A study was made to investigate the effect of 0, 400, 800 and 1200 mg l-1 of soluble silica (SiO2) on organic matter disappearance of forage (Rhodes grass, Chloris gayana) incubated in vitro with rumen fluid or cellulase. Sodium silicate (Na2SiO3) was used as the form of soluble silica. The pH was adjusted to 7.0 and 4.6, respectively, after the addition of sodium silicate and before mixing with either rumen fluid or cellulase. Organic matter disappearance of forage samples was measured after incubating with rumen fluid or cellulase containing the four levels of silica at 39°C for 48 h. Increased levels of silica caused a small decrease in the organic matter disappearance when forage samples were incubated with rumen fluid or cellulase. However, soluble silica had less effect with cellulase than with rumen fluid. It is suggested that soluble silica affected not only the activity of cellulolytic microbes of the rumen, but also the activity of cellulolytic enzymes.
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