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Evidence Supporting the Hypothesis that Ambient Temperature and Dietary Composition Influence the Relative Efficacy of Methionine and Its Hydroxy Analogues for Broilers: A Review

D. Balnave^* and J. Brake

^* Faculty of Veterinary Science, University of Sydney, Camden, New South Wales, Australia 2570
Department of Poultry Science, College of Agriculture and Life Sciences, North Carolina State University, Raleigh, North Carolina 27695-7608

Correspondence: J. Brake, E-mail: jbrake{at}ncsu.edu

Many comparisons have been made between DL-methionine (DLM) and various methionine hydroxy analogues (MHA) as sources of dietary methionine activity for broilers, with conflicting results between studies often reported. A review revealed that certain dietary factors may influence the outcome of such efficacy studies. Purified crystalline amino acid diets, which are generally characterized by a low Arg:Lys ratio of 1.07 or less and high levels of dietary salt (NaCl or NaHCO3), typically have shown better growth performance with DLM than with MHA supplements. When practical or when semipurified diets that contain intact proteins were fed under thermoneutral conditions, the advantages observed, when using DLM in purified crystalline amino acid diets, were greatly reduced. Indeed, in many cases the differences in biological performance between the 2 sources of methionine activity were often not significant. These latter diets generally have had Arg:Lys ratios of 1.05 and greater and much lower concentrations of NaCl than the purified crystalline amino acid diets. With practical diets under heat stress conditions, the MHA supplements appeared to perform statistically as well as, or better than, DLM supplements as long as the Arg:Lys ratio was approximately 1.20 or greater. Thus, data exist that demonstrate that particular dietary factors and ambient temperature could inadvertently influence the outcome of efficacy studies. Further, these data suggest that the selection of methionine activity source in practical conditions should be tempered by knowledge of the prevailing Arg:Lys ratio, dietary salt inclusion levels, and the thermal environment.

Key Words: methionine • methionine hydroxy analogue • arginine • lysine • sodium chloride