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This study aimed to examine and compare the effectiveness of a novel spray-dried inoculant and a commercial freeze-dried additive on the fermentation quality, aerobic stability and bacterial population of vetch-oat silage. An entirely random design used a 3 × 4 factorial arrangement of treatments, with and without lactic acid bacteria (LAB) inoculants and four fermentation periods. Physicochemical parameters, microbiological counts and 16S rRNA gene sequencing analysis on Nanopore MinION were conducted to characterize the ensiling process. Both LAB inoculants increased dry matter, crude protein, lactic, acetic and propionic acid contents, while reducing pH, neutral detergent fibre, ammonia nitrogen/total nitrogen and ethanol concentrations compared to the control group. Overall, the native inoculant decreased the cell load of coliforms, yeasts and moulds. In addition, bio-inoculants enhanced the aerobic stability of vetch-oat intercrops. After ensiling, bacterial alpha diversity decreased noticeably; inoculation reduced the number of observed operational taxonomic units and the Shannon and Simpson indices. Notably, the relative abundance of Lactobacillus in the control group was lower than in treated silages, while the relative values of Staphylococcus increased sharply in the uninoculated group. In conclusion, the native strains showed promise for usage as a bio-inoculant in the ensiling of vetch-oat at a mixture rate of 1:1, producing an immediate impact as well as a favourable effect on the post-opening phase. This represents the first report on 16S rRNA gene-based nanopore metagenomics applied to the bacterial analysis of vetch-oat silage, providing a microbiological insight where native and commercial strains dominate the natural epiphytic community. [ABSTRACT FROM AUTHOR]