Rumen manipulation for the improved utilization of tropical forages

This article reviews present or potential practices of rumen manipulation to increase the nutritional value of ruminant feeds, particularly relevant in tropical animal production. Modification of rumen microbial composition and microbial activities can be achieved by means of additives that selectively affect rumen symbionts, by introducing foreign microbes into the rumen, or potentially, by genetically manipulating rumen microorganisms. Rumen defaunation has been shown to increase the protein outflow to the intestine and a number of tropical plants have been shown to have strong defaunating activity. They could be useful both as protein supplements and as natural defaunating agents. Microorganisms from systems other than the rumen, such as digestive bacteria from wild herbivores and nitrogen-fixing bacteria from legume roots or fungi, may well represent a source of enzymes to increase degradation of recalcitrant components or detoxification of the diet. Molecular techniques have great potential for achieving the goals of rumen manipulation. They could allow the introduction of new or enhanced, fermentation activities. Considerable research is being done to transfer and express foreign genes in rumen bacteria. Suitable markers other than antibiotic resistance for tracking the genetically manipulated bacteria in the rumen and cloning vectors from rumen bacteria, are being sought. In practical terms, the potential of molecular techniques to improve rumen fermentation is enormous, in spite of the limitations imposed by the complexity of the rumen ecosystem.