Apparent ileal digestibility (AID) of amino acids (AA) is usually corrected for basal endogenous amino acid losses (BEL) to yield standardized values (SID) because the latter is preferred due to its additivity in diet formulation. Fat digestibility depends on the physical and chemical properties of fat sources, which influence the digestibility of other dietary components. This complicates the comparison of the results of SID assays using animal or plant fats. We previously reported that BEL was not significantly different for broiler chickens that received nitrogen-free diets in which tallow or soybean oil was used, but it is unclear if the same will hold in SID determination. A total of 320 male broiler chicks were allocated to 64 cages with four treatments (2 × 2 factorial) and eight replicates per treatment to study the influence of fat types (soybean oil added at 15g/kg) or (tallow added at 20g/kg) on SID of AA of two feedstuffs varying in AA profile (corn and SBM) and on the morphology of small intestine. All the birds were fed a basal corn-soybean meal diet for 16 d and experimental diets from d 16 to 21. On day 21, all the birds were euthanized, and the digesta were collected from the distal ileum to calculate the AA digestibility. Sections of the jejunum and ileum were collected to measure villi height (VH), crypt depth (CD), and their ratio (VH: CD). Data were analyzed as a two-way ANOVA using JMP, and significance was set at P < 0.05. There were no feedstuff × fat-type interactions for any of the responses. The coefficients of SID were significantly greater (P < 0.01) for SBM than for corn, except for Leu, Met, Ala, Cys, and Pro. Although not significantly different, except for Ala, the SID values were numerically greater for birds receiving semi-purified diets with soybean oil as the fat type. The jejunal and ileal VH and jejunal VH: CD were significantly greater (P < 0.01) for the birds that received the semi-purified SBM-based assay diets. Our studies on fat-type effects on BEL and SID consistently showed numerically higher values for birds that received soybean oil as dietary fat. However, these differences reflected a minimal, non-statistically significant effect of fat type within the inclusion levels used in the current experiment. In conclusion, using soybean oil or tallow as fats in the diets had only marginal effects on BEL and SID of AA, and the morphological characteristics of the small intestine in the experiment were protein feedstuff- rather than fat type-dependent. Therefore, comparing SID values determined with assay diets using these fat types should not lead to erroneous conclusions.

Veeraganti, S., M.  Pilevar, B. Kasireddy and O. Olukosi. 2025. Assessment of the interactive effect of dietary fat and feedstuff types on their standardized ileal amino acid digestibility and the morphology of small intestine in broiler chickens. 2025 Poultry Science Association Annual Meeting, Abstract 130