Manipulation of rumen fermentation and methane production with plant secondary metabolites

Methane produced by ruminants represents a loss of energy for ruminants and contributes to the greenhouse gas emissions which have a global warming potential. This review describes the basic conceptual aspects of ruminal methanogenesis, which is a way of keeping a low H₂ pressure in the rumen by reducing CO₂, and steps where it may be possible to intervene to reduce CH₄ production. Numerous studies have been completed on use of plant secondary metabolites (PSM) in substitute for chemical feed additives because some of them modify rumen fermentation and reduce CH₄ production. This review aims to illustrate the multiplicity of possible effects of the major groups of PSM (i.e., saponins, tannins, essential oils) on ruminal microbiota (i.e., protozoa, fungi, bacteria), ammonia N concentration and fermentation end products (i.e., volatile fatty acids). Finally, a comprehensive review on effects of PSM on CH₄ production is included, with a discussion of possible underlying mechanisms and key factors affecting the observed effects of PSM.