Effects of essential oils and their components on in vitro rumen microbial fermentation

The objective of this study was to investigate the effects of essential oils (EO) and essential oil compounds (EOC) on in vitro rumen microbial fermentation. Treatments were: control (no additive), cinnamon leaf oil (400 mg L-1), clove leaf oil (200 mg L-1), sweet orange oil (200 mg L-1), oregano oil (200 mg L-1), thyme oil (200 mg L-1), carvacrol (400 mg L-1), cinnamaldehyde (400 mg L-1), eugenol (800 mg L-1), and thymol (400 mg L-1). Treatments were evaluated using in vitro 24-h batch culture of rumen fluid with a 51:49 forage:concentrate dairy ration [16.7% crude protein (Cl?), 34.4% neutral detergent fibre (NDF)]. Incubations were conducted in triplicate with gas production (GP) measured at 0, 2, 6, 8, 12, and 24 h, while pH, ammonia (NH3), volatile fatty acid (VFA), in vitro dry matter (IVDMD) and neutral detergent fibre (IVNDFD) digestibilities were determined after 24 h of incubation. Among the EO and EOC evaluated, only the phenolic compounds, carvacrol, thymol, and eugenol affected ruminal fermentation, relative to the control. Carvacrol (400 mg L-1) and eugenol (800 mg L-1) increased pH and molar proportion of butyrate, and decreased molar proportion of propionate, IVDMD, IVNDFD, and GP. At the concentration of 200 mg L-1, thymol increased final pH, and reduced molar proportion of propionate, IVNDFD, and GP. None of the EO or EOC affected NH3 concentration, suggesting that at the doses tested in this study, protein degradation was not affected by EO and EOC. This study showed that among the EO and EOC investigated only phenolics exhibited antimicrobial activity as exemplified by reduced diet fermentability and a shift in VFA profile from less propionate towards more butyrate. These changes in fermentation may not be nutritionally beneficial to dairy cattle. The present study suggests that the types and concentrations of EO and EOC employed to alter ruminal fermentation must be fully defined before their widespread use in ruminant nutrition can be recommended.