Oxalic Acid Supplementation in Different Hemicellulose Diets Affects In Vitro Rumen Fermentation by Regulating Nutritional Digestibility, Microbial Diversity and Metabolic Pathways

Determining hemicellulose (HM) degradation is crucial for evaluating the nutritional value of ruminant diets. Our previous study showed that oxalic acid (OA) regulates rumen fermentation. Building on this research, the present study examined the effects of OA supplementation in different hemicellulose diets on sheep rumen fermentation, microbial diversity, and metabolite production in vitro. Diets with low and high HM levels (10.3% and 17%, respectively) and supplemented with seven concentrations of OA (0, 2.5, 5, 10, 20, 40, 80 mg/kg DM) were evaluated. Tests were conducted under both low (HM10.3%) and high (HM17%) hemicellulose conditions; however, the addition of 10 mg/kg DM oxalic acid could have better effects under low hemicellulose (HM10.3%), with higher concentrations of acetic, propionic, and butyric acids, as well as total acids. A 2 × 2 factorial design was used to collect rumen fluid after 12 h of fermentation to analyze microbial populations and metabolites. OA supplementation at 10 mg/kg DM significantly increased the relative abundances of several bacterial genera, including <i>Prevotella</i>, <i>Butyrivibrio</i>, <i>Ruminococcus</i>, <i>Sharpea</i>, <i>RFN20</i>, <i>Bulleidia</i>, <i>Olsenella</i>, and <i>Bifidobacterium</i> (<i>p</i> < 0.05). A positive correlation was observed between Butyrivibrio and Sharpea and the production of isobutyric and isovaleric acids (<i>p</i> < 0.01), indicating that these bacteria play a role in volatile fatty acid (VFA) production. Furthermore, rumen metabolites involved in mineral absorption and lipid metabolism, including α-tocopherol, L-glutamic acid, and ginkgolide B, were upregulated. In summary, supplementation with oxalic acid in HM diets alters rumen fermentation, enhances nutrient digestibility, promotes microbial diversity, and influences metabolic pathways. Thus, OA supplementation should be tailored to specific dietary conditions for optimal effects.