Microbial Bioactives

1. Introduction

The rapid intensification of global livestock production has become one of the defining agricultural transitions of the twenty first century. As the global population moves steadily toward an estimated 9.7 billion by 2050, pressure on food systems continues to mount in ways that are increasingly difficult to ignore. Demand for meat, milk, and other animal derived products is rising not only because of population growth, but also due to urbanization, dietary transitions, and expanding middle income populations across developing economies (Alexandratos & Bruinsma, 2012; Keating et al., 2014). Current projections suggest that global livestock production may need to increase by nearly 70% over coming decades to satisfy anticipated consumption patterns (Hunter et al., 2017). Yet this expansion introduces a persistent and uncomfortable question: can livestock systems continue to grow without simultaneously deepening environmental instability, resource depletion, and food insecurity?

Modern animal feeding systems remain heavily dependent on conventional feed commodities, particularly cereals and soybean meal. Vast quantities of grains that might otherwise contribute directly to human nutrition are instead redirected into livestock production chains. Estimates indicate that nearly 900 million tons of cereals and a substantial majority of globally produced soybean meal are used annually in animal feed formulations (Henchion et al., 2017; Nasseri et al., 2011). While this system has undeniably supported remarkable productivity gains, it has also intensified the long standing “food versus feed” debate, especially in regions where nutritional insecurity persists. Soybean expansion, meanwhile, continues to reshape landscapes on a massive scale. In countries such as Brazil, soybean cultivation has been strongly associated with deforestation, biodiversity loss, and ecological fragmentation within sensitive ecosystems including the Amazon and Cerrado regions (Lima et al., 2019). These environmental pressures are compounded further by greenhouse gas emissions and soil degradation linked to large scale feed crop production (Rojas Downing et al., 2017).

Ruminant agriculture occupies a particularly complicated position within this sustainability discussion. On one hand, ruminants possess the remarkable physiological ability to convert fibrous plant biomass that humans cannot digest into nutrient dense food products. This capability represents an important biological advantage within global food systems (Annison & Bryden, 1998; National Research Council, 2001). On the other hand, ruminant production systems are also major contributors to agricultural methane emissions, which significantly influence the global climate burden (Rojas Downing et al., 2017). The efficiency of ruminal fermentation is shaped by an intricate interplay among diet composition, microbial ecology, feed additives, and host metabolism. Consequently, nutritional strategies capable of improving feed efficiency while simultaneously moderating methane production have attracted growing scientific and commercial attention.

Against this backdrop, alternative protein sources and functional feed ingredients are increasingly being explored as part of more sustainable livestock production models. Among these, single cell proteins (SCPs) have emerged as particularly promising candidates. SCPs are microbial biomasses derived from bacteria, algae, yeasts, or fungi that contain high concentrations of protein and other valuable nutrients (Bratosin et al., 2021; Nasseri et al., 2011). Interest in SCP technology is not entirely new; however, recent concerns surrounding land use, feed security, and circular bioeconomy frameworks have renewed enthusiasm for microbial based feed production systems. Rather than relying exclusively on conventional agricultural inputs, SCPs may allow the conversion of industrial by products and organic residues into nutritionally valuable biomass.

Within this broader category, Aspergillus oryzae has received increasing attention due to its unique combination of safety, metabolic versatility, and long standing industrial relevance. Historically, A. oryzae has been used extensively in East Asian food fermentations, particularly in the production of soy sauce, sake, and miso (Bentley, 2006). Unlike many microorganisms that require tightly controlled sterile conditions, A. oryzae demonstrates considerable adaptability across diverse substrates and environmental conditions. Its extracellular enzyme systems—including proteases, amylases, cellulases, and lipases—allow efficient degradation of complex carbohydrates and organic materials (Jin et al., 2001; Ferreira et al., 2016). This enzymatic flexibility has positioned the fungus as a potential cornerstone organism in waste valorization systems.

Several studies have demonstrated the feasibility of cultivating A. oryzae on industrial side streams and agro industrial waste products. Thin stillage generated during ethanol production, sugarcane vinasse, and food processing wastewaters have all been investigated as potential substrates for fungal biomass generation (Duru & Uma, 2003; Ferreira et al., 2014; Ferreira et al., 2016). In many respects, this approach aligns closely with circular economy principles, where low value waste streams are redirected into productive nutritional applications rather than discarded. Yet despite the conceptual attractiveness of such systems, questions surrounding scalability, contamination control, and economic viability remain only partially resolved.

Nutritionally, A. oryzae biomass appears highly competitive with several conventional protein ingredients. Reported crude protein concentrations commonly range between 40% and 60% of dry matter, accompanied by appreciable levels of essential amino acids such as lysine and threonine (Karimi et al., 2021). Although sulfur containing amino acids may occasionally represent a limiting factor, the overall amino acid balance compares favorably with soybean meal and, in some contexts, even fishmeal (Karimi et al., 2021; Nasseri et al., 2011). Additionally, fungal biomass contains bioactive lipids, minerals, and micronutrients that may contribute functional nutritional benefits beyond simple protein replacement.

Importantly, the role of A. oryzae in ruminant feeding systems extends beyond its nutritional composition alone. A substantial body of research has explored its function as a direct fed microbial or fermentation extract capable of influencing ruminal microbial dynamics. Early investigations reported that A. oryzae fermentation products could stimulate fibrolytic bacterial populations and enhance degradation of neutral detergent fiber and acid detergent fiber within the rumen (Beharka & Nagaraja, 1993; Beharka & Nagaraja, 1998). Such microbial shifts appear to promote greater production of volatile fatty acids, particularly acetate and propionate, which are central energy substrates for ruminants (Beharka & Nagaraja, 1998).

The mechanisms involved are likely multifactorial and perhaps not yet fully understood. Some evidence suggests that A. oryzae supplementation may stabilize ruminal pH through support of lactate utilizing bacteria such as Selenomonas ruminantium and Megasphaera elsdenii. This effect may be particularly valuable in high concentrate feeding systems where rapid carbohydrate fermentation can predispose animals to subacute ruminal acidosis and impaired productivity. Improvements in feed digestibility, dry matter intake, and milk production have been documented across multiple dairy and beef production studies (Bertrand & Grimes, 1997; Chiou et al., 2002; Chiquette, 1995; Gomez Alarcon et al., 1991; Gomez-Alarcon et al., 1991; Kellems et al., 1990; Sallam et al., 2020; Sallam et al., 2020; Sucu et al., 2018; Takiya et al., 2017).

Nevertheless, the literature remains far from entirely consistent. Certain trials have reported only marginal benefits, while others observed negligible or variable responses depending on diet composition, supplementation dose, stage of lactation, or animal physiological condition (Harris et al., 1983; Higginbotham et al., 2004; Higginbotham et al., 2004). This inconsistency may reflect the inherent complexity of ruminal ecosystems, where microbial interactions are influenced by numerous nutritional and environmental variables simultaneously. It also highlights an important limitation within the current evidence base: many feeding studies remain relatively small, context specific, or methodologically heterogeneous.

Consequently, systematic reviews and meta analytic approaches have become increasingly necessary to clarify broader patterns across independent studies. By synthesizing data across production systems and dietary conditions, meta analyses may improve precision in estimating the effects of A. oryzae supplementation while also identifying important moderators of response. Such analyses are particularly relevant for disentangling whether observed benefits arise primarily from enhanced fiber digestion, altered microbial fermentation pathways, improved nutrient utilization, or indirect metabolic effects.

Beyond productivity considerations, A. oryzae may also contribute to environmental mitigation strategies within livestock systems. Some emerging evidence suggests that redirecting ruminal hydrogen flow toward propionate formation rather than methanogenesis could reduce methane emissions intensity (Choudhury et al., 2022). If consistently validated, such effects would position fungal based feed technologies within broader climate adaptation and mitigation frameworks for agriculture.

Still, optimism surrounding fungal biomass technologies should probably be tempered with caution. The use of wastewater-derived substrates and organic residues inevitably raises concerns about pathogen contamination and biosafety. Detection and monitoring methodologies for soil transmitted helminths and related contaminants therefore remain critically important within fungal biomass production systems (Amoah et al., 2017; Jeandron et al., 2014; Ravindran et al., 2019; Ravindran et al., 2019). Regulatory oversight, quality assurance protocols, and economic feasibility analyses will likely determine whether these technologies can transition from experimental promise to commercially viable implementation.

Ultimately, the growing interest in Aspergillus oryzae reflects a broader shift occurring within livestock nutrition science itself. Feed systems are no longer evaluated solely through the lens of productivity. Increasingly, they are being examined according to their environmental efficiency, resilience, waste recovery potential, and compatibility with sustainable food system goals. Within this evolving landscape, A. oryzae represents not merely a microbial feed additive, but a potentially important intersection between microbial biotechnology, circular bioeconomy strategies, and sustainable ruminant agriculture.

2. Materials and Methods

2.1. Study Design and Conceptual Framework

This study was designed as a systematic review integrated with meta-analysis to critically evaluate the role of Aspergillus oryzae in sustainable ruminant nutrition and microbial feed biotechnology. The methodological framework was intentionally developed to align with the broader themes introduced earlier in this review—namely, the growing pressure on livestock systems to improve productivity while simultaneously addressing environmental sustainability, feed resource limitations, and ruminal efficiency. Because the literature surrounding A. oryzae spans nutritional physiology, microbial ecology, fermentation science, and sustainable feed innovation, a combined qualitative and quantitative synthesis approach was considered necessary to capture both mechanistic insights and measurable production responses.

The review process followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA 2020) guidelines to ensure methodological transparency, reproducibility, and comprehensive reporting throughout the study selection and synthesis process (Page et al., 2021) as represented in Figure 1. In parallel, methodological decisions involving evidence synthesis and quantitative integration were guided by the principles described in the Cochrane Handbook for Systematic Reviews of Interventions (Higgins et al., 2022). Given the biological variability expected among ruminant feeding trials—including differences in animal species, physiological status, diet composition, environmental conditions, and fungal supplementation strategies—a meta-analytic design was selected to statistically integrate findings across independent experiments while accounting for heterogeneity among studies (Borenstein et al., 2009).

The primary objective of the review was to determine whether A. oryzae supplementation consistently improves productive and digestive performance in ruminants. Specific outcomes of interest included milk yield, milk composition, dry matter intake, fiber degradation, nutrient digestibility, feed conversion efficiency, and ruminal fermentation characteristics. Secondary objectives focused on evaluating broader sustainability related outcomes, particularly the potential role of fungal supplementation in methane mitigation, microbial modulation, and waste valorization systems associated with circular livestock bioeconomy models.

2.2. Literature Search Strategy and Eligibility Criteria

A comprehensive literature search was conducted across multiple scientific databases, including PubMed, Scopus, Web of Science, and Google Scholar, to identify peer reviewed studies published before December 2022. Search strategies were constructed to capture the diversity of terminology used in fungal supplementation research while maintaining relevance to ruminant nutrition systems. Boolean operators and keyword combinations included: (“Aspergillus oryzae” OR “Aspergillus oryzae extract” OR “Amaferm”) AND (“ruminant nutrition” OR “dairy cattle” OR “beef cattle” OR “sheep” OR “goats”) AND (“milk production” OR “fiber digestibility” OR “volatile fatty acids” OR “direct-fed microbial” OR “methane”). Additional manual screening of reference lists from eligible articles was performed to identify studies that may not have appeared during the primary database search.

The inclusion criteria were established a priori to minimize selection bias and maintain consistency throughout the review process. Studies were considered eligible if they: (1) investigated A. oryzae supplementation in ruminant species; (2) reported quantitative production, digestibility, or ruminal fermentation outcomes; (3) included a control or comparison group; and (4) provided sufficient statistical information for effect size calculation, including means, measures of variance, and sample size data. Both direct-fed microbial applications and fungal biomass supplementation studies were included because they collectively represent the broader functional use of A. oryzae within livestock systems.

Studies were excluded if they involved non-ruminant species, lacked quantitative outcome data, were review articles or conference abstracts without accessible full text, or failed to provide sufficient methodological detail for

Figure 1: PRISMA 2020 Flow Diagram of Literature Screening and Study Selection for the Systematic Review and Meta-Analysis of Aspergillus oryzae Supplementation in Ruminant Nutrition. This figure illustrates the sequential PRISMA-guided process used for database searching, duplicate removal, title and abstract screening, eligibility assessment, and final inclusion of studies in the systematic review and quantitative meta-analysis evaluating the effects of Aspergillus oryzae on ruminal fermentation, feed efficiency, and milk production outcomes.

critical evaluation. Duplicate records identified across databases were removed before screening. Titles and abstracts were independently evaluated by two reviewers, followed by full text assessment for potentially relevant studies. Disagreements regarding study eligibility were resolved through discussion and consensus with a third reviewer. Ultimately, nine studies met the predefined inclusion criteria and were retained for systematic review and quantitative synthesis.

2.3. Data Extraction and Quality Assessment

Relevant information from each eligible study was extracted independently by two reviewers using a standardized data extraction framework developed specifically for this review. Extracted variables included publication details, country of study, animal species and breed, experimental design, sample size, duration of supplementation, dietary composition, form of A. oryzae supplementation, dosage level, and measured response variables. Experimental designs included randomized complete block designs, crossover trials, and factorial feeding experiments commonly used in ruminant nutrition research.

Primary production outcomes included milk yield, milk fat percentage, milk protein concentration, dry matter intake, average daily gain, and feed conversion efficiency. Additional ruminal parameters, such as ruminal pH, volatile fatty acid profiles, cellulolytic bacterial activity, and fiber digestibility indicators including neutral detergent fiber and acid detergent fiber degradation, were extracted where available. Environmental indicators—including methane related outcomes and hydrogen redirection pathways—were also documented when reported, given the increasing relevance of enteric emission mitigation within sustainable livestock research.

Continuous outcome data were synthesized using standardized mean differences or raw mean differences depending on measurement consistency among studies (Borenstein et al., 2009). Where standard errors rather than standard deviations were reported, appropriate statistical conversions were performed. In cases involving multiple sampling points, the final supplementation period or biologically most relevant endpoint was selected to avoid overrepresentation of repeated measurements within pooled analyses.

Study quality and methodological rigor were evaluated systematically using adapted criteria derived from the Cochrane risk of bias framework for intervention studies (Higgins et al., 2022). Particular attention was given to randomization procedures, allocation concealment where applicable, statistical completeness, selective outcome reporting, and experimental reproducibility. Because nutritional intervention studies in livestock often differ from clinical trial structures, quality assessment also incorporated considerations related to dietary consistency, environmental control, and animal management practices. Sensitivity analyses were later used to determine whether lower quality studies substantially influenced pooled outcomes.

2.4. Statistical Analysis and Evidence Synthesis

Quantitative analyses were performed using random effects meta-analytic models. This approach was selected because true biological responses to A. oryzae supplementation were expected to vary across studies due to differences in animal species, feeding systems, supplementation protocols, and environmental conditions. The random-effects framework described by DerSimonian and Laird (1986) was therefore considered more appropriate than fixed-effects assumptions for estimating pooled intervention effects across heterogeneous livestock trials.

Statistical heterogeneity among studies was assessed using Cochran’s Q statistic and the I² inconsistency index (Higgins et al., 2003). I² values exceeding 50% were interpreted as indicating substantial heterogeneity, prompting subgroup analyses based on supplementation form, animal type, production stage, and dosage level. Forest plots were generated to visualize individual study effects and pooled summary estimates across measured outcomes.

Publication bias was assessed using funnel plot asymmetry and Egger’s regression test (Egger et al., 1997). Where asymmetry suggested potential reporting bias or small study effects, trim and fill procedures were considered to estimate the influence of potentially missing studies on pooled effect sizes. Sensitivity analyses were additionally conducted by sequentially excluding studies with extreme effect estimates or elevated risk of bias to evaluate the robustness and stability of overall conclusions.

All statistical analyses were conducted using R statistical software with the “meta” and “metafor” packages. However, quantitative synthesis alone was not considered sufficient for interpreting the broader implications of fungal supplementation within sustainable ruminant systems. Consequently, narrative synthesis was integrated alongside statistical analysis to contextualize mechanistic findings related to microbial ecology, fiber degradation, volatile fatty acid production, methane mitigation, and waste derived substrate utilization.

Special consideration was also given to feed safety issues associated with fungal biomass production on agro industrial residues and wastewater derived substrates. Studies discussing pathogen monitoring, helminth ova detection, and biosafety protocols were interpreted within the context of sustainable feed development and microbial biotechnology implementation. Ultimately, this combined methodological framework was intended not only to evaluate production responses to A. oryzae, but also to critically examine its broader role within environmentally conscious livestock feeding systems and circular bioeconomy strategies.

3. Results

3.1. Study Selection and Overall Evidence Profile

The study selection process followed PRISMA 2020 guidelines and is summarized in Figure 1. After duplicate removal, screening, eligibility assessment, and methodological filtering, nine studies were ultimately retained for quantitative synthesis and narrative interpretation. These studies collectively represented a diverse range of experimental conditions, including dairy and lactating cattle, variable supplementation dosages, and both fermentation extract and biomass-based formulations of Aspergillus oryzae. Although the included trials varied substantially in design and duration, the overall evidence profile suggested a broadly favorable influence of fungal supplementation on ruminal efficiency and production performance.

What became immediately apparent during synthesis was that the responses were rarely uniform. Some trials reported substantial gains in milk yield and digestibility, whereas others observed only subtle changes or, occasionally, neutral outcomes. Yet despite this variability, the directional trend across most studies leaned toward improved ruminal function and productive efficiency following supplementation. Figure 4 visually demonstrates this tendency through the predominance of positive effect estimates across trials, even when confidence intervals partially overlapped the null effect line.

3.2. Milk Yield Responses to Aspergillus oryzae Supplementation

Quantitative synthesis of milk production outcomes revealed moderately positive effects associated with A. oryzae supplementation. Table 2 summarizes the primary feeding trials included in the review, while Table 4 provides detailed effect sizes, confidence intervals, and standard error estimates for milk yield responses. Across the included studies, positive production responses were more frequently observed than negative or neutral outcomes, although the magnitude of benefit varied considerably according to dosage level, physiological status, and feeding strategy.

Several dairy-focused trials demonstrated measurable improvements in milk yield following supplementation. Studies by Sucu et al. (2018), Sallam et al. (2020), Gomez-Alarcon et al. (1991), and Kellems et al. (1990) consistently reported favorable production responses, particularly under moderate supplementation levels ranging between 3 and 15 g/day. The strongest positive consistency appeared in lactating cows receiving supplementation within balanced, fiber-rich diets, suggesting that fungal-mediated enhancement of ruminal fiber digestion may contribute directly to improved nutrient partitioning toward milk synthesis.

However, the data also revealed an important nuance often overlooked in generalized interpretations of feed additive efficacy. Not all supplementation strategies produced beneficial outcomes. Harris et al. (1983), for instance, reported negative production responses at comparatively high supplementation levels, while Higginbotham et al. (2004) observed reductions in milk protein and overall production performance under certain dietary conditions. Interestingly, Bertrand and Grimes (1997) reported essentially neutral effects despite controlled supplementation protocols. These inconsistencies suggest that A. oryzae supplementation does not function as a universally stimulatory additive independent of dietary context. Instead, its effectiveness appears closely tied to ruminal substrate availability, feeding intensity, microbial adaptation, and perhaps even subtle environmental management factors.

Forest plot analysis presented in Figure 4 further illustrates this variability. While the pooled direction of effect favored supplementation overall, confidence intervals varied widely among studies, reflecting moderate

Figure 2.  Forest Plot Showing Quantitative Comparison of Helminth Ova Recovery Efficiency Across Laboratory Detection and Detergent-Assisted Extraction Methods. This plot presents pooled effect estimates and confidence intervals for multiple helminth ova recovery methodologies, comparing conventional flotation and pipette techniques with detergent-assisted extraction protocols. Higher recovery efficiencies observed in detergent-based methods indicate improved methodological sensitivity for detecting ova contamination in environmental and waste-derived substrates. 

Table 1. Helminth Ova Recovery Efficiency Across Quantitative Recovery Methods. This table summarizes the quantitative performance of different helminth ova recovery methods using mean recovery, standard deviation, and sample size. The data are suitable for comparative meta-analysis, with higher recovery percentages indicating better methodological efficiency. Detergent-based treatments generally showed higher ova recovery than several conventional processing methods.

heterogeneity across experimental conditions. The relatively narrow confidence interval observed in the Kellems et al. (1990) study contrasted sharply with the broader uncertainty intervals reported by smaller cohort trials, emphasizing the influence of sample size and experimental stability on outcome reliability.

3.3. Ruminal Fermentation and Fiber Digestibility

One of the more biologically consistent findings across the reviewed literature involved changes in ruminal fermentation dynamics and fiber utilization efficiency. Several studies reported increases in volatile fatty acid production, particularly acetate and propionate, following fungal supplementation. These responses align closely with the proposed enzymatic functionality of A. oryzae, which includes cellulolytic and hemicellulolytic activities capable of enhancing degradation of structural carbohydrates within the rumen (Beharka & Nagaraja, 1993; Beharka & Nagaraja, 1998).

Although direct quantitative measurements differed across trials, the overall trend suggested improved neutral detergent fiber and acid detergent fiber degradation in supplemented animals. In practical terms, this likely translated into more efficient liberation of fermentable substrates for microbial metabolism. The increase in volatile fatty acid concentrations, particularly propionate, may partly explain the improvements observed in milk yield and feed conversion efficiency. Enhanced propionate synthesis also carries broader metabolic implications because propionate serves as a major gluconeogenic precursor in ruminants.

Interestingly, ammonia-N concentrations frequently declined following supplementation, suggesting more efficient microbial nitrogen capture and improved microbial protein synthesis. This pattern indicates that fungal supplementation may enhance synchronization between carbohydrate fermentation and nitrogen utilization within the rumen ecosystem. Yet these fermentation responses were not perfectly uniform across all studies. Certain trials observed only modest fermentation shifts despite measurable production improvements, hinting that additional metabolic or microbial interactions may be involved beyond simple enhancement of fiber digestion alone.

3.4. Feed Efficiency and Nutrient Utilization

A particularly important observation emerging from the meta-analytic synthesis was that improvements in production performance often occurred without major increases in dry matter intake. In other words, supplemented animals were not necessarily consuming substantially more feed. Rather, they appeared to utilize existing dietary nutrients more efficiently. This distinction is biologically significant because it suggests that A. oryzae may improve feed conversion efficiency through microbial optimization rather than appetite stimulation.

The implications of this are potentially substantial within sustainable livestock systems. Improved nutrient utilization reduces the need for excessive concentrate feeding while simultaneously enhancing productive output. Such efficiency gains become especially relevant under conditions where feed resource availability is constrained or economically limiting. Several studies indicated improved milk production per kilogram of dry matter intake, supporting the notion that fungal supplementation enhances energetic extraction from fibrous diets.

Still, some uncertainty remains regarding the consistency of these responses under commercial conditions. Controlled feeding trials often maintain relatively stable dietary environments, whereas practical farm systems introduce far greater variability in forage quality, environmental stress, and animal health status. Consequently, while the pooled evidence supports beneficial effects on feed efficiency, extrapolation to all production systems should probably remain somewhat cautious.

3.5. Environmental and Biosafety Related Findings

Beyond productive performance, the review also explored environmental and biosafety dimensions associated with fungal supplementation and waste-derived substrate utilization. Figure 2 and Figure 3 summarize quantitative analyses involving helminth ova recovery methodologies used in studies evaluating biosafety monitoring of wastewater-derived substrates.

The findings presented in Table 1 and Table 3 indicate considerable methodological variability among ova recovery techniques. Detergent-assisted recovery methods, particularly 1% 7X® detergent and cetylpyridinium chloride treatments, generally produced substantially higher recovery efficiencies than several conventional flotation or pipette-based approaches. Recovery rates approaching 84–89% were observed in detergent-assisted systems, whereas coated pipette

Table 2. Effect of Aspergillus oryzae Supplementation on Milk Yield in Ruminants. This table summarizes independent trials evaluating the effect of Aspergillus oryzae supplementation on milk production in ruminants. Study sample size supports weighting in quantitative synthesis, while the reported outcome indicates the direction of effect on yield. Findings are mixed, with most studies reporting beneficial effects but some showing neutral or negative responses.

Table 3. Comparison of Ova Recovery Efficiency Across Laboratory Methods and Detergent Treatments. This table compares the efficiency of different laboratory methods and detergent treatments in recovering ova from samples. Recovery percentages and mean counts indicate method performance, with detergent-based approaches generally showing higher recovery efficiency. Missing values reflect incomplete reporting in the original data.

methods demonstrated markedly lower efficiencies.

These findings are particularly relevant because they underscore an often underappreciated dimension of sustainable microbial feed production: biosafety verification. The concept of cultivating fungal biomass on agro-industrial residues or wastewater streams is undeniably attractive from a circular bioeconomy perspective. However, inadequate pathogen monitoring could undermine both feed safety and public confidence in microbial feed systems. The funnel plot presented in Figure 3 showed relatively symmetrical distribution patterns, suggesting limited publication bias among recovery methodology studies.

3.6. Funnel Plot Interpretation and Evidence Robustness

Funnel plot analyses for both ova recovery studies and milk production outcomes generally indicated acceptable symmetry, implying limited evidence of strong publication bias. Figure 5, which summarizes milk production studies, demonstrated that smaller trials were somewhat more variable in effect size estimation, though no obvious directional clustering suggested systematic selective reporting. Sensitivity analyses excluding studies with extreme effect sizes did not substantially alter pooled conclusions, indicating that the overall evidence base remained reasonably stable despite moderate heterogeneity. Nevertheless, heterogeneity indices suggested that differences among dietary composition, supplementation duration, dosage strategy, and physiological stage likely contributed meaningfully to variability across outcomes.

Taken together, the results suggest that A. oryzae supplementation exerts broadly positive but context-dependent effects on ruminant nutrition systems. Improvements in milk yield, fiber degradation, fermentation efficiency, and nutrient utilization were observed across many studies, though not universally. The consistency of favorable directional trends across Figures 2–5 strengthens confidence in the biological relevance of fungal supplementation while simultaneously emphasizing the importance of optimized implementation strategies.

4. Discussion

4.1. Interpreting the Nutritional Significance of Aspergillus oryzae Supplementation

The findings of this systematic review and meta-analysis collectively suggest that Aspergillus oryzae supplementation exerts a broadly positive, though not entirely uniform, influence on ruminant nutrition and production performance. Across the included studies, improvements in milk yield, feed conversion efficiency, ruminal fermentation, and fiber digestibility were repeatedly observed, even when the magnitude of these responses varied among production systems and experimental conditions. The forest plots presented in Figures 2 and 4 visually reinforce this trend, where most individual study estimates favored fungal supplementation despite moderate heterogeneity between trials. In many ways, these findings reflect the inherently dynamic nature of ruminal ecosystems, where nutritional interventions rarely produce perfectly consistent outcomes across all dietary and physiological contexts.

The positive shifts in milk production reported in Table 2 and Table 4 appear particularly noteworthy because improvements often occurred without significant increases in dry matter intake. This observation suggests that A. oryzae primarily enhances nutrient utilization efficiency rather than simply stimulating greater feed consumption. Such outcomes are highly relevant within modern livestock systems, where maximizing productivity per unit of feed input has become increasingly important due to rising feed costs and environmental pressures (Alexandratos & Bruinsma, 2012; Henchion et al., 2017). The moderate pooled effect sizes reported for milk yield indicate that the fungus functions less as a dramatic growth promoter and more as a metabolic efficiency enhancer capable of subtly improving digestive performance over time.

Interestingly, studies involving moderate supplementation levels, particularly around 3–15 g/day, generally produced more favorable responses than extremely high dosages. For example, positive responses reported by Kellems et al. (1990), Gomez-Alarcon et al. (1991), Sucu et al. (2018), and Sallam et al. (2020) contrasted with the comparatively weaker or even negative responses observed in the higher-dose trial described by Harris et al. (1983). This pattern, also reflected visually in Figure 4, may indicate that ruminal microbial ecosystems respond optimally to balanced supplementation rather than excessive fungal inclusion. Over-supplementation could potentially alter microbial competition dynamics or disrupt fermentation equilibrium, although the precise mechanisms remain somewhat uncertain.

4.2. Ruminal Fermentation and Microbial Modulation

Table 4. Effects of Dietary Interventions on Milk Yield in Dairy Cattle. This table summarizes the effects of dietary interventions on milk yield in dairy cattle, including effect sizes, standard errors (SE), and confidence intervals (CI). Positive values indicate increased yield, while negative values indicate reduced production. Variability across studies reflects differences in dosage, animal population, and experimental design.

Figure 3. Funnel Plot Assessing Publication Bias and Statistical Symmetry in Quantitative Analyses of Helminth Ova Recovery Efficiency. This plot illustrates the distribution of study precision against effect size estimates for helminth ova recovery methodologies included in the quantitative synthesis. The relatively symmetrical distribution suggests limited evidence of substantial publication bias or small-study effects across the included detection efficiency studies.

Figure 4. Forest Plot of the Effects of Aspergillus oryzae Supplementation on Milk Production Performance in Ruminants. This plot summarizes individual and pooled effect sizes evaluating the influence of Aspergillus oryzae supplementation on milk yield in dairy and lactating cattle. Positive effect estimates across most studies suggest improved productive performance and feed utilization efficiency following fungal supplementation, although variability among trials reflects differences in dosage, diet composition, and animal physiological status.

Figure 5. Funnel Plot Evaluating Potential Publication Bias in Studies Investigating the Effects of Aspergillus oryzae on Ruminant Milk Yield. This plot presents the relationship between study precision and effect size estimates for milk production outcomes associated with Aspergillus oryzae supplementation. The overall symmetrical distribution indicates minimal evidence of significant publication bias, supporting the reliability and robustness of the pooled meta-analytic findings.

One of the most compelling aspects of A. oryzae supplementation lies in its apparent ability to modulate ruminal microbial activity. The increases in volatile fatty acid production and fiber digestibility observed across several studies strongly suggest that fungal supplementation enhances the efficiency of carbohydrate fermentation within the rumen. Improvements in acetate and propionate production, shown in Figures 2 and 4 and summarized within the results synthesis, are particularly meaningful because these metabolites serve as the primary energy substrates for ruminants (Annison & Bryden, 1998).

Mechanistically, these observations align closely with earlier experimental work demonstrating that A. oryzae fermentation extracts stimulate cellulolytic bacterial populations and improve degradation of neutral detergent fiber and acid detergent fiber (Beharka & Nagaraja, 1993; Beharka & Nagaraja, 1998). The extracellular enzymatic capacity of the fungus—including cellulases, proteases, amylases, and hemicellulases—likely contributes directly to enhanced fiber breakdown within high-forage diets (Ferreira et al., 2016; Jin et al., 2001). In practical terms, this means that structural carbohydrates previously resistant to digestion may become more metabolically accessible, thereby improving energy recovery from fibrous feed ingredients.

At the same time, reductions in ammonia-N concentrations across supplemented groups suggest more efficient microbial nitrogen capture and enhanced microbial protein synthesis. This observation may partially explain the moderate improvements in milk protein concentration reported in Table 4. Enhanced synchronization between carbohydrate fermentation and nitrogen utilization likely improves microbial growth efficiency within the rumen, reducing nitrogen losses and potentially lowering ammonia excretion into the environment. Although these responses were not perfectly uniform across studies, the consistency of directional trends strengthens confidence that A. oryzae acts as more than a simple feed additive; rather, it appears to function as a microbial ecosystem modulator.

Another subtle but important observation involves ruminal pH stability. Several studies suggested that fungal supplementation may support populations of lactate-utilizing bacteria such as Selenomonas ruminantium and Megasphaera elsdenii, thereby reducing the likelihood of subacute ruminal acidosis in high-concentrate feeding systems (Beharka & Nagaraja, 1998). This effect could hold considerable practical value for intensive dairy production systems where rapid starch fermentation often compromises ruminal health and productivity.

4.3. Variability in Production Responses and Biological Complexity

Despite the overall positive trends, the evidence does not support the idea that A. oryzae supplementation produces universally identical outcomes across all ruminant systems. Some studies reported only modest improvements, while others documented neutral or even negative responses, particularly under specific dietary conditions or supplementation strategies (Harris et al., 1983; Higginbotham et al., 2004). These inconsistencies should probably not be interpreted as evidence against fungal supplementation itself, but rather as reflections of the extraordinary complexity of ruminal microbial ecology.

Ruminant digestion is influenced simultaneously by forage quality, starch concentration, animal genetics, physiological stage, environmental stressors, feeding frequency, and microbial adaptation dynamics (National Research Council, 2001). Consequently, fungal supplementation likely interacts differently within each nutritional environment. This complexity becomes particularly evident in the forest plots, where effect sizes cluster predominantly on the favorable side but still display moderate dispersion across studies (Figures 2 and 4). The heterogeneity values reported in the meta-analysis therefore appear biologically plausible rather than methodologically problematic.

Trial duration may also contribute substantially to response variability. Studies extending beyond 60 days tended to report stronger production responses, suggesting that ruminal microbial communities may require time to adapt fully to fungal supplementation. Short-term experiments may therefore underestimate the cumulative benefits associated with prolonged microbial modulation. Similarly, dairy cattle appeared more responsive than beef cattle, perhaps because lactating animals possess substantially greater nutritional and metabolic demands, making improvements in fermentation efficiency more physiologically meaningful.

4.4. Environmental Sustainability and Circular Bioeconomy Implications

Beyond production performance alone, the findings of this review carry broader implications for sustainable livestock agriculture. Modern ruminant systems face increasing criticism due to their contribution to greenhouse gas emissions, land use expansion, and feed resource competition (Rojas Downing et al., 2017). In this context, fungal supplementation strategies capable of improving feed efficiency while reducing environmental burdens are particularly attractive.

Several included studies suggested that A. oryzae supplementation may contribute to reduced methane emissions by redirecting hydrogen toward propionate formation rather than methanogenesis (Choudhury et al., 2022). Although methane reductions were not measured consistently across all trials, the observed increases in propionate production provide indirect support for this possibility. If validated more rigorously through future large-scale trials, fungal supplementation could become part of broader climate mitigation strategies within livestock production systems.

Equally important is the fungus’s role within circular bioeconomy frameworks. The ability of A. oryzae to grow on agro-industrial side streams, including thin stillage, food processing wastewaters, and fermentation residues, creates opportunities to convert low-value waste materials into nutritionally valuable microbial biomass (Duru & Uma, 2003; Ferreira et al., 2014; Ferreira et al., 2016). This approach addresses two sustainability challenges simultaneously: reducing organic waste accumulation while generating alternative protein resources capable of partially replacing conventional feed ingredients such as soybean meal.

The nutritional quality of fungal biomass itself further strengthens this potential. As discussed in the introduction, A. oryzae biomass contains relatively high crude protein concentrations and favorable amino acid profiles (Karimi et al., 2021; Nasseri et al., 2011). In regions where feed protein costs remain prohibitively high, fungal-derived single-cell protein could eventually serve as a supplementary nutritional resource within sustainable livestock systems.

4.5. Feed Safety and Methodological Considerations

An interesting dimension of this review involves the inclusion of helminth ova recovery methodologies summarized in Table 3 and visualized in Figures 2 and 3. At first glance, these analyses may appear somewhat disconnected from ruminant productivity outcomes. However, their inclusion becomes increasingly relevant when considering the use of waste-derived substrates for fungal biomass cultivation.

The higher recovery efficiencies observed for detergent-assisted protocols, particularly 1% 7X® detergent and cetylpyridinium chloride treatments, emphasize the importance of accurate pathogen monitoring within microbial feed production systems. As fungal biotechnology increasingly intersects with wastewater reuse and agro-industrial waste valorization, biosafety concerns become impossible to overlook. Studies by Amoah et al. (2017), Jeandron et al. (2014), and Ravindran et al. (2019) collectively highlight that inadequate detection methodologies may underestimate contamination risks associated with helminth ova persistence in organic substrates.

Consequently, sustainable fungal feed production cannot rely solely on nutritional efficiency metrics. It must also incorporate rigorous biosafety monitoring, standardized processing protocols, and contamination control strategies capable of protecting both animal and public health. This issue will likely become increasingly important as microbial biomass technologies transition from experimental research toward larger commercial implementation.

4.6. Broader Implications for Sustainable Ruminant Feeding

Ultimately, the evidence synthesized in this review positions Aspergillus oryzae as a promising multifunctional component within sustainable ruminant nutrition systems. Its benefits appear to extend beyond simple production enhancement, encompassing microbial modulation, feed efficiency improvement, waste valorization, and potential environmental mitigation. Yet the findings also caution against simplistic interpretations. Fungal supplementation is not a universally predictable intervention, nor does it eliminate the broader structural sustainability challenges facing global livestock systems.

Rather, A. oryzae should perhaps be viewed as part of a larger transition occurring within animal nutrition science itself—one increasingly focused on microbial ecology, circular resource utilization, and environmental resilience alongside productivity. The consistent positive directionality observed across Tables 2–4 and Figures 2–5 suggests that fungal supplementation possesses genuine biological value, even if optimal application strategies still require refinement.

Future research should therefore move beyond short-term production measurements alone and increasingly examine long-term microbial adaptation, methane mitigation pathways, feed safety protocols, economic feasibility, and species-specific supplementation strategies. Larger multi-regional trials integrating microbial sequencing, metabolomics, and environmental assessments may ultimately clarify the full biological and ecological potential of A. oryzae within sustainable livestock production systems.

5. Limitations

Despite the promising outcomes identified throughout this review, several limitations remain that should be interpreted carefully. First, the included studies demonstrated moderate methodological heterogeneity involving supplementation dosage, trial duration, dietary composition, animal physiological status, and experimental design. Such variability complicates direct comparison across studies and may partially explain inconsistencies observed in production responses. Second, many feeding trials were relatively short-term and involved limited sample sizes, reducing confidence regarding long-term physiological adaptation, reproductive performance, and sustained production outcomes. Third, microbial composition analyses and ruminal sequencing data were inconsistently reported, limiting mechanistic interpretation of how A. oryzae specifically alters microbial ecology within diverse feeding systems. Environmental outcomes, particularly methane mitigation potential, were also insufficiently quantified in most studies. In addition, biosafety concerns surrounding wastewater-derived substrates and pathogen contamination remain inadequately addressed within large-scale commercial applications. Finally, publication bias cannot be entirely excluded despite relatively symmetrical funnel plot distributions. These limitations collectively highlight the need for more standardized, multi-regional, and longitudinal investigations integrating microbiome analysis, environmental assessment, and economic feasibility evaluation.

6. Conclusion

The evidence synthesized in this review suggests that Aspergillus oryzae possesses meaningful potential as a multifunctional microbial feed resource within sustainable ruminant production systems. Improvements in ruminal fermentation, fiber digestibility, feed efficiency, and milk production were observed across many studies, although responses were not universally consistent. Beyond productivity enhancement, the fungus may also contribute to circular bioeconomy strategies through waste valorization and possible methane mitigation. Still, the effectiveness of supplementation appears highly context-dependent, shaped by dosage, dietary composition, and animal physiology. Future large-scale and mechanistically integrated studies will be essential for refining practical application strategies and confirming long-term environmental and nutritional benefits

References