The growing interest in improving rumen fermentation and mitigating methane emissions necessitates the use of essential oil blends (EOB) and fumaric acid (FA). This study evaluated the synergistic effect of four EOB with or without FA supplementation on in vitro dry matter digestibility, greenhouse gas emission, and total volatile fatty acid production using inoculum from three rumen-cannulated Black Angus beef cows. The study was arranged in a 4 × 2 + 1 factorial design to evaluate the effects of the four EOB and two FA levels on a total mixed ration (TMR). The EOB dosage was 100 µL while FA was added at 3% of total mixed ration. The EOB × FA interaction (p < 0.05) influenced the dry matter, neutral detergent fiber, and hemicellulose degradabilities. All the EOB and FA (EFA) treatments decreased (p < 0.001) the dry matter degradability compared to the control (TMR substrate only). The EFA4 treatment reduced the neutral detergent fiber and hemicellulose degradabilities compared to the control. The ruminal pH was influenced (p < 0.001) by both the EOB and FA inclusion, and the EOB × FA interaction was significant. The microbial mass was higher (p < 0.001) in the EFA1, EFA4, and EOB4 compared to the control and the EOB3 treatments. The EFA1 and EOB1 produced less (p < 0.001) gas than the control by 29.1 and 32.1%, respectively. Compared with the control, the EFA1 and EOB1 treatments decreased (p < 0.001) methane gas by 90.8% and 86.4%, respectively, while the carbon dioxide was reduced (p = 0.004) by 65.7 and 57.9%, respectively. The EOB × FA interaction was significant (p < 0.001) for the total and individual volatile fatty acid concentrations. The inclusion of FA increased the propionate concentration by 9.5% and decreased (p = 0.02) the acetate concentration by 4%. In summary, the synergistic effect of the EOB and FA offers an effective way to reduce greenhouse gas emission and enhance total volatile fatty acids.

The present study evaluated the inclusion of fumaric acid and essential oil blends (EOBs) containing anise, cedarwood, clove, cumin, eucalyptus, garlic, ginger, lavender, lemongrass, nutmeg, oregano, and peppermint at different proportions on in vitro dry matter (DM) disappearance (DMD), fiber fraction disappearance, the efficiency of microbial production, and the total volatile fatty acids (VFAs). Ten treatments without (control treatment) or with different EOB/fumaric combinations were used in the study with eight replicates. The EOB inclusion level was 200 μL/g of feed (total mixed ration, (TMR)) while fumaric acid was administered at 3% of the TMR (DM basis). The highest DMD, in vitro true degradable DM, partitioning factor (PF₂₄), and in vitro apparent degradable DM were recorded for the fumaric only treatment and the control. Neutral detergent fiber disappearance was reduced with the inclusion of EOB/fumaric combinations. The production of microbial mass and undegraded DM were higher (p < 0.001) for all EOBs and EOB and fumaric treatments. The inclusion of EOB and fumaric combinations reduced (p < 0.001) the total gas production, methane, and ammonia, with a higher PF₂₄ value noted for EOB3 treatment. The inclusion of individual EOB1 containing garlic, lemongrass, cumin, lavender, and nutmeg in a ratio of 4:2:2:1:1 or combined with fumaric acid yielded the highest propionate concentration across all treatments. We concluded that EOBs decreased methane production and nutrient degradability with better results with the individual EOB1 or EOB1/fumaric combination, which showed a potential enhancement in energy production.

This study investigated the combined impact of essential oil blends (EOBs) and fumaric acid (FA) on ruminal fermentation in dairy cows using the rumen simulation technique (RUSITEC) system. Three rumen-cannulated, non-lactating Holstein Friesian cows served as inoculum donors. The substrate, a total mixed ration (TMR), comprised corn silage, alfalfa hay, and a concentrate mix in a 3:1:1 ratio. The four treatments evaluated were Control (TMR without additives), EFA1 (TMR + EOB1 + FA), EFA2 (TMR + EOB2 + FA), and EFA3 (TMR + EOB3 + FA). Sixteen fermentation chambers were randomly assigned to the treatments, each with four replicates, following a completely randomized design during a 9-day experimental period. EOBs and FA were added at 10 µL/g feed and 3% of TMR, respectively. After a 4-day adaptation, samples were collected for 5 days. Results revealed that EFA1 significantly reduced (p = 0.0351) CH₄ emissions by 60.2% without negatively impacting dry matter disappearance, fiber fraction digestibility, pH, or gas volume. All EFAs increased (p < 0.001) the propionate molar proportion and decreased (p < 0.001) the acetate-to-propionate ratio. EFA2 decreased (p < 0.05) the acetate proportion by 3.3% compared to the control. In conclusion, EFA1 is recommended as an effective nutritional intervention to mitigate CH₄ emissions and optimize ruminal fermentation in dairy cows.

This study evaluated the synergistic effects of prebiotics containing galacto-oligosaccharides (GOS) and/or mannan oligosaccharides (MOS), essential oil blend (EOB), and onion peel (OPE) on fermentation characteristics using the rumen simulation technique (RUSITEC) system. Three rumen-cannulated, non-lactating Holstein Friesian cows were the inoculum donors. The substrate used for the study was a total mixed ration (TMR), which consisted of corn silage, alfalfa hay, and concentrate at 6:2:2, respectively. Sixteen fermentation vessels were randomly allotted to four treatments with four replicates each over a 9-day period in a completely randomized design. The treatments assessed include: control [TMR only], GEO [TMR + GOS + EOB + OPE], MEO [TMR + MOS + EOB + OPE], and OLEO [TMR + OLG + EOB + OPE]. OLG comprises GOS and MOS in equal proportion. EOB was included at 3 µL/g, while OPE, GOS, MOS, and OLG were added at 30 mg/g TMR. Results showed that pH, gas volume, effluent volume, and ammonia-N were not affected (p > 0.05) by the different additives. Similarly, greenhouse gas (GHG) emissions and nutrient digestibility were not affected by the treatments. Compared to the control, total volatile fatty acids (VFA) were decreased (p < 0.05) by 14.8, 10.8, and 8.5% with GEO, MEO, and OLEO inclusion, respectively, while the molar proportion of acetate was increased (p = 0.011) by 3.3, 1.1, and 3.8% with GEO, MEO, and OLEO inclusion, respectively. MEO increased isobutyrate (p = 0.001) and branched chain VFA (p = 0.013) contents; however, GEO and OLEO inclusion reduced them. Overall, the interaction of EOB, OPE, GOS, and/or MOS did not affect nutrient digestibility or GHG emissions but reduced VFA production. Further research is recommended to assess the dose effect of the additives on GHG emissions and VFA production; and to determine the long-term effects of these interventions on the rumen microbiome and animal performance.

Essential oils are natural feed additives that improve animal health and enhance their performance. This study investigated the effects of the rumen infusion of five essential oil blends (EOBs) on greenhouse gas (GHG) emissions, rumen fermentation parameters, and rumen metabolome and metabolic pathways in Black Angus cows. Using a 6 × 6 Latin Square experimental design, a 90-day study was conducted with six cattle. A daily dosage of 4 mL of EOBs was administered during each infusion. Volcano plot analyses between the control (CON) and each of the EOBs (EOB1, EOB2, EOB3, EOB4, and EOB5) revealed several differentially abundant (p ≤ 0.05; absolute fold change ≥1.5) metabolites. The EOB5 treatment exhibited the most significant impact, with 26 differentially abundant metabolites, including elevated valine and reduced gallic acid. Volatile fatty acids (VFAs), including valerate, isobutyrate, and isovalerate, were significantly increased (p < 0.05). GHG emissions were not significantly affected, but a numerical decrease was observed in the animals infused with the EOB5 treatment. Ammonia nitrogen concentrations remained within the suitable range for rumen microbes’ growth, indicating a normal internal environment for microbial crude protein synthesis. In conclusion, the study has demonstrated that the direct infusion of EOBs significantly improved the generation of VFAs and impacted the energy production, protein synthesis, and microbial activity of the animals.

Background and Objectives: Dairy products provide vital energy, high-quality protein, and micronutrients for over six billion people worldwide, with dairy cows contributing nearly 81% of global milk production. Sustainable strategies to enhance productivity are therefore critical. Feed additives such as essential oil blends (EOB), onion peel (OPE), and prebiotics including mannan oligosaccharides (MOS) and galacto-oligosaccharides (GOS) have been proposed to improve rumen fermentation, modulate microbial ecology, and mitigate greenhouse gas emissions. This study evaluated the combined effects of EOB, OPE, MOS, and GOS on rumen metabolism using the rumen simulation technique (RUSITEC). Materials and Methods: Rumen inoculum from three cannulated Holstein Friesian cows was incubated across 16 vessels (four treatments × four replicates) for nine days. Treatments included a control (CON; TMR only), GEO (TMR + GOS + EOB + OPE), MEO (TMR + MOS + EOB + OPE), and OLEO (TMR + a 1:1 mixture of GOS and MOS + EOB + OPE). Additives were included at 3 µL/g TMR for EOB and 30 mg/g TMR (3% w/w) for OPE, GOS, MOS, or OLG. Rumen effluents were collected for untargeted metabolomic profiling by liquid chromatography–mass spectrometry, identifying 661 metabolites. Results: Partial least squares-discriminant analysis revealed clear separation between CON and additive groups, confirming distinct metabolic shifts. GEO primarily enhanced tryptophan, tyrosine, and purine metabolism; MEO stimulated phosphonate and pyrimidine pathways and bile acid biosynthesis; OLEO promoted phosphonate, nicotinamide, and taurine metabolism. Microbial analysis showed enrichment of taxa such as Lachnospira, Succinivibrionaceae, Macellibacteroides, Lysinibacillus, and Christensenellaceae, indicating complementary effects on fermentation and microbial stability. Conclusions: These results demonstrate that dietary supplementation with GEO, MEO, or OLEO modulates rumen metabolism and microbial ecology without impairing fermentation, supporting improved nutrient utilization, antioxidant defenses, and metabolic resilience in dairy cows, with potential benefits for productivity and sustainability.