Effect of different levels of citrus essential oil and its active component on rumen microbial fermentation and methane emission in vitro

The fermentation characteristics of adding different levels (0, 25, 50 and 75µ l/75ml buffered rumen fluid) of citrus essential oil or its bioactive component (limonene, 0, 30, 45 and 60 µl/75ml buffered rumen fluid) to a basal substrate (50% roughage: 50% concentrate) were evaluated in vitro by semi automatic gas production (GP) technique. The investigated essential oil was Citrus reticulata (CR25, CR50, CR75), and Limonene (L30, L45, L60). The analyses of the citrus essential oils by GC/MS showed that the main components were dl-Limonene (83.9%) and y- Terpinene (10.75%). There were significant differences (P 0.05) in cumulative GP after subtracting the blank gas volume for different levels of citrus essential oil or limonene. All levels of citrus essential oil and limonene significantly (P0.05) decreased the GP compared with the substrate with no additive. The second and third dose of citrus essential oil or limonene decreased (P0.05) methane emission when expressed on dry matter basis, but when expressed on the basis of digested organic matter the third dose from citrus essential oil decreased (P0.05) methane emission only in vitro. The inhibition of methane production was accompanied with a significant reduction in protozoal count. Partitioning factor (PF) was used as an index of the efficiency of microbial protein synthesis in vitro. There was no significant effect of citrus essential oils on PF, while limonene supplementation decreased (P0.05) the PF values. The inclusion of citrus essential oil or limonene affected negatively the true digestibility of dry and organic matter. The NH3-N concentration dramatically declined with the inclusion of high level of citrus or limonene. This study suggested that the citrus essential oil has the potential to affect ruminal fermentation efficiency, and could be a promising methane mitigating agent due to its rich content of limonene.