Effects of citric acid and heat-treated soybean meal on rumen fermentation characteristics, methane emissions, and microbiota: an in vitro study.

Abstract

This study aimed to assess the impact of citric acid (CA) and heat-treated soybean meal (SBM) on rumen fermentation characteristics, methane production, and microbiota through an in vitro experiment. Untreated SBM, heat-treated SBM (HSBM), CA-treated SBM (CSBM), and SBM treated with a combination of heat and CA (HCSBM). Parameters assessed in in vitro were gas production, methane emissions, dry matter degradability (IVDMD), crude protein degradability (IVCPD), ammonia nitrogen (NH₃-N), microbial crude protein (MCP), volatile fatty acids (VFA), pH, and microbiota composition. The HCSBM exhibited the lowest gas production and theoretical maximum gas production (p < 0.01). Methane production (%) was significantly reduced in both CSBM and HCSBM (p < 0.01), with the lowest methane emissions (mL/g dry matter, DM) observed in HCSBM (p < 0.01). The IVCPD was significantly reduced in both the HSBM and HCSBM groups (p < 0.01). HCSBM had the lowest NH₃-N and MCP concentrations (p < 0.01). Total VFA production was the lowest in HCSBM (p < 0.01), with a higher proportion of acetate and lower proportions of propionate (p < 0.01). HCSBM reduced the enrichment of Thermoplasmatota compared to HSBM (p < 0.05) and decreased the enrichment of the coenzyme M biosynthesis pathway in the microbial functional profiles compared to SBM and CSBM (p < 0.05). Additionally, an increase in fiber-degrading bacteria, particularly Fibrobacterota, was observed in HCSBM (p < 0.05). These findings suggest that the HCSBM may effectively reduce ruminal protein degradation and methane emissions. Further in vivo studies are necessary to validate these results and assess their practical application in ruminant nutrition.