Evaluation of Essential Oils as Additives during Fermentation of Feed Products: A Meta-Analysis

Essential oils (EOs) are secondary metabolites of plants with antimicrobial functions that can be used as silage additives, but their effectiveness has been inconsistent. Therefore, this study aimed to evaluate the effects of EOs as silage additives on nutritional quality, fermentative products, microbial population, and in vitro rumen fermentation. A total of 17 articles consisting of 113 studies (comparisons) were selected for meta-analysis. The results showed that EO supplementation increased several parameters of nutrient contents such as dry matter (DM), crude protein (CP), and ether extract (EE) (p < 0.05), but decreased crude fiber content and mold population (p < 0.05). EO supplementation also affected rumen fermentability, namely reduced gas production, organic matter digestibility, and some fermentation products such as total VFA, iso-butyrate, iso-valerate, acetic, and succinic acid (p < 0.05) but did not affect methane production. Sub-group analysis based on the source of EOs revealed that only cumin and oregano reduced mold population during ensiling (p < 0.05). These results showed that EO supplementation improved several nutrient qualities such as DM, CP, and EE, inhibiting mold growth and stabilizing rumen pH.