Newbold et al. (2006) and Calsamiglia et al. (2007) described essential oils (EO) as follows: volatile aromatic compounds with an oily appearance extracted from plant materials typically by steam distillation; alcohol, ester or aldehyde derivatives ofphenylproponoids and terpenoids; some of the more common EO compounds available include thymol (thyme and oregano), eugenol (clove), pinene (Juniper), limonene (dill), cinnamaldehyde (cinnamon), capsaicin (hot peppers), terpinene (tea tree), allicin (garlic), anethol (anise), etc.; antimicrobial activity; modify rumen microbial fermentation. With regard to EO as modifiers of rumen microbial fermentation, Calsamiglia et al. (2007) from an extensive review of the literature (primarily in vitro, in situ or continuous culture based) concluded the following: inhibition of deamination and methanogenesis, which results in lower ammonia-N, methane and acetate and higher propionate and butyrate concentrations; effects may vary depending on the specific EO or combination of EO supplemented; effects of some EO are pH and diet dependent. Readers are referred to Calsamiglia et al. (2007) for an in depth review of EO and effects on rumen microbial fermentation. The purpose of this paper is to review the availableresearch reports involving EO as dietary supplements for dairy cows.
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