Nine Holstein dairy cows (630±50 kg) were used in a completely randomized design with repeated meas- ures (two 21 d periods) to investigate the effects of disodium fumarate (DSF, 160 g/d), thyme essential oil (TEO, 9 mL/d) and simultaneous use of TEO and DSF (SIMTF, 160 g/d DSF and 9 mL/d TEO) on per- formance, apparent digestibility, blood metabolites, rumen microbial communities and fermentation charac- teristics. Dry matter intake and apparent digestibility (except for TEO), milk production and composition were not affected by the treatments. Relative to the control (CON, no additive), organic matter apparent digestibility tended to decrease by TEO supplementation (578 vs. 599 g/kg). Serum urea nitrogen concen- tration was lower in TEO than CON (9.8 vs. 10.9 mg/dL). Also, DSF supplementation significantly in- creased the molar proportion of propionate and the glucogenic: non-glucogenic ratio of volatile fatty acids (VFA) and decreased the molar proportion of butyrate. Supplementation of DSF and SIMTF resulted in a significantly decrease in the acetate: propionate ratio. Relative to the CON, DSF and SIMTF supplementa- tion significantly increased serum glucose concentration. TEO supplementation decreased rumen fluid am- monia nitrogen and increased large peptides and small peptides plus amino acid nitrogen concentration. Rumen total and cellulolytic bacteria and protozoa abundance were not affected by treatments (except for total bacteria that decreased by TEO). Results of the present study demonstrated that the simultaneous use of DSF and TEO relative to DSF alone could not have synergistic effects on performance, ruminal fermen- tation and serum metabolites of dairy cow. DSF can be used as a ruminal glucogenic precursor and resulted in an increase in the serum concentration of glucose in dairy cows.
