M. Taghavi Nezhad - Department of Animal Science, Faculty of Agriculture, Bu-Ali Sina University, Hamadan, Islamic Republic of Iran.
D. Alipour - Department of Animal Science, Faculty of Agriculture, Bu-Ali Sina University, Hamadan, Islamic Republic of Iran.
M. Torabi Goudarzi - Jundishapour Research Center of Medicinal Plants, Kashan, Islamic Republic of Iran.
P. Zamani - Department of Animal Science, Faculty of Agriculture, Bu-Ali Sina University, Hamadan, Islamic Republic of Iran.
G. Khodakaramian - Department of Plant protection, Faculty of Agriculture, Bu-Ali Sina University, Hamadan, Islamic Republic of Iran.

The aim of this study was to assess the effect of several doses of spearmint essential oil (SEO; ۰, ۲۵۰, ۵۰۰, ۷۵۰ or ۱,۰۰۰ µg ml-۱ buffered rumen fluid) on the fermentation kinetic and digestibility using in vitro gas production technique. A total mixed ration (۳۰% roughage: ۷۰% concentrate) was incubated with buffered rumen fluid. In vitro gas production, asymptotic gas production (A), rate of gas production (µ), partitioning factor (PF), microbial biomass (MB), ammonia concentration and digestibility were determined. Increasing the dose of SEO decreased the parameters A and µ. Adding SEO, however, increased PF, ammonia concentration, apparent in vitro dry matter digestibility and true in vitro organic matter digestibility at the lower levels of SEO (۲۵۰ and ۵۰۰ µg ml-۱). But, at the level of ۱,۰۰۰ µg ml-۱, a decrease was observed for these parameters. The increment in PF and digestibility illustrate that SEO has a potential to modulate the rumen fermentation, which may be beneficial (at low doses) for improving nutrient utilization.

Essential oil, Carvone, in vitro gas production, Partitioning factor, Spearmint