Abstract: The aim of this experiment was to study ruminal environment and rumen bacterial diversity of grazing Dongbao blackhead goats in grass withering period.The forge grass were collected and the common nutrition contents were determined.Six healthy annual grazing Dongbao blackhead goats (3 males and 3 females) with the body weight of (45.24±1.25) kg were selected and marked using ear tags with numbers,and the rumen fluids of them were collected via oral cavity before free-range with empty stomach in grass withering period to measured the ruminal environment parameters and rumen microbial diversity.The results showed that the pH of rumen fluid of Dongbao blackhead goats was 6.65,the concentration of ammonia nitrogen (NH₃-N) was 50.30 mg/L,and the ratio of acetic acid/propionic acid was 4.43.A total of 290 868 valid 16S rDNA sequences and 479.67 operational taxonomic unit (OTU) across 6 samples were obtained.The richness indexes Ace and Chao1 indexes of ruminal bacterial were 533.79 and 523.31,respectively,and alpha diversity indexes Shannon and Simpson indexes were 6.39 and 0.96,respectively.The rumen bacteria of grazing Dongbao blackhead goats belonging to 15 phyla,23 classes,29 orders,34 families and 87 genera were identified via taxonomic summary.Among these rumen bacterial populations,the dominant bacterial phyla were Bacteroidetes (75.75%) and Firmicutes (15.62%).The dominant bacterial families were Rikenellaceae (45.89%),Prevotellaceae (19.18%).Rikenellaceae_RC9_gut_group (45.64%) and Prevotella_1 (15.46%) were the dominant bacterial genera.In conclusion,the rumen bacterial diversity of grazing Dongbao blackhead goats was low in grass withering period,and the ratio of acetic acid/propionic acid was high.Bacteroidetes is the predominant phylum and Firmicutes is the second most abundant phylum.Rikenellaceae_RC9_gut_group is the most abundant genus in the rumen bacterial community.

Key words: grass withering period, grazing Dongbao blackhead goats, ruminal environment, bacterial diversity, MiSeq sequencing