Abstract A mathematical model is developed to study the effect of cellulase and amylase additives on the rate and extent of silage fermentation and on the changes in fiber concentrations over long storage periods. Based on published results, equations are derived for the effects of temperature, pH, water activity, and substrate and end-product concentrations on the activity and stability of Trichoderma cellulases and Bacillus subtilis amylases. Apparent activity rates in silage are obtained by fitting the model's predictions to the results of published enzyme-additive silage experiments, and are estimated to be .01 to .00001 times those determined in laboratory assays, possibly as a result of the structure of the substrates, diffusion limitations of enzymes and products, and the presence of proteases. The model predicts that cellulase addition levels of 5000 g/t silage are required to influence substantially the fermentation process; however, at 100 g/t silage, a significant fraction of the cellulose may be hydrolyzed over long storage periods. Amylase addition at 100 g/t silage is predicted to affect final silage pH in cases where the level of indigenous water-soluble carbohydrates is low enough to limit the extent of fermentation. Cellulases, amylases, and inoculants combined into a single additive are predicted to have a complementary effect on silage preservation and quality.