Abstract: This study investigated the effects of different dietary protein sources and nisin on rumen fermentation and rumen microbiota in fattening Hu sheep. Thirty-two male Hu lambs ((23±2) kg initial BW) were assigned to 4 dietary treatments in a randomized block design with a 2×2 factorial arrangement. Two blocks were designed according to body weight with 16 lambs each. Two protein sources (soybean meal (SBM) and dried distillers grains with solubles (DDGS)), and two levels of nisin (0 and 30.5 mg·kg^-1 DM) were used to formulate 4 isonitrogenous and isoenergetic diets. The feeding trial was conducted for 10 weeks, with the first 1 week for adaptation followed by 9 weeks of dietary treatment. At the end of the experimental period, 6 lambs (3 lambs per block) were randomly selected from each group, and slaughtered for rumen contents collection. Metagenomic DNA of rumen content was extracted, and rumen microbiota were analyzed by Illumina MiSeq sequencing and quantitative PCR. No interaction (P>0.05) of protein×nisin was found on all measured indicators (rumen fermentation characteristics, ruminal microbial populations, rumen bacterial diversity and relative abundance, etc.) except for the relative abundance of Ruminococcaceae NK4A214 group and unclassified Bacteroidetes. Nisin supplementation had no impact on all measured indicators (P>0.05). Lambs receiving DDGS had lower concentrations of ruminal acetate, ammonia and branched-chain VFA (BCVFA) compared with lambs fed SBM (P<0.05). The DDGS-fed lambs had a smaller (P<0.05) population of protozoa and C. aminophilum than those fed SBM, but the population of total bacteria, fungi, and methanogens were similar (P ≥ 0.053). For alpha diversity, ACE and Chao1 index were higher (P<0.05) in the DDGS-fed lambs than in those fed SBM. At phylum level, the dominant bacteria in different diet treatment groups were Bacteroidetes and Firmicutes. At genus level, the dominant bacteria in each treatment group were Prevotella 1, Christensenellaceae R-7 group and Ruminococcaceae NK4A214 group. None of the relative abundance of bacteria at phylum level was affected (P ≥ 0.14) by protein sources. Compared with those fed SBM, the relative abundance of Pseudobutyrivibrio and Roseburia were higher (P<0.05), while the relative abundance of Butyrivibrio 2 and Ruminococcaceae UCG-005 were lower (P<0.05) in DDGS-fed lambs. Replacing SBM in an isonitrogenous lambs diet with DDGS changed the rumen fermentation characteristics and the microbial community structure, and the reduction of the population of protozoa and C. aminophilum may be the main reason for the reduction of rumen ammonia concentration. Nisin supplementation at 30.5 mg·kg^-1 DM had no effects on rumen fermentation and rumen microbiota in fattening lambs.

Key words: dried distillers grains with solubles, soybean meal, nisin, rumen fermentation, microbiota structure