Evaluation of the Chick Bio-Assay for Estimating Sulfur Amino Acid, Lysine, and Tryptophan Availability in Soybean Meal

Abstract

Chick growth assays based on 9 to 13 day, 9 to 15 day and 9 to 17 day posthatching weight gains were used to determine the effect of the bio-assay time periods, using the slope-ratio technique, on the relative availability of sulfur amino acids, lysine, and tryptophan in soybean meal. Eight day bio-assays produced the highest availability estimates. Availability estimates for sulfur amino acids, lysine, and tryptophan in soybean meal were 88.46% (8 day-bioassay) vs. 74.41% (4 day-bioassay), 83.23% (8 day-bioassay) vs. 77.42% (6 day-bioassay) and 90.02% (8 day-bioassay) vs. 86.66% (6 day-bioassay), respectively. By measuring growth response of chicks fed reference standard diets as compared with that of chicks fed control diets containing equivalent levels of amino acids determined in soybean meal, the effect of excess amino acids in soybean meal on the sulfur amino acid availability estimate for soybean meal was determined. Excess amino acids did not influence the availability estimate for sulfur amino acids in soybean meal. The relative sulfur amino acid availability estimates of chicks fed excess crystalline amino acids which simulated the amino acidcomposition of soybean meal was 100.96%, as compared with 88.46% for the relative sulfur amino acid and availability estimate for soybean, as determined by the slope-ratio technique.