Methionine is considered the first limiting amino acid in poultry due to its high demand on protein synthesis and feather development. Methionine requirements, on an as-fed basis, during the production stage of broiler chickens varied 0.50–0.52% for starter phase, 0.38–0.45% for grower phase and 0.32–0.44% for finisher phase.
Thus, it is common practice to supplement synthetic methionine in poultry diets to balance the dietary amino acids. The commonly used synthetic methionine products include DL-methionine (DLM) and methionine-hydroxy analogue (MHA) and the latter is present either a free acid (MHA-FA, containing 88% of active substance) or a calcium salt (MHA-Ca, containing 84% of active substance).
Although MHA is not considered as true amino acid as it lacks amino group in their structure, it is known that it converts to biologically active L-methionine. Thus, there has been an on-going controversial debate concerning the biological availability of MHA compared to DLM in poultry.