The use of antibiotics in animal feeds was banned in the EU in 2006. As a result, there has been substantial interest in evaluating the potential of natural additives and supplements such as prebiotics, probiotics and plant extracts, including tannins, saponins and essential oils (EOs). Research has involved microbiologists and nutritionists exploring these additives to help modify rumen metabolism, improve feed efficiency and increase animal productivity (Benchaar et al., 2008; Busquet et al., 2006). In addition, more recently there has been increased interest in the ability of these additives to reduce greenhouse gas (GHG) emissions and, in particular, methanogenesis.