Effect of plant extracts on methanogenesis and microbial profile of the rumen of buffalo: a brief overview

Plants rich in secondary metabolites (saponins, tannins, essential oils, etc.) have antimicrobial activity which can be exploited for selective inhibition of a particular group of microbes in the rumen. We have screened a large number of plant extracts for their potential to inhibit methanogenesis and ciliate protozoa in an in vitro gas production test using buffalo rumen liquor as the inoculum. Out of 93 plant extracts tested, 11 inhibited in vitro methanogenesis to the extent of 25-50% and nine plant extracts inhibited methanogenesis more than 50%. Among 20 extracts exhibiting antimethanogenic activity, nine were ethanol extracts, 10 were methanol extracts and only one was a water extract. Some of these plant extracts inhibited ciliate protozoa as tested by microscopic examination and C-14-labelled radioisotopic technique, but the protozoa inhibition was not correlated with methane inhibition, indicating that the methanogens sensitive to plant secondary metabolites may or may not be having any symbiotic relationship with ciliate protozoa. Methane inhibition was accompanied by a drastic fall in the number of methanogens as determined by real time PCR. Plants that appeared to have some potential as feed additives to control methanogenesis by the ruminants are: (i) seed pulp of Sapindus mukorossi (rich in saponins) and Terminalia chebula (rich in tannins); (ii) leaves of Populus deltoides, Mangifera indica and Psidium guajava (rich in tannins and essential oils); and (iii) flower buds of Syzygium aromaticum and bulb of Allium sativum (rich in essential oils). Some of the plants reported in literature exhibiting antimethanogenic activity include Equisetum arvense, Lotus corniculatus, Rheum palmatum, Salvia officinalis, Sapindus saponaria, Uncaria gambir and Yucca schidigera.