Effects of essential oils on proteolytic, deaminative and methanogenic activities of mixed ruminal bacteria

The objective of this study was to evaluate in vitro the effects of three essential oils (EO) [cinnamon leaf (250 mg L^-1), garlic oil (100 and 250 mg L^-1), and juniper berry oil (20 mg L^-1)] and two EO compounds (EOC) [anethol (20 mg L^-1) and p-cymene (20 mg L ^-1)] on proteolytic, deaminative and methanogenic activities of mixed ruminal bacteria. Concentrations of total VFA were similar (P > 0.05) among treatments. With the exception of cinnamon and garlic oils, which reduced (P < 0.05) the proportion of propionate, the other EO and EOC had no effect on the proportions of individual VFA, compared with the control. Proteolytic activity of ruminal bacteria was unaffected (P > 0.05) by treatments; however, bacterial deaminative activity and NH₃ concentration were increased (P < 0.05) by the addition of EO (except for cinnamon leaf oil and garlic oil at 250 mg L^-1) and EOC. Except for anethol, methanogenic activity of ruminal bacteria was reduced (P < 0.05) by EO and EOC, which was reflected by a marked decrease in methane concentration. This study shows that at the concentrations evaluated, anethol, garlic oil (100 mg L^-1), juniper berry oil, and p-cymene may not be beneficial to improve efficiency of N utilization in ruminants because they enhance deaminative activity, while cinnamon and garlic oil (250 mg L^-1) could be good alternatives to antibiotics because they reduce methanogenic activity of ruminal bacteria.