* Based on C. Holland and W. Kezar, The Pioneer Forage Manual - Nutritional Guide. Pioneer Hi-Bred Interational Inc. 1999.
PHASE 1
At the time of harvesting, aerobic microorganisms predominate on the forage surface. Aerobic respiration by freshly cut plant material and aerobic bacteria begins at harvesting and continues after the forage is piled and packed. Aerobic respiration by bacteria and plant material consumes soluble carbohydrates needed by the beneficial lactic acid bacteria (or the animal consuming the forage). Aerobic respiration consumes the oxygen contained within and between the forage particles creating the desired anaerobic conditions. The respiration process produces water and heat in the silage mass. Excessive heat build-up resulting from an extended Phase 1 period can greatly reduce the digestibility of nutrients such as proteins.
Another important chemical change that occurs during this early phase is the breakdown of plant proteins called proteolysis. Proteins are first reduced to amino acids and then to ammonia and amines. Up to 50 percent of the total plant protein may be broken down during this process. The extent of protein breakdown is dependent on the rate of pH decline in the silage. The acid environment of the silage eventually reduces the activity of the enzymes that break proteins down.
Phase 1 ends once the oxygen has been eliminated from the silage mass. Under ideal crop and storage conditions, this phase will last only a few hours; with improper management, this phase may continue for several weeks. The primary objective for putting up silage is to minimize air infiltration to shorten the time required to achieve an anaerobic environment. Key management practices are crop choice, proper crop maturity, moisture, chop length, and rapid filling with adequate packing and proper sealing of the storage structure.
PHASE 2
Phase 2 begins after the oxygen in the ensiled forage has been utilized by the aerobic bacteria. Anaerobic bacteria take over. These bacteria ferment soluble carbohydrates into acetic acid. Acetic acid production is desirable because it reduces pH and because it can be used as an energy source for rumen microbes. As the pH of the ensiled mass falls below 5.0, the acetic bacteria decline in numbers. This signals the end of Phase 2 which usually lasts no longer then 24 to 72 hours.
PHASE 3
The lower pH enhances the growth of an anaerobic group of bacteria that produces lactic acid.
PHASE 4
This is a continuation of Phase 3 as the lactic acid bacteria increase, fermenting soluble carbohydrates and producing lactic acid. Lactic acid is the most desirable of the fermentation acids and after efficient preservation, should comprise greater than 60 percent of the total silage organic acids produced. Lactic acid can be utilized by cattle as an energy source. Phase 4 is the longest phase in the ensiling process as it continues until the pH of the forage is low enough to inhibit the growth of all bacteria. When this pH is reached, the forage is in a stable state so long as oxygen is excluded.
PHASE 5
The final pH of the ensiled forage depends largely on the type of forage being ensiled and the condition at the time of ensiling. Haylage should reach a final pH of around 4.5 and corn silage near 4.0. The pH of the forage alone is not a good indicator of the quality of the silage or the type of fermentation that occurred. Forages ensiled at moisture levels greater than 70 percent may undergo a different version of Phase 4 where clostridia bacteria proliferate rather than lactic acid bacteria. Clostridia bacteria produce butyric acid rather than lactic acid, which results in sour silage. With this type of fermentation the pH may be 5.0 or above.
PHASE 6
This phase refers to the silage as it is being fed out from the storage structure. This phase is important because up to 50 percent of the silage dry matter losses occur from secondary aerobic decomposition. Phase 6 occurs on any surface of the silage that is exposed to oxygen while in storage and in the feed-bunk. High populations of yeast and mould or the mishandling of stressed crops can lead to significant losses due to aerobic deterioration of the silage. Proper management is vital to reduce these losses and improve the bunk-life (aerobic stability) of the silage.
