Application of Apparent Metabolizable Energy versus Nitrogen-Corrected Apparent Metabolizable Energy in Poultry Feed Formulations: A Continuing Conundrum

Simple Summary Despite some limitations, the metabolizable energy system has been extensively used for describing the available energy in ingredients and for formulating complete poultry feeds. Three methods, namely direct, difference (substitution), and regression, or modifications thereof, have been employed to measure the apparent metabolizable energy (AME) of feeds and ingredients for poultry. The AME of feed ingredients are often corrected for zero nitrogen (N) retention to estimate the N-corrected AME (AMEn). Although the need for N-retention corrections has been intensely debated and challenged ever since the advent of the AME system, no definitive conclusion has been reached and the majority of poultry diets today are formulated to meet the requirements for AMEn rather than AME. There is limited information on the effect of zero N-retention correction on the energy value of major protein sources. The aim of this investigation was to understand the consequences of correction to zero N retention to the energy values of samples of several protein sources differing in protein quality. Based on the data presented herein, correcting AME values to zero N retention for modern fast-growing broilers penalizes the energy value of all major protein sources and is of higher magnitude for ingredients with higher protein quality. In the present investigation, N retention, AME, and AMEn data from six energy evaluation assays, involving four protein sources (soybean meal, full-fat soybean, rapeseed meal and maize distiller's dried grains with solubles [DDGS]), are reported. The correction for zero N retention, reduced the AME value of soybean meal samples from different origins from 9.9 to 17.8% with increasing N retention. The magnitude of AME penalization in full-fat soybean samples, imposed by zero N correction, increased from 1.90 to 9.64% with increasing N retention. The Delta AME (AME minus AMEn) in rapeseed meal samples increased from 0.70 to 1.09 MJ/kg as N-retention increased. In maize DDGS samples, the correction for zero N retention increased the magnitude of AME penalization from 5.44 to 8.21% with increasing N retention. For all protein sources, positive correlations (p < 0.001; r = 0.831 to 0.991) were observed between the N retention and Delta AME. The present data confirms that correcting AME values to zero N retention for modern broilers penalizes the energy value of protein sources and is of higher magnitude for ingredients with higher protein quality. Feed formulation based on uncorrected AME values could benefit least cost broiler feed formulations and merits further investigation.