Determination of ileal digestibility of tryptophan in tryptophan biomass for broilers using the direct and regression methods

A total of 372 Ross 308 male broilers were used in a 2-day trial to determine the ileal digestibility of tryptophan in tryptophan biomass using the direct and regression methods, and to compare the digestibility between the 2 methods. A nitrogen-free diet was prepared to estimate the basal endogenous losses of amino acids of birds. Two semi-purified diets were formulated to contain tryptophan biomass as the sole source of amino acids. Five corn-soybean meal-based diets consisted of a basal diet and 4 assay diets containing either 30 or 60 g/kg of tryptophan biomass A or B (TBA or TBB) at the expense of corn and soybean meal in the basal diet. The ratio of corn and soybean meal remained consistent across the basal and assay diets. Birds received a standard starter diet (193 g/kg crude protein) from day 0–19 post-hatch and 8 experimental diets for 2 days from day 19. On day 21 post-hatch, birds were asphyxiated with carbon dioxide and digesta samples from the distal section of the ileum were collected. The linear regression of tryptophan biomass-associated digested tryptophan against tryptophan biomass-associated tryptophan intake was conducted to determine the ileal tryptophan digestibility in tryptophan biomass. The apparent and standardized ileal digestibility of tryptophan determined by the direct method were 0.993 and 0.993 in TBA, and 0.995 and 0.996 in TBB, respectively. The apparent ileal digestibility of amino acids in corn-soybean meal-based diets quadratically increased (P < 0.05) as dietary levels of tryptophan biomass increased. The ileal digestibility of tryptophan determined by the regression method was 1.013 and 1.009 for TBA and TBB, respectively. The ileal digestibility of tryptophan in TBA derived from the regression method was greater (P = 0.033) than that of determined using the direct method, but no difference (P = 0.193) was observed in TBB. However, the ileal digestibility of tryptophan in the tryptophan biomass was close to 100 % irrespective of the method used.