Application of the slope‐ratio growth assay technique to estimate tryptophan availability in soybean meal fed to young rats

Abstract

The slope-ratio assay for rat was used to determine whether tryptophan (Trp) availability in soybean meal (SBM) is affected by the presence of other amino acids (AAs). In a preliminary study, rats were fed graded levels of Trp-supplemented diets to establish the Trp concentration range over which the weight gain response was linear. This range was found to be from 0.04% to 0.12% Trp. Subsequently, rats were fed basal (0.045% Trp) or Trp-supplemented diets from three different sources: l-Trp alone, SBM, or l-Trp mixed with other AAs to reflect AA levels in the test SBM (AA-mix). Weight gain in rats increased linearly with supplemental Trp intake (p<.05) for all Trp sources. Compared to the slope achieved with l-Trp alone, the estimated availability of Trp in SBM was 84.4%, while for the AA-mix it was 93.4%. It is evident that the 6.6% reduction in l-Trp availability in AA-mix is due to metabolic costs derived from excess levels of other AAs beside Trp, given that the absorption of crystalline l-Trp in the small intestine is 100%. In conclusion, the Trp availability of SBM was estimated to be around 90.4% (i.e., 84.4/93.4×100) after correcting for the effects of the other AAs in SBM.