Nutritional evaluation of wheat. 2. The sequence of limiting amino acids in wheats of different protein content as determined with growing rats

Abstract

SUMMARY1. Weanling male rats were used in three experiments to evaluate the sequence of limiting amino acids for maximum growth with diets based on wheats containing 10, 13 or 17% crude protein (N×5·71).2. Lysine was the first-limiting, threonine the second-limiting, and valine or methionine the third- or fourth-limiting amino acid for growth rate and feed-conversion ratio of rats offered the diet based on wheat containing 10% crude protein. A further four amino acids (isoleucine, histidine, tryptophan and tyrosine) were limiting for feed-conversion ratio only.3. For the diet based on wheat containing 13% crude protein the sequence of limiting amino acids was lysine, threonine and methionine.4. Only lysine was limiting in the diet based on wheat containing 17% crude protein.5. Plasma-free amino-acid concentrations were found to be a valuable criterion for prediction of the first-limiting amino acid in diets.6. Improvement of growth and feed-conversion ratio was observed with diets based on wheat containing 17 or 13% crude protein when the limiting amino acids were supplemented to provide the recommended allowance. Further improvement in feed-conversion ratio was obtained when lysine and threonine were provided in excess of the recommended allowances.7. There was a lysine-threonine interaction affecting growth rate and feed-conversion ratio, and a lysine-me.thionine interaction affecting feed-conversion ratio.