Plant extracts to manipulate rumen fermentation

The removal of antibiotic growth-promoters has led to an increased interest in alternative means of manipulating rumen fermentation. In this review we consider one of the possible alternatives: natural plant products including essential oils, saponins and related compounds. Essential oils have been used by man for many years. Their main effects in the rumen involve reduction of protein and starch degradation and an inhibition of amino acid degradation, due to selective action on certain rumen microorganisms, specifically some bacteria. One mode of action suggested for essential oils is an effect on the pattern of bacteria] colonisation of, in particular starch rich, substrates as they enter the rumen. A second possible mode of action is their inhibition of "hyper ammonia producing bacteria" involved in amino acid deamination. However, the effect of essential oils depends on the chemical make up of the essential oils used, which is not always sufficiently described in the literature. Saponins are secondary compounds produced mainly by plants. A wide range of biological effects of saponins have been described, although the majority may be ascribed to their action on membranes. Here the effects of saponins on protozoa and fermentation end products are reviewed. There seems to be inconsistency in the effects of saponins in the rumen environment. It is suggested that this is due in part to the development of other microbial populations capable of degrading saponins in the rumen. This transient effect of saponins might limit their use in practical conditions. Organosulphurous supplements prepared from garlic have experienced increasing popularity in the last decade. Because the structure of the organosulphurous constituents in whole garlic is complicated, due to their volatility and rapid breakdown, their final concentration in different garlic preparations varies significantly. Effects have been reported on volatile fatty acid production, increased total volatile fatty acid production, with a stimulation in propionate, and decreased methane production, with a subsequent decrease in rumen methanaogens. Plant extracts thus have the potential to be exploited as rumen manipulating agents. The use of molecular techniques is helping to better describe the changes in the rumen environment to fully understand modes of action and fully exploit the use of plant extracts. (C) 2007 Elsevier B.V. All rights reserved.