Evaluation of Plant Essential Oils as Natural Alternatives to Monensin in In Vitro Ruminal Fermentation

Extensive growth promoter use in livestock production has raised concerns about their role in selective pressure on resistant microorganisms, driving interest in natural alternatives such as essential oils (EOs). This study aimed to evaluate the effects of tea tree, holy wood, and citronella EOs on in vitro ruminal fermentation. The study follows a completely randomized design with the following five treatments: control, monensin (5 μM), tea tree EO (50 mg/L), holy wood EO (50 mg/L), and citronella EO (50 mg/L), each conducted in triplicate. Incubations were performed at 39 °C for 48 h in the rumen fluid collected from fistulated cattle fed a 20:80 forage-to-concentrate diet. Notably, EOs exhibited no significant effects on pH, microbial protein production, total volatile fatty acids, or in vitro dry matter digestibility (<i>p</i> > 0.05). Tea tree and holy wood EOs enhanced deamination activity, and all treatments increased ammonia concentration compared with that in the control. Monensin treatment increased acetate concentration and reduced in vitro neutral detergent fiber digestibility; holy wood EO exhibited a similar trend. Altogether, the findings of this study suggest that EOs can selectively modulate the ruminal microbiota, influencing nitrogen metabolism and fermentation patterns without impairing rumen stability.