The nature and determination of metabolizable energy : a thesis presented for the partial fulfilment of the requirements for the degree of Master of Agricultural Science in Animal Science at Massey University

Abstract

Part 1 of this thesis involves a review along quantitative lines of investigations concerning metabolizable energy (ME) in poultry nutrition. A model for the mechanism of nitrogen (N) excretion is presented and examined in an experimental section comprising Part 2.

In Part 1, Chapter is an exposition of the distribution and utilization of feed energy and raises the subject of additivity and questions of standardization of ME assay procedure. Chapter 2 consists of 2 sections. Section 1 provides a literature review of the bioassay determination of apparent metabolizable energy (AME). It covers an analysis of the nature of AME and explains basic concepts, provides mathematical definitions and perspectives and N corrected AME values (AMEn) are discussed. Further it describes methods of determination and provides an examination of the factors involved in AME variation. Section 2 deals with the nature of true metabolizable energy (TME) in which definitions and derivations of TME are provided, the relationship between TME and AME given, deviations from linearity of the energy excreted (EO) on energy input (EI) regression investigated and N corrected TME values (TMEn) discussed. Additionally, methods and evidence bearing on the central premise to TME are presented and other areas that have gained attention reviewed.

In Part 2, the subject of Chapter 3 is a linear experimental model developed by King (1984) to explain deviations in linearity of the relationship between N excreted (NO) and N intake (NI) as it may apply to adult cockerels and the nature of the correction of TME values to zero N balance (ZNB). Chapter 4 deals with 2 experiments, LN 202 and LN 204. The primary objective of Experiment LN 202 was to examine and investigate the regression relationship, EO on EI obtained for adult cockerels, and to assess the effect on it of correcting EO to ZNB. Experiment LN 204 was set up to study the impact of diet and assay procedure on TME of meat and bone meal (M & B) and to examine the effect on these values of correcting to ZNB. The effect of assay procedure on TME and TMEn of a whole diet was also explored.

In Experiment LN 202 2 slopes were obtained for the regression relationship, EO on EI, and on correcting EO to ZNB slope satisfactorily represented the relationship. This was consistent with expectations arising from the model. The model suggests that this slope contains a bias element that causes TME values to deviate from unbiassed TME.

In Experiment LN 204 the TME values of M & B as determined by dietary inclusion and by direct supply were compared and assessed in terms of the model. Correction of the TMEs to ZNB resulted in a single value. TME assessment of a whole diet by 2 different assays resulted in similar values when the values, according to the model, estimated the same quantity and different values when the quantities measured were, as predicted by the model, different. Correction to ZNB caused like values to deviate and unlike values to come closer together.

Chapter 5 provides an overview of the model and the experimental findings and outlines the conclusions that have been drawn from this work.