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Methane is the single largest source of anthropogenic greenhouse gases produced in ruminants. As global warming is a main concern, the interest in mitigation strategies for ruminant derived methane has strongly increased over the last years. Methane is a natural by-product of anaerobic microbial (bacteria, archaea, protozoa, and fungi) fermentation of carbohydrates and, to a lesser extent, amino acids in the rumen. This gaseous compound is the most prominent hydrogen sink product synthesized in the rumen. It is formed by the archaea, the so-called methanogens, which utilize excessive ruminal hydrogen. For the past several decades, nutritionists and microbiologists have been trying to manipulate the rumen microbial ecosystem to improve the energy use efficiency of the feed by using several chemical and synthetic feed additives. None of these has been proved to be useful consistently and these additives have been criticised by researchers and consumers for microbial resistance and feed safety. In this context, the phytogenic feed additives seem to be the most developed means in mitigating methane from enteric fermentation in ruminants.
