Thyme and cinnamon essential oils: Potential alternatives for monensin as a rumen modifier in beef production systems

•We test the potential of essential oils as alternatives for monensin in beef cattle.•The acetate to propionate ratio decreased with monensin, thyme and cinnamon oil.•Monensin and essential oils reduced ruminal population of protozoa and methanogens.•Ruminal populations of F. succinogenes and R. albus were affected by additives.

Four ruminally cannulated Holstein steers (540 ± 35 kg initial body weight) were used in a 4 × 4 Latin square design over 21-day periods to examine effects of supplementing a basal diet (CON) with thyme oil (THY, 500 mg/kg DM) or cinnamon oil (CIN, 500 mg/kg DM) on nutrient digestibility, ruminal fermentation characteristics and rumen microbial populations. Monensin (MON, 33 mg/kg DM) was used as positive control. Steers were fed a basal diet as total mixed ration (TMR) ad libitum. The results indicated that dry matter intake (DMI) and apparent digestibility of nutrients were not affected by additives. Dietary supplementations did not affect ruminal pH and ammonia nitrogen concentration. Total volatile fatty acid (VFA) concentrations and proportions of acetate and butyrate were not affected by essential oils (EO), but there was a trend (P<0.10) for a reduced concentration of total VFA and proportion of butyrate with MON supplementation compared to CON. Propionate proportion increased (P<0.05) with THY and MON supplementation and the ratio of acetate to propionate decreased (P<0.05) with EO and MON. The relative abundances of protozoa and methanogens decreased (P<0.01) in the rumen of steers when supplemented with additives. Ruminal population of Ruminococcus flavefaciens was not affected by treatments, but populations of Fibrobacter succinogenes and Ruminococcus albus decreased (P<0.05) by supplementation of EO or MON, respectively. Results from this study suggest that THY or CIN can be considered as potential alternatives to MON and might be useful as rumen fermentation modifiers in beef production systems.