Historical Flaws of Metabolisable Energy Bioassays for Poultry – A Mini Review

Dietary energy available to animals is key to formulating feed as it is required for all aspects of animal life. In poultry, apparent (AME) and true (TME) metabolisable energy (ME) values have been developed for feed formulation with or without correction for nitrogen balance. Over the past 50 years, the accuracy of ME systems has been an ongoing debate and the data produced from different bioassays have been found not to be comparable (Farrell, 1999). Overall, the ingredient matrix ME values used in feed formulation are not consistent and to some extent confusing (Mateos, et al., 2018; Pesti and Edwards Jr, 1983). Therefore, a thorough review of the bioassay methods and the values published in the literature becomes necessary.

A recent review discussed the reasons for the discrepancies among AME values from different sources, focusing on factors such as physico-chemical characters of diets and ingredients, heat processing, feed form and particle size, dietary fibre and fat content, antinutritional factors, and supplementation with additives (Mateos et al., 2018). However, aspects related to the experimental design, data analysis and even human errors present in the literature have not been evaluated. Thus, we examined the ME data produced over the past century to elucidate pros and cons of different ME bioassays, the practice of correcting ME to zero nitrogen retention, and other methodological errors such as equations used in the calculations.

It is concluded that accurate ME values have to be generated by in vitro bioassays to ensure accurate utilisations of other approaches, namely prediction equations, table values, in vitro assays and NIR analysis, to estimate ME of feedstuffs for feed mills. We suggest that the multiple linear regression method or basal diet substitution method, with several levels of the test ingredient, be used to generate ingredient matrix ME values. Until a systematic reevaluation of the ME values of feed ingredients used for poultry, feed formulation will continue to rely on data with large deviations, leading to less than satisfactory practical outcomes in precision feeding.