Metabolism of Energy and Implementation of Net Energy System in Laying Hens

Abstract

Dietary energy is an expensive component of poultry diet formulations and may be as high as 60% of diets costs in certain situations. Different energy evaluation systems have been used for poultry diets formulations. The apparent metabolizable energy system (AME) is widely accepted and has been applied in the industry for 50 years in most world areas. The system compares the total gross energy of the diet to that present in excreta to determine what is retained. Development of energetic measurement techniques such as open and closed circuit calorimetry have enabled researchers to measure the wasted heat energy to determine the true available amount of energy for different body functions as net energy (NE). While the NE system has been proposed as a more accurate system for expressing feed energy and birds energy requirements compared to the AME system some difficulties remain in the poultry area with respect to the effect of age, environmental conditions and lack of data. Net energy based feeding systems are in successful use for pig and cattle feed formulation. For laying hens most nutritionists use the same AME values used for broilers. While this may be adequate, the use of broiler NE values for laying hens would not likely be acceptable as broilers are growing at a much higher rate than layers. This thesis examined the application of the NE system in laying hens. Chapter 1 provides general information about energy metabolism in body with more focus on the objectives of this study experiments. Chapter 2 is the literature review that provides the scientific background for the comparison of different feed energy evaluation systems and their limitations in practice. Included is a discussion on the metabolism of energy in chickens, partitioning of energy for different metabolic activities (maintenance, growth, and production) and dietary energy utilization for various body functions. The effect of the dietary profile, age, genotype, physiological status and environment on the energy metabolism, specifically on the net energy of the diets are considered and discussed.<br/>Chapter 3 studies the application of the bioassay method for measuring the AME, AMEn (AME adjusted for zero nitrogen retention) and AMEs (AME adjusted to 50% nitrogen retention) values of common dietary ingredients in layers feed as specific ingredients AME values are rarely available for laying hens. The bioassay evaluation used the reference diet substitution method and compared the data with the regression estimation method. The results confirmed that the in vivo measured AME values of ingredients using laying hens were close to those calculated from proximate composition using the European prediction equation and tabulated values based on adult cockerels. The results showed a good agreement between the reference diet substitution and regression methods to estimate ingredients AME content. In conclusion, the AMEn values are not representative of production conditions, in particular for the high-protein ingredients. In addition, AME values as obtained from the difference method should be interpreted with caution as it is affected by the CP content of the test diet. AMEs would then be the most representative of productive conditions.<br/>Chapter 4 evaluates the energy efficiency and net energy prediction of feed in laying hens. Using closed-circuit calorimetry chambers by feeding different diets with various nutrient contents to the laying hens in different ages in the production phase enabled the measurement of gas exchange, heat production, AME and NE of diets. Then AME and NE equations were generated based on diets and applied to or ingredients. The equations were further validated in calorimetry chambers. It was confirmed that the NE of diets can be predicted from AME or AMEn, crude protein and ether extract levels in laying hen diets.<br/>Chapter 5 describes two production experiments that were conducted to investigate the influence of different energy ratios (NE/ AMEn) by increasing dietary ether extract (EE) levels on birds performance and egg quality parameters. This chapter examined the effect of formulating diets based on the NE system compared to the default system (AME) and is intended to provide recommendations for nutritionists serving the layer industry. The results indicate that higher NE/AMEn diets with added EE improved hen performance and egg quality with higher albumen and Haugh units and darker yolk color score.<br/>Chapter 6 examines the energy metabolism at the molecular level. The effect of dietary NE/AME levels on messenger RNA (mRNA) expression of genes involved in energy metabolism and lipogenesis in laying hens was examined. Feeding laying hens diets with different NE/AME and levels of EE over time increased mRNA expression of peroxisome proliferator-activated receptor gamma (PPARG) a gene involved in fatty acid storage and glucose metabolism, in jejunal mitochondria. The different dietary treatments did not alter the mRNA expression of genes involved in cellular energy metabolism, oxidative phosphorylation or fatty acid synthesis. Furthermore, mitochondrial content per cell remained unchanged as a result of changes in dietary NE/AME ratio.<br/>This studies conducted in this thesis have provided the data necessary for nutritionists to begin implementation of an NE based formulation system for layer feed. An NE database of ingredients has been provided along with equations that can be applied to ingredients not present in the database so the NE value can be generated. The system gives higher NE values to ingredients with higher EE levels and lower NE values to ingredients with high protein levels relative to the AME system. This should give nutritionists operating in the layer industry to formulate diets more efficiently than before with improved performance and lower dietary costs. Further study is warranted to further confirm the benefits of the NE system with the existing AMEn system for layers