To determine the optimal drying duration for forage oat （Avena sativa L.） hay preparation and mitigate fungal contamination risks， this study monitored microbial counts and the succession of fungal communities on oats and the attached topsoil during the drying process （days 1-5）. The results showed that with prolonged drying time， the counts of molds， yeasts， coliforms， and aerobic bacteria in both oats and topsoil generally exhibited a logarithmic growth trend. Specifically， aerobic bacteria counts remained at their lowest levels on days 1-2， while mold counts peaked on day 3. Notably， from days 2 to 5， the counts of molds and aerobic bacteria significantly exceeded the limits specified in the Hygienic Standard for Feeds （GB 13078-2017）. Regarding community structure， Ascomycota （72.24%） and Basidiomycota （21.10%） were identified as the dominant phyla. The dominant genera on the oat surface exhibited a distinct succession pattern： the relative abundances of Bulleromyces （19.22%） and Sarocladium （16.12%） decreased with prolonged drying， whereas those of Microdochium （15.14%） and Cryptococcus （11.07%） significantly increased. Correlation network analysis indicated that the fungal community network underwent a topological shift from a simple， sparse structure in topsoil to a complex， dense structure on oat plants. Furthermore， correlation analysis revealed a succession path of potential pathogens： the community dominated by potential toxin-producing or endophytic pathogenic taxa （represented by Sarocladium and Acremonium） in the topsoil gradually evolved into a community dominated by soil-borne root rot pathogens （represented by Plectosphaerella and Idriella） on the plants. In conclusion， to inhibit pathogen colonization and ensure the hygienic quality of hay， it is recommended to control the drying duration within 1-2 days in production practice.

Feed oats  /  Hay modulation  /  Fungi  /  Fungal community