Animal Feed Science and Technology 132 (2007) 186–201 Effects of dose and adaptation time of a specific blend of essential oil compounds on rumen fermentation L. Castillejos a , S. Calsamiglia a,∗ , A. Ferret a , R. Losa b a Animal Nutrition, Management, and Welfare Research Group, Departament de Ci` encia Animal i dels Aliments, Universitat Aut` onoma de Barcelona, 08193 Bellaterra, Spain b AKZONOBEL/CRINA SA, 15 Chemin de la Combe, 1196 Gland, Switzerland Received 9 October 2005; received in revised form 21 March 2006; accepted 30 March 2006 Abstract Essential oils can be used as natural additives in animal feeds. The present study evaluated the effects of three different doses and different adaptation times of a specific blend of essential oils (BEO) on rumen microbial fermentation. Eight dual flow continuous culture fermenters (1320 ml) were used in two periods of 9 days each to study the effects of increasing doses of BEO. Treatments were: CTR (no BEO), D5 (5 mg/l of BEO), D50 (50 mg/l of BEO) and D500 (500 mg/l of BEO). During the last 3 days, samples were taken at 0, 2, 4 and 6 h after the morning feeding and analyzed for large peptide (LPep), small peptides plus amino acid (SPep + AA) and ammonia N concentrations, and at 2 h after feeding for volatile fatty acids (VFA) concentration and profile. The D5 increased total VFA concentration, acetate proportion and acetate to propionate ratio, and decreased propionate and valerate proportion, compared with CTR. The concentration of LPep N tended (P=0.08) to be lower Abbreviations: ADBEO, rumen fluid from adapted sheep; ADBEO + BEO, rumen fluid from adapted sheep plus a single dose of BEO; ADFom, acid detergent fibre; BCVFA, branch-chained volatile fatty acid; BEO, blend of essential oil compounds; CP, crude protein; CTR, control treatment and rumen fluid from control sheep; CTR + BEO, rumen fluid from control sheep plus a single dose of BEO; DM, dry matter; EMPS, efficiency of microbial protein synthesis; aNDFom, neutral detergent fibre; LPep, large peptide; SPep + AA, small peptide plus amino acid; OM, organic matter; VFA, volatile fatty acid ∗ Corresponding author. Tel.: +34 935811495; fax: +34 935811494. E-mail address: Sergio.Calsamiglia@uab.es (S. Calsamiglia). 0377-8401/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.anifeedsci.2006.03.023