An experiment was conducted to evaluate the effects of silage additives (microbial inoculant plus enzymes) on the fermentation characteristics of Johnson grass (Sorghum halapense) ensiled after two regrowth periods. Johnson grass was harvested at 45 [22.64% dry matter (DM)] and 110 days of regrowth (43.84% DM) at the Lajas Agricultural Experiment Station, University of Puerto Rico. Forage at each vegetative stage was placed into polyvinylchloride (PVC) laboratory silos and assigned to two treatments: no additive (control) and enzymes plus inoculant. Enzymes were applied at 0.1% of fresh material and the microbial inoculant at 106 colony forming units (cfu) g-1 of fresh material. Three silos per treatment at each regrowth period were opened after seven ensiling periods (0, 1, 3, 7, 14, 21, and 100 days) and analysed for pH, lactic-acidproducing bacteria populations (LAB), fermentation end-products, and water-soluble carbohydrates. Structural carbohydrate content [neutral-detergent fibre (NDF), acid-detergent fibre (ADF), hemicellulose, and cellulose] was determined at 0 and 100 days post-ensiling. Fermentation characteristics in Johnson grass silage varied due to regrowth period. For both stages of regrowth, Johnson grass treated with microbial inoculant plus enzymes had lower pH and higher LAB populations, and higher lactic acid contents than control silages. Silage additives also decreased butyric acid content in Johnson grass ensiled at 45 days and ethanol content in the more mature forage.