Post-ingestion metabolism of fresh forage

It is generally assumed that breakdown of plant material in the rumen is a process mediated by gut microorganisms. This view arose because of the iden tification of a pre-gastric fermentation in the rumen, brought about by a l arge and diverse microbial population. The extensive use of dried and groun d feed particles in forage evaluation might have helped to promote this ass umption. However, although the assumption might be correct in animals feedi ng on conserved forage (hay and silage) where the cells of ingested forage are dead, it is possible that with grazed (living) forage, the role played by plant enzymes in the rumen hi-is been overlooked. In a grazing situation , plant cells that remain intact on entering the rumen are not inert, bur w ill respond to the perceived stresses of the rumen environment for as long as they are metabolically viable. Metabolic adjustments could include anaer obic and heat-shock responses that could promote premature senescence, lead ing to remobilization of cell components, especially proteins. Moreover, co ntact of plant cells with colonizing microorganisms in the rumen might prom ote a type of hypersensitive response, in much the same way as it does outs ide the rumen. After fresh plant material enters the rumen and prior to ext ensive plant cell-wall degradation, there is often a phase of rapid proteol ysis providing N in excess of that required to maintain the rumen microbial population. The inefficient use of this ingested N results in generation o f ammonia and urea in exhaled breath and urine, which promotes welfare and environmental pollution concerns. Therefore an important research goal in l ivestock agriculture is to find ways of decreasing this initial rate of pro teolysis in the rumen. This will benefit the farmer financially (through de creased use of feed supplements), but will also benefit the environment, as N pollution can adversely affect pasture diversity and ecology. This revie w considers the possible responses of plant metabolism to the rumen environ ment, and how such considerations could alter current thinking in ruminant agriculture.