Abstract: 【Objective】 The purpose of this experiment was to investigate the effect of yeast selenium supplementation on the gastrointestinal microflora of sheep under grazing conditions.【Method】 Fourteen healthy 7-8 months old Dolphin sheep with similar body weight were selected and divided into two groups by single factor complete randomized trial design,with seven sheep in each group.One group was the control group without adding selenium,and the other group was the selenium adding group.The selenium (yeast selenium) adding level was 0.2 mg/d.A 30 day feeding experiment was conducted under natural grazing conditions.Rumen juice and feces samples were collected on the 15th and 30th days of the experiment,respectively.The 16S rRNA sequencing method was used to analyze the changes of rumen and hindgut microflora of sheep.【Result】 The results of principle coordinate analysis showed that rumen and hindgut microbes of sheep in two groups had significant differences in OTU and genus levels after 30 days of selenium supplementation (P＜0.05).The results of discriminant analysis of species differences showed that eight rumen microorganisms had significant differences between groups at phylum and genus levels on the 15th day of feeding (P＜0.05),but the abundance of these species was lower than 1%. At the 30th day of feeding,there were 17 rumen microorganisms with significant differences between groups at the phylum and genus levels (P＜0.05),among which Firmicutes,Bacteroidetes,Prevotella-1,Ruminococcaceae_NK4A214 and Bacteroidales_BS11_gut_group had relative abundance greater than 1%.Compared with the control group,the relative abundance of Firmicutes,Ruminococcaceae_NK4A214 and Bacteroidales_BS11_gut_group in the rumen in selenium supplementation group was significantly increased (P＜0.05),while the relative abundances of Bacteroidetes and Prevotella_1 was significantly decreased (P＜0.05).At the genus level,on the 15th day,there were significant differences in five species of posterior intestinal microorganisms between groups (P＜0.05),but the abundance of these species was lower than 1%.On the 30th day,there were 18 species of posterior intestinal microorganisms with significant differences between groups at phylum and genus level (P＜0.05),among them,the relative abundance of Firmicutes,Bacteroidetes,Bacteroidetes,Ruminococcaceae_UCG-009,Ruminococcaceae_UCG-010,Ruminococcaceae_UCG-013 and Ruminococcaceae_NK4A214,norank_f_Ruminococcaceae,Lachnospiraceae_AC2044 and norank_f_Muribaculaceae were greater than 1%.Compared with the control group,the relative abundances of Bacteroidetes,Bacteroidetes,Ruminococcaceae_UCG-010 and Ruminococcaceae_UCG-013 in intestinal tract of selenium supplementation group were significantly increased (P＜0.05).The relative abundances of Firmicutes,Lachnospiraceae_AC2044,Ruminococcaceae_NK4A214 and Ruminococcaceae_UCG-009,norank_f_Ruminococcaceae and norank_f_Muribaculaceae were significantly decreased (P＜0.05).【Conclusion】 Selenium supplementation of sheep under grazing conditions could improve rumen microbial diversity and hindgut microbial richness,increase the abundance of fiber-degraded bacteria in rumen as well as protein polysaccharide-degraded bacteria in hindgut.In addition,selenium supplementation could promote probiotics growth and inhibited the platation of harmful microorganisms in hindgut of sheep.

Key words: yeast selenium; sheep; microflora