Critical evaluation of essential oils as rumen modifiers in ruminant nutrition: A review

Ruminant livestock systems contribute significantly to emission of methane, a potent greenhouse gas as they waste a portion of the ingested energy (2-15%) as methane and a large proportion (75-95%) of the ingested nitrogen as ammonia. Recently, numerous researches have been conducted to evaluate plant secondary metabolites, including essential oils (EO), as natural feed additives in ruminant nutrition and to exploit their potential to improve rumen fermentation efficiency. Essential oils appeared to be very promising compounds as they selectively reduced methane production and protein breakdown in both in vitro and in vivo studies. However, in some studies, the use of EO as feed additives was accompanied with decreased feed degradability and lowered volatile fatty acid. These adverse effects could be attributed to their broad and often non-specific antimicrobial activities within the rumen. Future research should be directed to identification of the active and useful EO compounds, optimization of EO doses, and use of a whole-farm approach with a focus on animal welfare, performance and economic benefits.