PSI-11 Effect of thymol on forage utilization, rumen fermentation, and rumen microorganisms in beef steers
Authors: Fukuda Emma , Lu Yuan , Jessup Russell , Drewery Merritt L 
Published: September 2024

Abstract
Essential oils (EOs) have been extensively researched for their ability to modulate rumen microbial populations and, subsequently, rumen fermentation and enteric methane production. However, biological changes from dietary EOs are inconsistent due to the varying presence and concentrations of terpenes, the main bioactive component of EOs. Further, mechanisms by which terpenes exert their actions are not well understood due to structural variety and complexity. Therefore, terpenes should be researched individually and at known dosages. The objective of this study was to determine the effects of thymol on forage utilization and rumen microbial populations in beef steers. Thymol is a terpene from thyme and oregano EOs which is thought to reduce methanogenesis in ruminants. As thymol has potent antimicrobial activity, we aimed to identify a dose that favorably modulates microbial populations without interfering with forage utilization. Accordingly, we utilized beef steers (n = 4) in a 4×4 Latin square design experiment with four 28-d periods. Four doses of thymol [0 (CON), 120, 240, and 480 mg thymol/kg intake] were administered on alfalfa cubes to a basal hay diet which was provided ad libitum. Intake and digestion were determined on d 9 to 12 and rumen contents were collected then separated into liquid and solid fractions on d 14 of each period. DNA was extracted from the liquid and solid rumen contents and analyzed with whole genome shotgun sequencing. Forage organic matter intake, total digestible organic matter intake, and organic matter digestibility were not affected by treatment (P ≥ 0.66). Ruminal ammonia, total volatile fatty acids, and rumen pH were also not affected by treatment (P ≥ 0.30). However, for 240 mg thymol/kg intake, molar proportions of acetate were significantly increased compared with 120 mg thymol/kg intake (P ≤ 0.02) and tended to be greater than for 480 mg thymol/kg intake (P = 0.08). Propionate was significantly decreased (P ≤ 0.02) for 240 mg thymol/kg intake compared with other treatments. Numerous lactic acid bacteria (LAB) including multiple Lactobacillus, Enterococcus, and Pediococcus species were increased for thymol treatments compared with CON. The ammonia producing bacteria Prevotella bryantii was increased at 240 and 480 mg thymol/kg intake compared with 120 mg thymol/kg intake (P ≤ 0.05). Methanogenic microbial species, uncultured Methanobrevibacter sp., and total archaeal abundances were increased in the solid microenvironment versus CON for 240 mg thymol/kg intake (P ≤ 0.04). Ultimately, our data indicate that thymol did not negatively affect forage utilization or fermentation products, although microbial populations were affected. While methanogenic archaea were not reduced with thymol supplementation, the increases in LAB may indicate an alternative mechanism for thymol to modulate microbial populations and methanogenesis. These data provide insight into the impact of specific doses of thymol on cattle and provide a foundation for future research that specifically studies the mechanisms by which terpenes and EOs exert biological actions.