Corn forage at the milk, soft dough, and black layer stages of maturity (22, 28, and 44% DM, respectively) was treated with an enzyme additive at 1, 10, or 100 times the recommended dose and ensiled in laboratory silos at 26 degrees C. By in vitro assay, the additive contained a full complement of cellulase and hemicellulase activities. The pH and temperature optima for cellulase and hemicellulase activities were 4.8 and 50 degrees C, respectively. Regardless of dose, the additive had no effect on fermentation acids or nitrogenous compounds in silage at any maturity; however, high doses increased the glucose content of silage at the milk stage of maturity and increased ethanol content at the soft dough stage. Across maturities, addition of the enzyme additive resulted in a linear decrease in ADF, NDF, and hemicellulose content of corn silage but decreased the acid detergent lignin content of silage only at the milk and black layer stages of maturity. The enzyme additive had no consistent effect on in vitro NDF digestion.