| Summary: | The study was conducted to investigate the effects of dietary NFC/NDF ratio change on rumen bacteria in sheep. Twelve Karakul sheep were assigned randomly into four groups fed with four dietary NFC/NDF ratios of 0.54, 0.96, 1.37, and 1.90 and they were assigned into groups 1, 2, 3, and 4, respectively. The experiment was divided into four periods: I (1−18 d), II (19−36 d), III (37−54 d), and IV (55−72 d). In each period, the first 15 d were used for adaption, and then rumen fluid was collected for 3 d from each sheep before morning feeding. The fluid was analyzed with three generations of full-length amplifiers sequencing. Results showed that the bacterial diversity of group 4 was decreased in period III and IV. At the phylum level, Bacteroidetes (37−60%) and Firmicutes (26−51%) were the most dominant bacteria over the four periods. The relative abundance of Bacteroidetes, Firmicutes, Tenericutes, and Spirochaete changed with dietary NFC/NDF ratio change over the four periods, but there was no difference among groups over the four periods (<i>p</i> > 0.05). At the genus level, unidentified-<i>Lachnospiraceae</i> was the dominant genus, and its relative abundance in group 3 was high during the period I and III (<i>p</i> < 0.05). The relative abundance of <i>Mycoplasma</i> in group 4 was high in the period I and II (<i>p</i> < 0.05). The relative abundance of <i>Succiniclasticum</i> was high in group 2 of period II (<i>p</i> < 0.05). At the species level, the relative abundance of <i>Butyrivibrio-fibrisolvens</i> was found to be high in group 3 during periods I and III (<i>p</i> < 0.05). The main semi-cellulose-degrading bacteria and starch-degrading bacteria were low, and there was no significant difference among groups over four periods (<i>p</i> > 0.05). Taken together, the dietary NFC/NDF ratio of 1.90 decreased the diversity of bacteria as a period changed from I to IV. While the main phylum bacteria didn’t change, their relative abundance changed with the dietary NFC/NDF ratio change over the four periods. The most prevalent genus was unidentified-<i>Lachnospiraceae,</i> and its relative abundance was higher in dietary NFC/NDF ratio of 1.37 than other groups. Similarly, the main cellulose-degrading species was higher in the treatment of dietary NFC/NDF ratio of 1.37 than other groups.
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