This study was designed to evaluate effects of increasing doses of a blend of essential oils (EO; oregano, cinnamon, thyme, orange peel) on in vitro rumen fermentation in rumen liquor from dairy cows and fattening bulls, as influenced by acidity at the start of the fermentation. Two in vitro fermentation experiments, which differed only in the origin of rumen liquor (i.e., dairy cows [DC] or fattening bulls [FB]), were conducted in duplicate runs in 100 ml glass tubes containing 50 ml of diluted rumen fluid and 500 mg of a mixed ration substrate. The pH of the diluted rumen liquor at the beginning of each fermentation was adjusted to 7.0 with 5N NaOH or 5.5 with 3N HCl. The EO blend was added to each fermentation tube at doses of 8, 16, 24 and 32 μl/tube, equivalent to 160, 320, 480 and 640 μl/l, using 3 tubes for each dosage level plus 3 tubes without EO as controls (i.e., 30 tubes in total/run). After 24 h of incubation, fermentation liquor was collected and analyzed for volatile fatty acids (VFA), ammonia N and pH. In both experiments, pH and ammonia N concentration of the fermentation liquor were lower (P<0.01) at the initial pH of 5.5. The reduction of initial pH from 7.0 to 5.5 depressed VFA yield (138 mM versus 120 mM and 132 mM versus 111 mM, P<0.01, respectively, for DC and FB rumen liquors), and modified end products of fermentation by lowering the acetate:propionate ratio (4.3 versus 3.6 and 3.2 versus 2.5, P<0.01, respectively, for DC and FB rumen liquors). Butyrate concentration was higher at the lower pH in the DC rumen liquor fermentation (22.4 mM/100 mM versus 16.2 mM/100 mM, P<0.01), but not in the FB fermentation. In both experiments, addition of the EO blend did not depress total VFA concentrations, except at the highest dose in the experiment with rumen fluid from DC (122 mM versus 128 mM, P<0.01). In both experiments, there was an interaction (P<0.01) between EO addition and initial pH on acetate proportion; at the initial pH of 5.5, EO addition decreased acetate from 65.0 to 63.9 and from 62.6 to 61.8 mM/100 mM, respectively, for DC and FB rumen liquors, while there were no effects of EO at the initial pH of 7.0. In the experiment with DC rumen liquid, EO addition tended (P<0.08) to increase propionate proportion (from 16.1 to 16.6 mM/100 mM), while there was no effect in FB rumen liquid. In the FB experiment, EO decreased (P<0.05) the acetate:propionate ratio, while in the DC experiment it decreased only at low pH (interaction 'EO addition×initial pH': P<0.01). A moderate shift in end products of fermentation occurred with both types of rumen liquor, but mainly at low ruminal pH (i.e., 5.5), suggesting a selective toxicity of these EO against rumen bacterial strains that grow at low rumen pH.
