Three fibrous by-products were evaluated over a 35-day feeding period in 64 weaned piglets, randomly assigned to four groups: a control without by-products (CON) and three others with diets containing 8% carrot pomace (CRT), 8% brewers’ spent grain (BSG), or 8% carob pods (CRB). The growth performance, feed intake, feed conversion ratio, and apparent ileal digestibility of protein and amino acids were not affected. The jejunal and colonic morphology showed no statistical differences, although small numerical increases in the villus height and villus height-to-crypt ratio were noted with the by-products. Total short-chain fatty acid concentrations were stable, but their profile shifted: acetate increased in CRT and CRB (p < 0.001) mainly at the expense of propionate (p = 0.005). The microbiota composition in the proximal colon showed modest changes, with the highest Bifidobacterium spp. abundance in CRT and lowest in CRB (p = 0.042), reduced Ruminococcaceae UCG 005 with all the by-products (p = 0.008), and greater microbial richness in CRB (p = 0.009). These results suggest that a moderate inclusion of fibrous by-products may influence intestinal microbial ecology and fermentation patterns without negatively affecting performance or nutrient digestibility in weaned piglets, with no source appearing superior, thereby highlighting their potential as sustainable feed ingredients.
