Amino acid bioavailability: a comparative evaluation of several assay techniques

A fish meal and a soybean meal sample were bioassayed for available amino acid content according to five different published techniques. The techniques consisted of: 1) a two week chick growth assay (GA) for available lysine; 2) a digestibility assay performed by total fecal collection (TFC); 3) a digestibility assay based on the analysis of ileal contents IA); 4) a procedure based on partial fecal collection and analysis of a dietary marker using gnotobiotic chicks (GF); and 5) procedure 4) using conventional chicks (CONV). The GA gave high lysine availability estimates for both proteins, but values for fish meal were significantly reduced when the level of dietary incorporation was increased. The TFC assay proved difficult in collecting adequate sample and, as a consequence, abnormally high availability values were obtained. Modification of this collection procedure resulted in values comparable to the IA which gave the lowest average values of all the assays (90.5 per cent for fish meal and 91.1 per cent for soybean meal). Mean availabilities calculated from GF and CONV assay were significantly higher than the modified TFC and IA techniques. Significant differences were obtained in several individual amino acids when the GF and CONV assay were compared. A growth assay, using diets formulated on the results of these assays, showed that the available amino acid profile obtained from the CONV assay resulted in significantly improved weight gains and feed conversions as compared to a similar diet based on the chemical analysis of amino acids alone. All diets utilizing available amino acid data resulted in improved feed conversions.