Chick bioassay of available methionine and methionine plus cystine. Development of assay procedure

A series of experiments has been conducted with young chicks to establish a valid assay procedure1 for assessing available sulphur amino acids (SAA) in protein concentrates. A basal diet was established in which soya bean meal provided 20% crude protein. This diet was shown to be limiting in only SAA. When adequately supplemented with methionine or methionine plus cystine it promoted excellent chick growth. The adaptation of the diet for the assay purposes is described. The linear portion of the regression curve of response on a logarithmic dose scale for methionine alone was found to lie between 0.015 and 0.25%; and 0.009 and 0.09% for methionine in the presence of 0.4% cystine. These regions constituted the potential ranges for the bioassay of methionine and SAA.