Partial replacement of soybean meal with vitamin B12 fermentation waste modulates rumen microbiota, immune response, and growth performance in finishing lambs.

This study aimed to evaluate the effect of substituting soybean meal (SBM) with vitamin B12 fermentation waste (VFW) on growth performance, nutrient digestibility, immune response, ruminal fermentation characteristics and microbiota in finishing lambs. Forty-eight male Hu lambs aged 80 ± 4 d, with an initial body weight of 26.5 ± 0.59 kg, were randomly assigned to one of the four groups (n = 12): Control, receiving a basal diet, while the other three groups were treated with 20% (low), 40% (medium) and 60% (high) VFW instead of SBM, with a 14-d adaptation and 56-d collection of data and samples. Feeding diets supplemented with VFW quadratically changed the dry matter intake (P = 0.045) and linearly increased the feed conversion ratio (FCR) of lambs (P = 0.048). Furthermore, the digestibility of neutral detergent fiber decreased with VFW supplementation (P = 0.034). Dietary VFW did not change blood metabolite concentrations. However, increasing doses of VFW linearly increased the concentrations of immunoglobulin M and total superoxide dismutase (P < 0.05) and linearly decreased the concentration of catalase (P = 0.030). Increasing dietary VFW doses linearly elevated ruminal pH, ammonia nitrogen, as well as the proportion of isobutyrate (P < 0.05), while linearly decreasing the concentration of total volatile fatty acid (P < 0.001). The ruminal alpha diversity indexes remained unaffected by VFW, whereas principal coordinates analysis (PCoA) based on unweighted UniFrac illustrated distinct separation of the ruminal clustering regarding bacterial microbiota between Control and VFW. At the phylum level, VFW increased the Bacteroidota abundance (P = 0.034) and decreased the Firmicutes abundance (P = 0.013), thereby reducing the Firmicutes-to-Bacteroidota ratio (F:B ratio; P = 0.031). At the genus level, VFW supplementation increased the abundances of Prevotella, Selenomonas, Bacteroidaceae _unclassified and Erysipelatoclostridiaceae _ UCG-004 (P < 0.05), while decreasing the abundances of Rikenellaceae _RC9_gut_group, NK4A214_group, Veillonellaceae _ UCG-001, Christensenellaceae _R-7_group, WCHB1-41_unclassified, Butyrivibrio, and [Eubacterium]_coprostanoligenes_group_unclassified (P < 0.05). Together, dietary VFW can reduce FCR and ruminal fermentation in a dose-dependent manner, while improving immune response and altering the ruminal bacterial composition in lambs. The findings indicate the potential of VFW as an alternative to SBM for sheep production.