate for fat deposition varies between 0.78 and 0.84, depending on the animal species and metabolic pathways, and whether the ATP yield from glucose is accounted for or not. When the ATP synthesis from glucose is not included in the efficiency calculation, the energy efficiency of triglyceride synthesis from glucose is 0.80: and when the ATP synthesis from glucose is included the efficiency is 0.84 (Baldwin, 1995).
The theoretical efficiency of fat deposition from dietary fat is very high and close to unity (Van Milgen, 2006). According to Baldwin (1995), after hydrolysis of dietary fat to monoglycerides and fatty acids, reformation of triacylglycerol and hydrolysis of blood triacylglycerol to glycerol and fatty acids, followed by re- esterification in adipose tissue, the net energy cost for fat synthesis is:
Reference Breed kp kf
Petersen, (1970) White Plymouth Rock 0.51 0.78
Scheimann et al. (1972) Unknown 0.61 0.84
De Groote, (1974) Hubbard 0.35- 0.70 0.70 -0.84
Boekholt et al. ( 1994) Hybro strain 0.66 0.86
Nieto et al. (1995) White Rock 0.40 -0.58 0.64 -0.9
Sakomura et al. (2005) Ross broilers 0.36-0.58 0.55-0.92
Zoone et al. (1991) Broiler 0.39 0.70
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12 × 0.074 MJ/mol ATP)/ 31.74 MJ/ mol tripalmitin = 0.03. The energy expenditure of 0.074 per mol ATP calculated as (180.2 g/mol glucose × 0.0156 MJ/g glucose)/38 ATP/mol glucose. Then, the result of an efficiency of fat deposition from a dietary fat source is equal to 0.97.
Roux, (2009) estimated the theoretical efficiency of protein deposition from dietary protein as:
kp = (22.6 × PR)/[(22.6 × PR) + (3.76 × PS)] where
PR is the protein retention (kg/d),
PS corresponds to the given rate of PR, allowing for turnover, and the coefficient 22.6 represents the energy content of protein (MJ/kg).
The cost of synthesis protein is 3.76 MJ/kg. A reasonable enough estimate can be derived, based on the assumption that 5 mol ATP is required to arrange one mol of peptide bonds (Van Milgen et al., 2001a), or 5/38 = 0.132 mol of glucose and then 0.132 mol of glucose × 2.82 MJ/mol glucose = 0.37 MJ of heat will be released per mol of peptide bond. Blaxter (1989) assumed that the average molecular weight of the constituent amino acids in protein is equal to 100g and then the energy cost per kg of protein deposition is 3.7 MJ, which is close to the value of Roux, (2009). If there is no protein turnover, then kp = 0.86. Similarly, Van Milgen (2001a) reported that the theoretical efficiency of deposition protein from amino acids is 0.87. However, the protein turnover, through repeated hydrolysis and synthesis of peptide bonds, contributed to a low efficiency of protein deposition. These terms are not easy to deal with and efficiency can change, depending on the number of turnover cycles. In pig, protein turnover typically occurs at a rate between two and three times (Reeds et al., 1980), which suggests an energy cost for turnover between 8 and 12 MJ/kg of protein deposition, and assuming that there is no other energy requirement, this implies an efficiency of protein deposition between 0.65 and 0.75. In chickens, based on 6.2 MJ/kg of synthesised protein, the efficiency of protein deposition would be 0.78 (Muramatsu et al., 1987).
Deposition of amino acid as fat deposition is implied with an additional energy cost for nitrogen excretion and energy for turnover cycle (assumed four times synthesis and three times breakdown). This situation would result in an efficiency of 0.63 for fat deposition (Van Milgen, 2001a).
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There are differences when comparing theoretical efficiency with experimental efficiency. Van Milgen, (2006) indicated that the experimental efficiency for fat deposition from dietary fat is much lower than the theoretical efficiency, and this may be due to the oxidation of dietary fat for ATP synthesis combined with de-novo lipid synthesis from other nutrients. In addition, part of the energy coming from dietary fat may be used for maintenance requirements (Van Milgen et al., 2001a). Consequently, the available energy for fat deposition from dietary fat may be decreased.
Wongsuthavas et al. (2008) reported that, when polyunsaturated fatty acids were fed to birds, the birds preferred to oxidise the fatty acids and yield ATP. So the carbohydrates had shifted from oxidation to the lipogenic pathway and therefore the energetic efficiency for fat deposition decreased: i.e. the conversion of glucose to triglycerides is less efficient in energy deposition than conversion of fatty acids to triglycerides.
The maximum theoretical efficiencies for protein deposition from amino acids range between 0.85 and 0.90 (Van Milgen, 2006). However, experimental values range between 0.48 and 0.63, so it is clear that repeated hydrolysis and synthesis of peptide bonds contribute to a low efficiency. In addition, the estimated experimental efficiency of protein deposition is 0.52, when amino acids are used for support costs and 0.63 when glucose is used for support costs (Van Milgen, 2006).