Determining other amino acids requirements
The amino acid requirements are usually estimated by dose-response studies and the most common approach is to express the requirements as an amino acid ratio. Amino acid ratio is a means of expressing the requirements for amino acids relative to the requirement for lysine. Lysine is used as a reference because is typically the first-limiting amino acid in most swine diets and the proper concentration of lysine and other amino acids is essential for protein synthesis ( Table 1 ).
In dose-response experiments, the first limiting amino acid in the diet must be the amino acid for which the requirement is wanted to be estimated and the second limiting amino acid must be lysine. The supply of other amino acids and nutrients should meet or slightly exceed the requirements to avoid being a limiting factor. Using this approach, the requirement is determined at the point which both the tested amino acid and lysine are equally limiting and can, therefore, be expressed relative to lysine (Simongiovanni et al., 2012).
The requirements of amino acids relative to lysine are often depicted as a diminishing returns model. This model can be used to determine which ratio provides 95 to 98% of the maximum performance and indicates the most economical amino acid ratio. Thus, the optimum amino acid ratio should be set by balancing the value accrued in performance to the incremental cost to increase the ratio.
A model has been developed to determine the tryptophan:lysine ratio for growing pigs (Tryptophan:lysine Economic Model).