Aspergillus oryzae as a Functional Microbial Bioactive in Ruminant Nutrition: Effects on Ruminal Fermentation, Feed Efficiency, and Sustainability

As pressure grows to produce more animal-derived food with fewer environmental costs, interest has intensified in microbial strategies that can improve both productivity and sustainability in ruminant systems. Among these, Aspergillus oryzae has emerged as a particularly intriguing candidate. Long recognized for its role in traditional fermentation, this filamentous fungus is increasingly being explored in animal feeding not merely as a nutritional additive, but as a functional microbial input capable of influencing ruminal fermentation, nutrient utilization, and overall production efficiency. Yet despite its widespread use, the evidence base has remained somewhat fragmented. This systematic review and meta-analysis therefore synthesized findings from 30 studies published before 2024 to evaluate the effects of A. oryzae supplementation on ruminal fermentation, feed efficiency, animal performance, and sustainability-related outcomes in ruminants. Across the available literature, A. oryzae supplementation was generally associated with favorable shifts in rumen function, including improved fiber degradation, enhanced volatile fatty acid production, and more stable ruminal pH. These changes were often accompanied by better nutrient digestibility and, in several cases, improvements in feed conversion, milk yield, milk composition, or average daily gain. At the microbial level, the fungus appeared to support a more fermentation-efficient rumen environment, likely through stimulation of fibrolytic populations and modulation of microbial activity, although the extent of these effects varied across diets, doses, and animal categories. Importantly, several studies also pointed toward sustainability benefits, particularly through improved utilization of agro-industrial substrates and the possibility of reducing enteric methane output. Taken together, the evidence suggests that Aspergillus oryzae may function as a valuable microbial bioactive in ruminant nutrition, with promising implications for fermentation efficiency, productive performance, and environmental sustainability. Still, methodological heterogeneity and inconsistent characterization of fungal preparations continue to limit direct comparability. More standardized, large-scale studies are needed to clarify optimal application strategies and strengthen translational confidence.