To examine the effect of lowering pH of goat rumen fluid on in vitro digestion of tropical pasture, an experiment measuring the disappearances of dry matter (DM), cell contents and cell wall components (NDF, ADF, cellulose and hemicellulose) in green panic (Panicum maximum Jacq. var. trichoglume Eyles) was carried out. Leaf and stem of green panic cut at 36 and 82 regrowth days were incubated for 48 h with rumen fluid collected from three goats fed alfalfa haycubes. The pH of the rumen fluid (rumen juice 1: artificial saliva 4) was 7.0, and lowered to 6.0 or 5.5 by the addition of lactic acid just before starting the in vitro incubation. The results obtained were as follows: Dry matter disappearance of each material significantly decreased as the pH of rumen fluid was lowered from 7.0 to 6.0, especially pH reduction from 6.0 to 5.5 induced a large decrease in DM disappearance. The disappearances of NDF, ADF, cellulose and hemicellulose were strongly inhibited by lowering rumen fluid pH to 5.5. The relative depression in fiber digestion caused by pH reduction to 6.0 was larger in stem than in leaf fraction. The decrease in the in vitro disappearance of fiber was larger than that of cell contents. It is concluded that a reduction in pH of goat rumen fluid has a great influence upon in vitro digestion of cell wall components in tropical pasture.
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