Niacin alters ruminal microbial composition and metabolites in sheep fed a high-concentrate diet

This study aimed to investigate the effects of niacin supplementation in a high-concentrate diet on growth performance, rumen fermentation, rumen microbiota, and metabolomics in sheep. Twelve sheep were randomly divided into two groups: (1) a control group (CON, n=6) fed a basal diet; (2) a niacin group (NA, n=6), sheep fed a basal diet supplemented with 130 mg/d niacin. The experiment lasted 26 days, a 14 days for the adaptive phase was followed by 12 days of sampling period. Growth performance (initial body weight, final body weight, and average daily gain) was measured on days 1 to 5 of the sampling period, and rumen fluid samples were collected on days 6 to 7 for the determination of rumen fermentation parameters, lactate metabolism-related enzymes, microbial diversity, and metabolomics.The results showed that 1) The dry matter feed intake of the niacin group was higher than that of the control group (p < 0.05). 2) The pH of the niacin group was significantly higher than that of the control group after 1 hour of feeding (p < 0.05), and it was significantly higher than that of the control group after 3, 5, and 7 hours of feeding (p < 0.01). The concentrations of NH3-N the niacin group (p <0.01), propionic acid (p <0.01), and butyric acid (p <0.05) were significant higher than those in the control group. 3) Compared with the control group, the pyruvate content in the rumen of the niacin group was significantly increased at 0 h before feeding (p < 0.05), and the lactic acid (p < 0.05) and lactate dehydrogenase (p < 0.01) activities were significantly decreased at 3 h and 5 h after feeding, respectively. 4) In terms of OTU, the number of specific OTUs in the control group and the niacin group were 26 and 37, respectively, a total of 1178 OTUs; PCoA (R2 = 0.172, pvalue = 0.007) and NMDS (Stress = 0.1646) results showed that the two groups of samples were significantly separated. 5) The species distribution bar graph showed that at the phylum level, the relative abundances of Bacteroidetes, Firmicutes and Proteobacteria were 43.70%, 36.25% and 12.77%, respectively. 6) OPLS-DA results showed that the two groups of samples were clearly separated in positive and negative ionization modes, with R2Y and Q2Y of 0.705, 0.857, 0.695 and 0.28, respectively. There were 72 metabolic pathways, mainly the citric acid cycle, pyruvate metabolism, cysteine and methionine metabolism. 7) Correlation analysis showed that many microorganisms (Succinivibrio, Prevotella, etc.) and differential metabolites (L-malic acid, propionic acid, succinic acid, pyruvic acid, etc.) participated in the tricarboxylic acid cycle metabolism.In summary, adding niacin to high-concentrate diets can significantly improve the growth performance of sheep, improve rumen fermentation and rumen microbial community structure, affect rumen metabolites, and thus alleviate the symptoms of rumen acidosis.