RESULTS
Total gas and methane production, degradability, pH, and ammonia concentrations.Total gas and methane production by the ruminal cultures decreased linearly with increasing doses of all EOs (Table 1). The most pronounced inhibition on methane production was noted for ORO, with the inhibition magnitude decreasing in the following order at the high dose (1.0 g/liter): ORO > GAO > CLO > PEO > EUO. Methane production in terms of ml/g digestible DM was also decreased by all EOs, but the dose-responses differed, with CLO, EUO, and PEO exhibiting a linear effect and GAO and ORO exhibiting linear and quadratic effects (Fig. 1). Similarly, apparent substrate DM degradability was decreased linearly with increasing EO doses (Table 1). However, true DM and NDF degradabilities were not (P > 0.10) affected by EOs, except for linear decreases in NDF degradability by ORO. Inclusion of CLO or ORO in the cultures decreased ammonia concentrations, whereas other EOs did not affect ammonia concentrations in the cultures (Table 1). The pH of the cultures was, in general, increased linearly when each of the EOs was added (Table 1).