Silage management
The fermentation process is the basis of good silage management. The goal is to maximize the amount of nutrients retained in the forage crop. Forage is stabilized through fermentation, which is the anaerobic conversion of plant sugars into organic acids by lactic acid bacteria (LAB).
The ensiling process requires four basic elements to insure high quality, stable silage: proper moisture sufficient plant sugars quick establishment of anaerobic conditions adequate numbers and types of bacteria The basic principles of silage can be broken down into four phases.
Phase I: Aerobic
The aerobic phase is mainly a process of respiration. It begins when the forage crop is cut, and continues through the time the forage is chopped and placed in the silo structure and all the oxygen in the plant material is depleted. Oxygen must be eliminated as quickly as possible to inhibit undesirable aerobic microorganism growth. Aerobic microorganisms consume plant sugars that might otherwise be utilized by lactic acid bacteria or by rumen microorganisms.
As long as oxygen is present, plant respiration continues and yeast and mold can grow. This can lead to excessive heating and subsequent protein, dry matter (DM), and energy loss.
The initial temperature rise in the forage mass normally occurs during Phase I, and it can reach 9 - 12°C/10 - 15°F above ambient. In a more acidic environment (< pH 5.5), plant enzyme activity is reduced, and protein and carbohydrate breakdown decreases.
Phase II: Anaerobic
Phase II begins when oxygen is depleted from the stored plant material. At this point, anaerobic bacteria begin the fermentation process and convert water-soluble carbohydrates into acetic acid, some lactic acid, alcohols, and carbon dioxide. Although acetic acid is weaker than lactic acid, it initiates the pH drop in the ensiled material. The continuous decline in pH will eventually inhibit acetic acid bacteria growth and favor lactic acid bacteria growth.
The initial pH of the ensiled material should drop below 5.0 in less than 24 hours in whole plant corn, and less than 72-96 hours in alfalfa. If the pH remains high, there will be increased DM loss, reduced availability of water-soluble carbohydrates for lactic acid production, and more opportunity for undesirable microorganism growth. Lactic acid production reduces the loss of plant nutrients during the anaerobic phase, and cattle also can use lactic acid as an energy source.
When the pH of the plant material reaches 5.0, the fermentation is dominated by LAB. The pH declines further as water-soluble carbohydrates are converted to lactic acid.
Phase II is completed when the pH is low enough to inhibit all bacterial growth. In properly managed forage that is ensiled at the recommended moisture level and inoculated with LAB, the fermentation typically is completed in about one to two weeks.
Various types of bacteria are found naturally on forage crops. There is no guarantee, however, that forage at harvest will have the adequate number or the desired species of bacteria needed for good fermentation. For this reason, bacterial inoculants containing LAB should be applied during harvest or at the silo to insure a rapid and uniform fermentation.
Phase III: Stable
Once the active growth of LAB stops and the ensiled crop reaches a terminal pH, the silage begins the stable phase. If the silo is properly sealed, there will be little biological activity and minimal losses of nutrients should occur.
Phase IV: Feed out
This phase begins when the silo is opened and continues until all the silage has been removed and fed. Oxygen has unrestricted access to silage at the unloading face. Silage on the face or in the feed bunk can undergo aerobic deterioration because yeasts, and sometimes molds, that were dormant throughout the stable phase now can grow and cause extensive loss of nutrients.
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