Ruminants, as an inseparable part of the food chain, will play a determining role in meeting future food demands and guaranteeing global food security for the ever-growing population. In this regard, supplying ruminants with feed material and feed additives that enhance animal health and products has attracted great research interest in the field of ruminant nutrition. Volatile fatty acids (VFAs), such as acetic, propionic and butyric acids, which are endogenously generated in the rumen, are mainly responsible for providing the animal with metabolizable energy and as precursors for animal products. In addition, VFA can be supplied to ruminants as ex situ additives. Extensive research on VFA additives has proven that VFA supplementation affects the development of ruminant gastrointestinal tract, hormone levels, milk yield and composition, adipose tissue, meat quality, weight gain, dry matter intake, etc. The current review provides a unique collection of studies on the effects of VFA additives on ruminants in addition to an in-depth overview of the ruminant digestion system and the fate of different fractions of feed material. Moreover, the potential for sustainable provision of VFA additives from organic-rich waste and residues through anaerobic digestion plus post separation and concentration is critically discussed in this review.

Conventional ruminant production systems are often characterized by inefficient feed utilization and the generation of large volumes of agro-food byproducts (AFBs), which are frequently disposed of as waste. Some biorefineries, such as anaerobic digestion (AD), can produce value-added products like volatile fatty acids (VFAs). This study aimed to produce and evaluate a bio-based VFAs mixture from AFBs as a sustainable feed ingredient for ruminants, hypothesizing that it could partially replace conventional feed, improve rumen fermentation, and maintain feed intake. A semi-continuous immersed membrane bioreactor (MBR) was applied to bioconvert apple pomace and potato protein liquor and recover VFAs at an optimal organic loading rate of 3.7 gVS/L.day (28.6 g/L VFAs). The MBR achieved long-term performance (114 days) with a maximum of 40 g/L total solids but fouling hindered further operation. This study demonstrates the technical feasibility of using MBRs to produce VFAs from AFBs, offering a potentially more sustainable alternative to conventional VFAs production methods. In vitro studies, including a modified Menke gas method and the Rumen Simulation Technique (RUSITEC), demonstrated that the VFAs mixture did not negatively change rumen fermentation key parameters such as pH and redox potential. It significantly reduced methane production in RUSITEC when replacing 20% of concentrate energy. 

Subsequently, a pilot-scale MBR continuously produced feed-grade VFAs for 105 days, yielding 35 liters of the mixture daily. This mixture was then evaluated in an in vivo trial with 24 Suffolk lambs, where it was partially mixed with concentrate in the diet at two inclusion levels. Importantly, VFAs supplementation did not negatively affect feed intake, growth performance and altered rumen VFAs profiles, suggesting improved rumen fermentation.  

These findings suggest that industrial-scale production of sustainable bio-based VFAs from AFBs is technically feasible. This VFAs mixture shows promise as a feed ingredient for ruminants, capable of partially replacing concentrate’s energy, not disrupting the normal function of rumen fermentation, and potentially reducing enteric methane emissions.

The transition from laboratory scale production to industrial scale implementation is a critical step in introducing innovative technologies into the industry. Pilot-scale studies play a pivotal role in ensuring a smooth transition with minimal risks. This study investigates the scalability of a pilot-scale submerged membrane bioreactor (MBR) to produce volatile fatty acids (VFAs) as a sustainable alternative feed ingredient for ruminants. Building upon previous laboratory-scale research, this study aims to validate the scalability and replicability of the technology, ensuring that the results obtained at a smaller scale can be consistently achieved at a larger scale. Through the acidogenic fermentation of potato protein liquor (PPL) and apple pomace (AP) inoculated with rumen microorganisms, VFAs have been produced, and recovered over 105 days. Strategic adjustments to the organic loading rate (OLR, 2 gVS/L.day) achieved high and stable total VFAs concentration (above 10 g/L) with average VFAs yield of 0.47 gVFAs/gVSadded by effectively inhibiting methanogenesis. The favorable characteristics of the VFAs effluent, including its pH, VFAs distribution (high acetic and caproic acids), and ammonium content (around 400 mg/L), suggest its potential as a sustainable feed ingredient for ruminants. Further research is necessary to explore the precise effects and optimal utilization of VFAs as a feed supplement, but this study presents a promising step towards sustainable VFAs production and its application in the livestock sector.

The scientific interest in volatile fatty acids (VFAs) as an energy source and chemical precursor in ruminant diets has been longstanding, as it has significant implications for animal physiology and well-being. The present study explores the substitution of volatile fatty acids (VFAs) derived from agro-food residues via acidogenic fermentation as an alternative energy source in ruminant feed. Utilizing the gas production method, rumen digestibility assays were conducted, wherein the recovered VFA effluent from the acidogenic fermentation of apple pomace and potato protein liquor was substituted for 10%, 20%, and 30% of the total mixed ration (TMR) energy. Various parameters such as gas, VFA yield and composition, VFA peak intervals, changes in pH, and ammonium nitrogen content were investigated. Based on the results obtained, provision of 20% and 30% of the energy with VFAs did not increase methane production or did not cause significant pH alternations. Nevertheless, such supplementation resulted in increased production and accumulation of VFAs in the rumen media. The bioconversion of agro-food side streams into VFAs opens a new path in sustainable nutrient recovery and feed production from low value agro-industrial residues.

The growing demand for sustainable ruminant feed alternatives has motivated the application of bioconversion approaches for the valorization of agro-food byproducts (AFB) into feed additives and supplements. The present study thoroughly investigated substituting volatile fatty acids (VFAs) obtained from acidogenic fermentation (AF) of AFB as an energy source in ruminant feed. Rumen in vitro digestibility assays were conducted utilizing the gas production method, wherein the VFAs obtained from AF of apple pomace and potato protein liquor was substituted with partial silage and concentrate energy at levels of 10%, 20%, and 30%. The results indicate that substituting 20% of the concentrate’s energy with VFA mixture significantly reduced methane production and had no adverse effect on the production and accumulation of VFAs in the simulated rumen media. Conversely, replacing 10% of the silage energy with VFAs led to a decrease in methane production and further enhanced the production of VFAs. Readily digestible VFAs in ruminant feed have the potential to enhance energy availability and sustainability in ruminant farming practices, aligning with the principles of circular economy and waste valorization. 

The resource-intensive nature of the ruminant farming sector, which has been exacerbated by population growth and increasing pressure to reduce feed antibiotics and growth promoters, has sparked interest in looking for sustainable alternative feed sources to enhance ruminant production efficiency. Edible filamentous fungi, rich in macronutrients like proteins, offer promise in reducing the reliance on conventional protein sources and antimicrobials to improve feed quality and animal performance. The inclusion of single-cell proteins, particularly filamentous fungi, in ruminant feed has long been of scientific and industrial interest. This review focuses on the potential application of the extensively studied Aspergillus oryzae and its fermentation extracts in ruminant nutrition. It provides an overview of conventional ruminant feed ingredients, supplements, and efficiency. Additionally, this review analyzes the re-utilization of organic residues for A. oryzae cultivation and examines the effects of adding fungal extracts to ruminant feed on ruminal digestibility and animal performance, all within a circular bioeconomy framework.

Background

As demand for high quality animal feed continues to raise, it becomes increasingly important to minimize the environmental impact of feed production. An appealing sustainable approach to provide feed fractions is to use organic residues from agro-food industry. In this regard, volatile fatty acids (VFAs) such as acetic, propionic and butyric acids, derived from bioconversion of organic residues can be used as precursors for production of microbial protein with ruminant feed inclusion potential. This study aims to investigate the in vitro digestibility of the Aspergillus oryzae edible fungal biomass cultivated on VFAs-derived from anaerobic digestion of residues. The produced fungal protein biomass, along with hay clover silage and rapeseed meal were subjected to various in vitro assays using two-stage Tilley and Terry (TT), gas, and bag methods to evaluate and compare its digestibility for application in ruminant feed.

Results

The produced fungal biomass contained a higher crude protein (CP) (41%–49%) and rather similar neutral detergent fiber (NDF) (41%–56%) compared to rapeseed meal. The rumen in vitro dry matter digestibility (IVDMD) of the fungal biomass in the TT method ranged from 82% to 88% (statistically similar to that of the gas method (72% to 85%)). The IVDMD of fungal biomass were up to 26% and 40% greater than that of hay clover silage and rapeseed meal, respectively. The type of substrate and bag method had pronounced effect on the fermentation products (ammonium-N (NH4+-N), total gas and VFAs). Fungal biomass digestion resulted in the highest release of NH4+-N (340–540 mg/L) and the ratio of acetate to propionate ratio (3.5) among subjected substrates.

Conclusion

The results indicate that gas method can be used as a reliable predictor for IVDMD as well as fermentation products. Furthermore, the high IVDMD and fermentation product observed for Aspergillus oryzae fungal biomass digestion, suggest that the supplementation of fungal biomass will contribute to improving the rumen digestion by providing necessary nitrogen and energy to the ruminant and microbiota.

Volatile fatty acids (VFAs) supplementation in ruminants’ diet as a source of energy and chemical precursors and their effect on animal’s physiology and well-being has long been of scientific interest. Production of VFAs through anaerobic digestion of agro-industrial residues not only creates value but also presents an alternative sustainable approach for ruminant feed supplementation. Therefore, this study aimed to investigate the bioconversion of agro-industrial residues produced in large quantities such as apple pomace (AP), thin stillage (Ts), and potato protein liquor (PPL) to VFAs, fully complying to regulations set for ruminant feed supplement production. In this regard, batch acidogenic fermentation assays (pH 6–10) and semi-continuous immersed membrane bioreactor (iMBR) were applied. In batch assays, at pH 10 the co-digestion of Ts and PPL produced the highest VFAs concentration (14.2 g/L), indicating a yield of 0.85 g CODVFAs/g volatile solids (VS)added. The optimum batch condition was then applied in the iMBR for in situ fermentation and recovery of VFAs at different organic loading rates (OLR). With increasing the OLR to 3.7 gVS/L.day, the highest VFAs concentration of 28.6 g/L (1,2 g CODVFAs /gVSadded) was achieved. Successful long-term (114 days) membrane filtration was conducted in a media with a maximum of 40 g/L of total solids (TS), facing irreversible membrane fouling in the final stages. Acidogenic fermentation using an iMBR has the potential to play an important role in the future of feed additive provision through the biorefining of agro-industrial wastes via the carboxylate platform, given the role of VFAs production from organic residues.