Abstract: 【Objective】 This experiment was aimed to study the effects of mannitol on the gastrointestinal microflora and serum antioxidant and immune indexes of grazing sheep.【Method】 Ten healthy 7-8 months old sheep with similar body weight (27.10 kg±3.55 kg) were selected and randomly divided into two groups by single factor complete randomized trial design,with five sheep in each group.Sheep in control group was supplemented with 100 g/d of corn powder without mannitol,and in experimantal group was supplemented with 100 g/d of corn powder containing 1 g mannitol.A 35 day feeding experiment was conducted under natural grazing conditions,with 5 d for adaptation period and 30 d for experimental period.Weights were weighed before grazing on the start and end days of the positive trial period and daily average weight gain (ADG) was calculated.On the last experimental day,rumen fluid,feces samples,and jugular vein blood were collected,respectively.The 16S rRNA sequencing method was used to analyze the changes in rumen and fecal microflora of sheep.Serum antioxidant and immune indexes were determined using relevant kits.【Result】 ①There was no significant difference in ADG,pH and NH₃-N content of rumen fluid between the control and experimental groups (P＞0.05),while the concentration of acetic acid,propionic acid and total SCFAs in rumen fluid in experimental group were significantly higher than those in control group (P＜0.05).②There were no significant difference in the number of OTUs and the Alpha diversity index of rumen and faeces microorganisms in the control and experimental groups (P＞0.05).However,PCoA based on abund_jaccard proximity distance for both rumen and facecal microorganisms were significantly separated in experimental group from control group (P＜0.05).③At the phylum level,there was no significant difference between control and experimental groups in terms of rumen microbes (P＞0.05),the abundance of Firmicutes was significantly lower,and the abundance of Bacteroidetes was significantly higher in the faeces of experimental group than that of control group (P＜0.05).At the genus level,the relative abundance of Ruminococcaceae_NK4A214 in the rumen of experimental group was significantly higher than that of control group (P＜0.05),the relative abundance of Unclassified_f_Achnospiraceae in the faeces of experimental group was significantly lower than that of the control group (P＜0.05).④There was no significant difference in the serum immune index between control and experimental groups (P＞0.05).The serum contents of glutathione (GSH) and malondialdehyde (MDA) were significantly lower in the experimental group than in control group (P＜0.05).【Conclusion】 Supplementation with mannitol to grazing sheep diets could increase the rumen fluid concentration of acetic acid,propionic acid and total SCFAs,as well as the relative abundance of the Ruminococcaceae_NK4A214 in the rumen;Reduce the relative abundance of Firmicutes phyla and Unclassified_f_achnospiraceae,and increase the relative abundance of Bacteroidetes phyla in faeces;Improve the body’s antioxidant capacity.

Key words: mannitol; grazing sheep; microbiome structure; immune; antioxidant