Effects of ELM supplementation on rumen fermentation efficiency and BUN are presented in Table 3. Ruminal temperature, pH and BUN were similar among treatments and the values were stable at 38.5°C to 38.6°C, 6.4–6.6, and 9.3–9.4 mg/dL, respectively. However, the concentration of NH3-N was found the lowest in the treatment with 120 g ELM/h/d supplementation and the highest was in the control group. Castillejos et al. (2007) studied long term effect of EO on rumen fermentation in vivo also found that supplementation EO resulted in lower ruminal ammonia-N when compared to that in control. Several studies (Newbold et al., 2004; Patra and Saxena, 2009) demonstrated that EO might inhibit the hyper-ammonia producing bacteria in the rumen, which results in decreased amino acid deamination, consequently, lowering rumen NH3-N.