Investigation of energy partitioning in modern broiler chickens : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Poultry Nutrition at Massey University, Palmerston North, New Zealand
Studies were conducted to estimate the energetic efficiencies for fat deposition from different energy sources (carbohydrate, protein, soybean oil and tallow) and to determine the maximum protein deposition (Pdmax) and minimum body lipid to protein ratio (minL/P) of the modern broiler chickens. Energetic efficiencies for fat deposition were assessed by feeding birds extra energy from different energy sources when protein was limiting in the diet. Comparison of birds slaughtered before and after the dietary treatments were applied allowed the determination of the energy retained as fat or protein. In the first experiment (Chapter 3), the energetic efficiencies of fat deposition from vegetable oil and starch were estimated to be 0.82 and 0.69, respectively. In the second experiment (Chapter 4), the energetic efficiencies of fat deposition were estimated to be 0.93 from soybean oil and 0.90 from tallow, but there was no significant difference between soybean oil and tallow. In the third experiment (Chapter 5), the efficiency of energy deposition as fat from non-essential amino acid intake was calculated to be 0.63.
In the fourth experiment (Chapter 6), the Pdmax and minL/P were determined by feeding diets not limited for protein with varying energy levels. The maximum daily protein deposition was predicted at 22 g/day. According to broken-line model, the rate of protein deposition increased when the apparent metabolisable energy intake above maintenance requirement (AMEIp) increased up to the break point of 1.2 MJ/day. Further increases of AMEIp did not lead to an increase in protein deposition rate whereas the fat deposition rate sharply increased. The body weight and energy intake affect the L/P ratio. Across all treatment groups, the minimum value of L/P ratio was observed at 0.31 for birds fed 1 MJ/day of AMEIp at 4 kg live body weight.
From the knowledge of net energy requirements and considering the efficiency of metabolisable energy for fat and protein deposition from all experiments, a simple mechanistic growth model was developed for modern broilers (Chapter 7). The model simulates the daily growth of broilers and it was able to predicting the broiler performance and carcass composition under a variety of nutritional conditions. Moreover, the model was evaluated with a range of experimental data (Chapter 8) and prediction values were in close agreement with observed values. The relative prediction errors were 3.8% and 7.3%, for prediction of slaughter live body weight for dependent and independent dataset, respectively.
In conclusion, the efficiencies of energy utilisation for fat deposition varied depending on energy sources with the highest values for soybean oil and tallow followed by starch and the lowest for protein. Modern broilers have an upper limit for protein deposition (22 g/day). The body weight and energy intake affect the L/P ratio and the minimal L/P ratio was observed at 0.31. The mechanistic growth model based on energy partitioning concepts can be a tool to predict the carcass composition and broiler performance and can deal adequately with the complexity of nutritional factors.
The finding of this thesis is that the broiler performance can be improved by formulating the diet to maximise the protein deposition with minimum fat deposition. The maximum protein deposition can be achieved when the birds consumed 1.2 MJ/day of AMEIp or 2.5 MJ/day of AME intake, further energy intake will deposited as lipid