The effects of different doses of thyme essential oil (TEO) [0.0 (as control), 100, 200, 300 and 400, µl/L of incubation medium] and di-sodium fumarate (DSF) [ 8, 10 and 12, mM/L] on in vitro total VFA concentration, total gas and methane production were evaluated after 24 h incubation. Rumen contents was collected from 2 adult ruminally fistulated, before the morning feeding and immediately strained through four layers of cheesecloth. Approximately, 500 mg of experimental diet (50:50, alfalfa hay: concentrate) were placed in a 125 ml bottles (N=9 per each treatment) containing 50 ml of buffered rumen fluid (buffer to rumen fluid ratio was 2:1), and incubated at 38.6º C. After 24 h of incubation, the bottles were transferred to an ice bath to stop fermentation. Then the bottles contents were filtered (48µm, pore size)and residues were oven dried (55ºC for 48 hours) and used to estimate the in vitro dry matter (IDMD), crude protein (ICPD) and organic matter (IOMD) disappearance and the nutrients use efficiency for microbial growth (PF). Data were analyzed in completely random design using GLM procedure of SAS (V8) and Tukey test was used to compare the means (P<0.05). Relative to the control, DSF and TEO had no effect on the ICPD and IDMD (except at 400 µl TEO/L). The addition of DSF at 8, 10 and 12 Mm/L resulted to increase (P<0.001) in IOMD (44.3, 42 or 45.2 %) and PF (//, // and ///, respectively) relative to those of the control. Relative to the control, the addition of 100 µl of TEO/L increased (P<0.05) the IOMD (47.6 %) and PF (///). Findings of the present study demonstrated that DSF can result to increase in feed use efficiency and improve in the ruminal fermentation at condition of the present study.
