Silage is green fodder conserved by lactic acid fermentation performed by epiphytic lactic acid bacteria under anaerobic conditions. To improve the ensiling process and the quality of the resulting silage, starter cultures are added to the fresh forage. A detailed analysis of the microbial community playing a role in grass ensiling has been carried out by high throughput sequencing technologies. Moreover, the influence of the inoculant Lactobacillus buchneri CD034 on the microbial community composition was studied. For this purpose, grass was ensiled untreated or inoculated with L. buchneri CD034. The fresh forage as well as silages after 14 and 58 days of fermentation were characterized physico-chemically. Characteristic silage conditions such as increased titers of lactic acid bacteria and higher concentrations of acetic acid were observed in the inoculated silage in comparison to the untreated samples. Taxonomic community profiles deduced from 16S rDNA amplicon sequences indicated that the relative abundance of Lactococci diminished in the course of fermentations and that the proportion of bacteria belonging to the phyla Proteobacteria and Bacteroidetes increased during the fermentation of untreated silage. In the inoculated silage, members of these phyla were repressed due to an increased abundance of Lactobacilli. In addition, metagenome analyses of silage samples confirmed taxonomic profiles based on 16S rDNA amplicons. Moreover, Lactobacillus plantarum, Lactobacillus brevis and Lactococcus lactis were found to be dominant species within silages as analyzed by means of fragment recruitments of metagenomic sequence reads on complete reference genome sequences. Fragment recruitments also provided clear evidence for the competitiveness of the inoculant strain L. buchneri CD034 during the fermentation of the inoculated silage. The inoculation strain was able to outcompete other community members and also affected physico-chemical characteristics of the silage.