The silage fermentation process takes several days and can be divided into five phases. Each of the five phases is characterized by different changes in the forage.
This phase begins from the time of harvesting the crop to the time oxygen is depleted from the sealed silo. Due to continued plant respiration, plant enzymes and aerobic bacteria cause nutrient losses by degrading plant proteins and converting sugars into carbon dioxide and water, and generating heat. The heat increases the silage temperature by 15° to 20°F or more, depending on the amount of air available. This phase progresses until the oxygen is depleted. It takes a few hours, ideally, but can last for up to 48 hours in poorly made silages. Harvesting at the correct plant maturity, chopping to the right particle length, packing the silo tightly and sealing within 12 hours of harvesting are key to minimizing nutrient losses during this phase.
This phase starts after the oxygen is used up, when anaerobic bacteria begin to ferment plant sugars into organic acids, alcohols, carbon dioxide and nitrogenous compounds. The organic acids lower silage pH from above 6 to about 5. If silage pH drops slowly and the moisture concentration is high due to harvesting too early, clostridial bacteria may grow. These bacteria degrade sugars and convert lactic acid to butyric acid, releasing strong offensive odors. They also break down protein to non-protein nitrogen and undesirable end products like amines. These changes lead to increased dry-matter loss and reduced palatability and quality of the silage.
Once the pH falls below 5, the lactic-acid-producing bacteria (LAB) dominate the fermentation and reduce the pH to about 4 or 4.5 in well-made silages and haylages (round-bale silage), respectively. Lactic acid is more effective than other organic acids at reducing the pH. Therefore, LAB that ferment sugars to lactic acid alone (homolactic LAB) are more efficient at causing a fast pH drop and preserving nutrients than others (heterolactic LAB) that produce lactic acid and other products. This phase can last for three days to four weeks, and it ends when fermentable sugars are depleted. This phase is often limited in warm-season grasses and other forages with low sugar concentrations.
Once the pH drops to or below 4.0 (4.5 in haylages), the silage becomes stable, and the growth of undesirable microbes is prevented. The quality of the silage can be maintained for the rest of the storage duration if the silo remains sealed and air does not penetrate the silo.
This phase begins after aerobic conditions are restored once the silo is opened during feedout. After air penetration occurs, yeasts and molds that were dormant during the fermentation are revived. These fungi use sugars, lactic acid and other nutrients for growth and produce carbon dioxide and heat as byproducts. Excessive heat accumulation denatures proteins and other nutrients in the silage. Collectively, these changes increase DM losses and reduce silage quality. Molds on the silage may also produce mycotoxins that, when consumed, reduce animal performance and cause various diseases.