Comparative evaluation of methods for measuring standardized ileal digestibility of amino acids for different sorghum...

This study investigated the effects of two evaluation methods (direct vs. substitution) on standardized ileal amino acid digestibility (SID_AA) and intestinal digestive physiology in broilers fed sorghum from six regional sources. There were 336 27-d-old Cobb broilers assigned to 14 treatments, including a nitrogen-free diet (NFD), a corn-soybean meal (CSM) basal diet, and 12 experimental diets arranged in a 2 × 6 factorial arrangement. In the direct method, sorghum served as the only source of dietary amino acids whereas in substitution method the test sorghum replaced 40% (w/w) of the basal diet. The six sorghums were Yingkou (YK), Chifeng (CF), Yangzhou (YZ), Liaoning (LN), Zhangjiakou (ZJK) from China, and one from the United States (US). The digestibility trial was conducted from 27 to 29 d of age. The substitution method improved jejunal villus height and crypt depth, α-amylase and chymotrypsin activities, compared with the direct method (P < 0.05). YK sorghum with higher tannin contents reduced villus height, crypt depth, and chymotrypsin activity but increased ileal Occludin and ZO‑1 expression (P < 0.05). However, US, YZ, CF, and LN sorghum with lower tannin contents improved broiler digestive physiology. Broilers fed US exhibited the highest weight gain, and those fed LN showed the greatest villus height, crypt depth, and chymotrypsin activity (P < 0.05). Significant sorghum source × evaluation method interactions were observed for the AID_AA and SID_AA of all evaluated amino acids (P < 0.001), with more extensive method-related differences observed in YK and ZJK sorghum than in LN and US sorghum. Sorghum-based diets altered ileal microbiota, reducing Lactobacillus abundance and increasing Streptococcus, Corynebacterium, and Enterococcus (P < 0.05). The direct method increased (P < 0.05) the abundance of Turicibacter and Brachybacterium, whereas the substitution method produced a microbial composition similar to the basal diet. In conclusion, LN and US sorghum showed favorable AA digestibility, and the substitution method generally yielded higher AID_AA and SID_AA values and more favorable short-term responses in digestive enzyme activity and ileal microbial profiles than the direct method. The method-related differences in high-tannin sorghum highlight the importance of considering anti-nutritional factor content when selecting a method for evaluating AA digestibility.