The aim of the present study was to investigate the effects of di-sodium fumarate (DSF) and thyme essential oil (TEO) solely and simultaneously on ruminal fermentation properties and microbial abundance. A dual-flow continuous culture system (DFCC) with eight 1400-mL fermenters was used in a period of 12 d that divided to 9 d for adaptation and 3 d for sampling. Fermenters were fed 100 g d-1 [dry matter (DM) basis] of a 50:50 alfalfa hay:concentrate ration. Treatments were no additive (control), 10 mM of DSF, 125 μL/L of TEO and simultaneous use of 10 mM of DSF, and 125 μL/L of TEO (SIMTF). Treatments had no effect on organic matter (OM) and neutral detergent fiber (NDF) disappearance, total and cellulolytic bacteria and protozoa abundance, large peptides and N-NH3 concentration of the effluent, N-NH3 and dietary N flow and molar proportions of acetate (C2), butyrate and isovalerate. DSF significantly increased crude protein (CP) degradation, the molar proportion of propionate (C3) and reduced C2:C3 ratio (P<0.05). TEO decreased (P<0.05) DM disappearance (-14.4%) and total volatile fatty acid (-19.4%) concentration, relative to the DSF. Relative to the control, small peptide plus amino acid N concentration was higher (P<0.05) in TEO treatment and DSF. SIMTF increased (P<0.05) the acid detergent fiber (ADF) disappearance and decreased the N-NH3 concentration from zero to 4 h after feeding. Total N, non-ammonia N and bacterial N flow and efficiency of microbial CP synthesis increased with SIMTF. Findings demonstrated that feed efficiency and ruminal fermentation were improved by the simultaneous use of DSF and TEO.
