Fermentation of hydrated corn grain and soybean meal alone or in combination: Impacts on nutrient preservation and greenhouse gas emissions.

Rehydrated fermented corn (RFC) is widely used in dairy production in Brazil; however, its fermentation often leads to excessive gas production and nutrient loss. This study investigated the effects of varying soybean meal replacement ratios (100:0 to 0:100), while maintaining a constant DM content, and fermentation durations (7 to 60 d) on nutrient preservation and GHG emissions in RFC. In addition, 16S rRNA sequencing was used to analyze changes in the microbial community composition and functional potential. The experiment was a 5 × 4 factorial design, completely randomized, with factor A as the soybean meal replacement ratios and factor B as the time of fermentation. Dry corn (DM: 89.74%, as-fed basis) was ground through a 2-mm screen hammer mill to prepare rehydrated corn, then water was added at a calculated rate to adjust its DM content to 60% and mixed thoroughly for 10 min. Increasing the proportion of soybean meal promoted lactic and acetic acid production, limited ammonia nitrogen accumulation, and reduced emissions of carbon dioxide, methane, and nitrous oxide. Microbial analysis revealed that with the increase in soybean meal, beneficial bacteria such as Weissella were enriched, while Klebsiella and Enterobacter were reduced, which are known to affect organic acid production and GHG emissions, respectively. Functional prediction via Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways indicated that soybean meal suppressed KEGG Orthology groups involved in pyruvate metabolism, methanogenesis, and nitrogen cycling. Higher soybean meal proportions also delayed GHG emission peaks while inhibiting associated metabolic activities. At most fermentation time points, the increase of soybean meal reduced nutrient degradation and gas emissions from RFC. At a high soybean meal ratio, losses were further decreased and gas emissions were reduced by shortening the duration. Overall, this study elucidates microbial and metabolic mechanisms underlying GHG emissions during feed fermentation and provides practical, sustainable strategies for producing high-quality RFC.