Effects of dietary inclusion of high- and low-tannin faba bean (Vicia faba L.) seeds on microbiota, histology and fermentation processes of the gastrointestinal tract in finisher turkeys

This study evaluated the effects of dietary replacement of soybean meal (SBM) with graded levels of faba bean (FB) seeds with high or low tannin content (HT or LT) on the gastrointestinal function and growth performance of turkeys at 13-18 weeks of age. Hybrid Converter turkeys were distributed into 7 treatments corresponding to 7 different finisher diets: a control wheat-soybean meal-based (FB0) diet and experimental diets where SBM was partially replaced with HT or LT seeds at 100, 200 and 300 g/kg. Each treatment comprised 210 turkeys, with seven replicate pens and 30 birds per pen. The LT treatment decreased jejunal crypt depth (vs. FB0; P = 0.049) and the experimental factors had no significant effect on the analysed caecal histological parameters. In comparison with the FB0 diet, diets containing HT and LT FB contributed to an increase in the total bacterial counts (P = 0.001 and P = 0.033) and Bacteria domain (P = 0.001 and P = 0.060), and a decrease in the counts of Bacteroides (P = 0.002 and P = 0.013). Diets containing LT FB reduced the abundance of Salmonella bacteria, relative to the FB0 diet (P = 0.011) and diets with HT FB (P = 0.023). The LT treatment decreased the counts of total bacteria and Bacteria domain (P = 0.005), in comparison with the HT treatment. The highest ileal short-chain fatty acid (SCFA) concentrations were observed in response to the LT200 diet. LT diets stimulated increased SCFA production in the caeca, relative to the FB0 diet (P = 0.022), and the opposite effect was noted when HT and SBM dietary treatments were compared. In comparison with HT diets, LT diets led to a desirable increase in the concentrations of all major fatty acids (acetic, propionic and butyric) in the caecal contents. It can be concluded that FB seeds enhanced fermentation processes in the gastrointestinal tract of turkeys. In comparison with HT FB, LT seeds improved selected parameters of intestinal function, including a decrease in the counts of Salmonella bacteria (P = 0.023), increased SCFA production (including butyrate; P = 0.001), and a decrease in the pH of intestinal digesta (P = 0.105). In conclusion, both LT and HT FB seeds, the latter containing up to 7.1 g/kg tannins, can be included in finisher turkey diets at up to 300 g/kg as a safe and effective substitute for SBM.