Effect of Plant Secondary Metabolites on Rumen Methanogens and Methane Emissions by Ruminants

Abstract

Methanogenesis occurs in the rumen to take care of reducing power generated during fermentation of feed and accounts for a significant loss of energy offered to the ruminants as feed. Once carbon dioxide is reduced to methane, it cannot be oxidized to release energy under the anaerobic conditions prevailing in the rumen. To save this energy loss, several chemicals have been tested and some of them are very effective in selectively inhibiting methanogenesis, but these chemicals cannot be used in practical feeding of livestock due to their adverse effects on other rumen microbes, health of the animals and the quality of livestock products. Therefore, plants containing secondary metabolites might be superior feed additives to control methanogenesis without affecting other microbes of the rumen. In vitro screening experiments conducted in many laboratories have indicated that methanogenesis can be inhibited by inclusion of plants/plant extracts in the substrate. Some of the plants which showed in vitro methane inhibition are : Allium sativum, Azadirachta indica, Emblica officinalis, Eugenia jambolana, Ficus benghalensis, Foeniculum vulgare, Lotus pedunculatus, Mangifera indica, Ocimum sanctum, Populus deltoides, Psidium guajava, Quercus incana, Sapindus mukorossi, Sapindus rarak, Sesbania sesban, Syzygium aromaticum, Trachyspermum ammi, Terminalia chebula and Yucca schidigera, but some of them do have adverse effects on rumen fermentation and feed digestibility. Several of the above plants have been tested in vivo as feed additives in different ruminants either alone or in a combination and have shown significant decrease in in vivo methane emission and no adverse effect on feed utilization when used at the rate of 1–2% of dry matter intake. There is a need to screen larger number of plants containing secondary metabolites and to study the effect of feeding these compounds on the feed utilization and the quality of livestock products.