PS8-3. Comparative Analysis of Soil Microbial Diversity and Communities Under Monocrop and Polycrop Forage Systems Swath-grazed During Winter.

Abstract The popularity of polycrop forage systems among livestock producers is increasing because of its perceived benefit to soil health through increased microbial diversity. Winter swath grazing has been reported to significantly reduce feed and labor costs as compared to drylot systems. Over 3 years, this study evaluated soil microbial diversity in paddocks under either an oat monocrop (OM) or a 4-species polycrop (PS; oat, turnip, forage pea and rapeseed) forage system. Each year, six 2-ha paddocks were seeded in June, swathed in August/September. Freshly weaned steers were randomly assigned to swath graze either OM or PS from November to January/February. Baseline and endline soil cores (0-7.62 cm) were collected from four predetermined GPS points per paddock at the beginning (2021) and the end (2024) of the trial. The compositions of prokaryotic and fungal communities were profiled by sequencing the V4 region of the 16S rRNA gene and the ITS1 region, respectively. Alpha-diversity indices were used to evaluate richness, evenness, and overall microbial diversity and beta-diversity based on Bray–Curtis dissimilarity was used to evaluate the effects of forage system and sampling year on microbial community composition. No significant difference in soil microbial community alpha diversity between OM and PS was observed after 3 years. By the end of the trial, the richness of prokaryotic soil microbiota increased significantly (P < 0.05) in both forage systems, while that of fungi decreased significantly (P < 0.05). The most abundant bacterial phylum changed from Proteobacteria in 2021 to Actinobacteria in 2024 in both systems. The most abundant archaeal and fungal phyla in both sampling years were Crenarchaeota and Ascomycota, respectively. Beta-diversity analysis revealed that samples clustered primarily by sampling year across all microbial communities, indicating that temporal variation strongly influenced microbial community composition in both forage systems. The interaction between sampling year and forage system was significant (P < 0.001) and explained 27%, 33% and 17% of the variation observed in bacterial (PERMANOVA-R² = 0.27, P < 0.001), archaeal (PERMANOVA-R² = 0.33, P < 0.001) and fungal (PERMANOVA-R² = 0.17, P < 0.01) communities, respectively. The bacterial phyla Actinobacteriota, Firmicutes, Chloroflexi and Myxococcota and fungal phylum Ascomycota were significantly enriched (LDA > 3.0, P < 0.05) by the endline. These findings suggest that the contribution of grazing cattle to soil microbial diversity is more profound and could have masked the contributions of the individual forage systems. This may also suggest that the PC requires more than three years to alter the diversity of microbial communities during grazing. The findings of this study can guide livestock operations in adopting new production strategies that enhance soil microbiota.