In vitro evaluation of effects of ten essential oils at three doses on ruminal fermentation of high concentrate feedlot-type diets

Use of antibiotics as growth promoters in animal feeds has been banned in the European Union due to increasing concerns about the appearance of residues in meat and milk and antibiotic resistant strains of bacteria. Some essential oils (EO) modify rumen microbial activity and appear to be a ‘natural’ alternative to modify rumen microbial fermentation. Effects of 10 EO (i.e., clove leave, hyssop, lavandin, lavender, thyme, oregano, rosemary, sage, savory, tea tree) were evaluated in in vitro 24 h batch culture of diluted rumen fluid [Tilley, J.M.A., Terry, R.A., 1963. A two stage technique for the in vitro digestion of forage crops. J. Brit. Grassland Soc. 18, 104–111] at pH 6.50. A 10:90 forage:concentrate diet (161 g/kg CP; 320 g/kg NDF; 380 g/kg starch on a dry matter basis) typically fed to beef cattle in a barley grain based beef system was used as substrate. Treatments were: negative control (CTR), positive control (10 mg/l of monensin), and three doses of each EO (5, 50, 500 mg/l). After 24 h, pH was determined in the culture fluid and samples were analyzed for ammonia N and volatile fatty acid (VFA) concentrations. Monensin increased VFA concentration, and propionate and valerate proportions, and decreased acetate and butyrate proportions, the acetate to propionate ratio and ammonia N concentration. Lavender oil did not modify rumen microbial fermentation and lavandin and oregano (500 mg/l) inhibited rumen microbial fermentation thereby decreasing VFA concentration, which suggests that these EO may not be beneficial in beef cattle nutrition. However, the lower doses of oregano oil increased VFA concentration by 39–56%. Thyme and savory oils increased VFA concentration, and decreased ammonia N concentration and final pH. The 500 mg/l doses of rosemary, hyssop, sage, tea tree and clove leaf oils acted similar to monensin by increasing propionate and valerate proportion, and reducing acetate and butyrate proportions and the acetate to propionate ratio without reducing VFA concentration. Clove leaf oil at 500 mg/l increased total VFA and was the only EO that increased final pH. Most of these EO modified rumen microbial fermentation and may allow manipulation of rumen fermentation to improve animal performance.