Inhibition of ruminal methanogenesis by tropical plants containing secondary compounds

Abstract

The secondary metabolites present in plants provide protection against predators, pathogens and invaders because of their anti-microbial activity. The majority of these compounds fall into the category of lignins, tannins, saponins, volatile essential oils, alkaloids, etc. The anti-microbial activity of these compounds is highly specific and therefore may be used for the manipulation of rumen fermentation by selective inhibition of a microbial group of the ecosystem. Different parts of plants like seed pulp, leaves and spices extracted in water, methanol and ethanol have been evaluated in in vitro gas production test for their anti-methanogenic and anti-protozoal activities. The ethanol extract of soapnut (Sapindus mukorossi) seed pulp completely inhibited in vitro methane production along with a significant reduction in protozoa count and acetate/propionate ratio. The methanol extract of seed pulp of harad (Terminalia chebula), leaves of poplar (Populus tremuloides), flower buds of cloves (Syzygium aromaticum), ethanol extract of guava (Psidium guayaba) leaves and both the ethanol and methanol extracts of garlic (Allium sativum) strongly inhibited in vitro methanogenesis. The effect on ciliate protozoa was variable with these plant extracts and there was no correlation between methane and protozoa inhibition. The presented in vitro results indicate that plant secondary compounds seem to have a potential to be used as feed additives for rumen manipulation to reduce methane emission.

Introduction

In the rumen microbial fermentation under anaerobic conditions, the monosaccharides released by hydrolysis of polysaccharides are further metabolized and reduced cofactors like NADH and NADPH are produced. For re-use in the anaerobic system, these reduced cofactors have to be oxidized to NAD⁺ and NADP⁺ by electron transfer to terminal acceptors other than oxygen like CO₂, SO₄, NO₃, fumarate, etc. Due to the high availability of CO₂ in the rumen, methane generation is preferred over the alternate hydrogen sinks and therefore a significant portion of energy is wasted [1]. As reported earlier [2], the Global Warming Potential (GWP) of methane is 23 times higher than that of carbon dioxide; it is essential that its level in the atmosphere is kept within safe limits. In addition to that, reduced methanogenesis in the rumen may result in a better feed conversion efficiency, making livestock production more economic.

In the past several decades efforts have been made to reduce methane emission by the animals keeping in view the above mentioned two objectives. The methane inhibitors like halogenated methane analogues, 2-bromoethanesulphonic acid, fatty acids, probiotics, monensin, hydroxymethylglutaryl-S-coA reductase inhibitors, sulphates and defaunation of animals have been tested [3], [4], [5], [6], [7], [8]. They inhibit methane generation in in vitro and in vivo, with varying degrees of success but some of these feed additives may also inhibit other biochemical reactions in the rumen or may have some adverse effect on the animal's physiology. Therefore, one needs to search for alternate feed additives which may selectively inhibit methanogens or ciliate protozoa in the rumen, as methanogens are closely associated with ciliate protozoa and reduction in protozoa population indirectly may affect methanogens.