Role of secondary plant metabolites on enteric methane mitigation in ruminants

The rumen microbiome plays a fundamental role in all ruminant species, it is involved
in health, nutrient utilization, detoxification, and methane emissions. Methane is a
greenhouse gas which is eructated in large volumes by ruminants grazing extensive
grasslands in the tropical regions of the world. Enteric methane is the largest contributor
to the emissions of greenhouse gases originating from animal agriculture. A large
variety of plants containing secondary metabolites [essential oils (terpenoids), tannins,
saponins, and flavonoids] have been evaluated as cattle feedstuffs and changes in
volatile fatty acid proportions and methane synthesis in the rumen have been assessed.
Alterations to the rumen microbiome may lead to changes in diversity, composition,
and structure of the methanogen community. Legumes containing condensed tannins
such as Leucaena leucocephala have shown a good methane mitigating effect when
fed at levels of up to 30–35% of ration dry matter in cattle as a result of the effect
of condensed tannins on rumen bacteria and methanogens. It has been shown that
saponins disrupt themembrane of rumen protozoa, thus decreasing the numbers of both
protozoa and methanogenic archaea. Trials carried out with cattle housed in respiration
chambers have demonstrated the enteric methane mitigation effect in cattle and sheep
of tropical legumes such as Enterolobium cyclocarpum and Samanea saman which
contain saponins. Essential oils are volatile constituents of terpenoid or non-terpenoid
origin which impair energy metabolism of archaea and have shown reductions of up to
26% in enteric methane emissions in ruminants. There is emerging evidence showing
the potential of flavonoids as methane mitigating compounds, but more work is required
in vivo to confirm preliminary findings. From the information hereby presented, it is clear
that plant secondary metabolites can be a rational approach to modulate the rumen
microbiome and modify its function, some species of rumen microbes improve protein
and fiber degradation and reduce feed energy loss as methane in ruminants fed tropical
plant species.