Abstract
BACKGROUND: Oat silage is a key feed source in animal husbandry due to its high nutritional value and adaptability. However, off-flavors caused by butyric acid fermentation significantly reduce its quality, with Clostridium bacteria being the main cause. The mechanisms underlying microbiota dynamics and volatile metabolite interactions remain unclear. In this study, we integrated microbial sequencing and metabolomics to analyze the key microorganisms and metabolic pathways involved in off-flavor formation, aiming to provide a theoretical basis for optimizing the fermentation process. RESULTS: The pH of the off-flavor group (UO) increased to 5.90, with reduced lactic acid content and elevated butyric acid and propionic acid. Nutritional analysis showed increased neutral detergent fiber and acid detergent fiber in the UO group, along with a decline in crude protein, indicating hindered fiber decomposition and increased nutrient loss. Microbial community analysis revealed decreased Lactobacillus spp. abundance, increased Clostridium spp. and Enterococcus spp., and a significant rise in the α-diversity index. Volatile metabolomics detected 734 metabolites, with a significant up-regulation of 454 compounds in the UO group of esters (19.71%) and terpenoids (19.86%), with an increase in the abundance of sweetness and fruity related substances such as methyl phenylpropionate and 3-methylphenylmethyl butyrate. Correlation analysis revealed that Clostridium (e.g. Clostridium tyrobutyricum) was positively correlated with butyric acid and off-flavor metabolites (e.g., 2-methoxy-4-vinylphenol) (p < 0.05), whereas Lactobacillus was strongly correlated with lactic acid content and indicators of high-quality fermentation (low pH, high DM).KEGG pathway analysis further revealed that secondary metabolite biosynthesis (e.g., terpenoids) and phenylpropane metabolic pathways are the core mechanisms of off-flavor formation. CONCLUSION: Oat silage off-flavors result from Clostridium-driven butyric acid fermentation, which converts carbohydrates and lactic acid into butyric acid and volatile esters. This process raises pH, affects fiber degradation (31.1% rise in NDF), and causes nutrient loss (16.3% drop in CP). The imbalance between Clostridium and Enterococcus, worsened by a 11.2% decrease in lactic acid bacteria, further degrades fermentation quality. The study innovatively used multi-omics analysis to clarify the molecular mechanisms of microbial dynamics and metabolite interactions behind off-flavor formation. It showed that boosting Lactobacillus abundance can suppress Clostridium activity.