Significance and limitations of the concept of dietary amino acid balance for the growing pig

Abstract

After being initially expressed by the dietary crude protein content, the protein requirement of the growing pig has been given a composite significance by considering the amino acid needs separately of each other, to end finally with the concept of a balanced (or "ideal") protein with constant ratios between the requirements for essential amino acids. The improvement of the dietary amino acid balance, with a resulting decrease in protein content, can now be obtained by an additional supply of industrial amino acids : first lysine alone, as the first limiting amino acid in most pig diets, then lysine in combination with the secondary limiting amino acids (threonine, tryptophan, methionine). Besides their limiting character, excesses of some amino acids may occur, as is the case for imbalance between branched amino acids, excess of arginine or imbalance between large neutral amino acids which may explain the depressive effect of excess protein on voluntary feed intake. In a second stage, the limitations of the concept of "ideal" protein were analyzed : non constancy of the amino acid composition of deposited protein, differences in the metabolisme of individual amino acids, changes in the availability of dietary amino acids. On the other hand, in addition to protein sparing, the improvement of the dietary amino acid balance allows a better utilization of dietary energy and exerts a stimulatory effect on voluntary feed intake, thus enabling a reduction of feeding cost and an optimal use of genotypes selected for a high rate of lean tissue growth.