An analysis of methods for the utilisation of net energy concepts to improve the accuracy of feed evaluation in diets for pigs

Accurate determination of feed energy value is pivotal to the efficiency of feed compounding and the production of meat from pigs. It can be argued that little may be gained from attempts to value feed in terms any lower in the hierarchy of energy use than their digestibility (DE); net realisation values for feed energy (NE) being subject to animal and other externalities resulting in non-transferability. However, the true value of a feed necessarily requires knowledge of final, net, energy utilisations. Choice of energy system relates to: (a) The influence upon digestibility of source of structural fibre, and the failure of current analytical method readily to distinguish these, (b) the different efficiencies of energy transfer when given dietary nutrients are transferred to given energy uses, (c) the efficiency of utilisation of work energy derived from DE disappearing as VFA rather than as glucose, (d) the need to account for the realised energy value of digested protein energy which may be either deposited in body protein or deaminated, and (e) the influence upon the efficiency of energy use of the balance of maintenance:protein deposition:lipid deposition occurring in the animal's body, as each of these functions are achieved with differing levels of efficiency. Feed energy description by correction factors for DE relating to pig age, or by prediction of NE from diet concentrations of fibre and lipid, introduce rigidities which fail to recognise the importance of animal end-product in the realisation of ultimate NE. It is concluded that NE systems require the iterative properties of simulation models which are able to incorporate the variable efficiencies of the flow of the different feed nutrient energies toward different body sinks, and account for the variable demands and efficiencies of the animal when accessing energy from various diet nutrients for various levels and ratios of end-products. (C) 1997 Elsevier Science B.V.