BackgroundThe fermentation of undigested material in the ileum is quantitatively important. However, the respective contributions of the microbial composition and the substrate to ileal fermentation are unclear.ObjectiveThis aim was to investigate the contribution of microbial composition and fiber source to in vitro ileal fermentation outcomes.MethodsThirteen ileal cannulated female pigs (Landrace/Large White; 9-wk-old; 30.5 kg body weight) were given diets containing black beans, wheat bread, chickpeas, peanuts, pigeon peas, sorghum, or wheat bran as the sole protein source for 7 d (100 g protein/kg dry matter diet). On day 7, ileal digesta were collected and stored at −80°C for microbial analysis and in vitro fermentation. For each diet, a pooled ileal inoculum was prepared to ferment different fiber sources (cellulose, pectin, arabinogalactan, inulin, fructooligosaccharides, and resistant starch) for 2 h at 37°C. Organic matter fermentability and organic acid production were determined following in vitro fermentation. Data were analyzed using a 2-way ANOVA (inoculum × fiber).ResultsForty-five percent of the identified genera in the digesta differed across diets. For instance, the number ofLactococcuswas 115-fold greater (P≤ 0.05) in the digesta of pigs fed the pigeon pea diet than for pigs fed the wheat bran diet. For both in vitro organic matter fermentability and organic acid production, there were significant (P≤ 0.05) interactions between the inoculum and the fiber source. For instance, pectin and resistant starch resulted in 1.6- to 31-fold more (P≤ 0.05) lactic acid production when fermented by the pigeon pea inoculum than other inocula. For specific fiber sources, statistically significant correlations were found between the number of bacteria from certain members of the ileal microbial community and fermentation outcomes.ConclusionsBoth the fiber source fermented and the ileal microbial composition of the growing pig affected in vitro fermentation; however, the effect of the fiber source was predominant.Curr Dev Nutr 2023;x:xx.