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Introduction
Silage is the feedstuff resulting from the preservation of green forage crops by
acidification. There are two main phases in the ensiling process. The first is the aerobic
phase, which occurs in the presence of oxygen (air). The oxygen that is present in the
forage, as it is placed into storage, is consumed by the plant material through the process
of respiration. Under aerobic conditions, plant enzymes and microorganisms consume
oxygen and burn up plant water-soluble carbohydrates (sugars), producing carbon
dioxide and heat.
The first phase should be as brief as possible to maintain the quality of the silage.
Excessive aerobic fermentation reduces the energy content of the silage and may cause
heat damage to proteins.
The second or anaerobic phase begins when available oxygen is used up by respiration
and aerobic bacteria cease to function. Anaerobic bacteria (bacteria that grow in the
absence of oxygen) then begin to multiply rapidly and the fermentation process begins.
The best silage is produced when the most rapidly growing microorganisms are
predominately of the lactobacilli species, as they produce lactic acid from the fermented
plant material. Lactic acid lowers the pH of the silage. Fermentation completely ceases
after three to four weeks when the pH becomes so low that all microbial growth is
inhibited.
The ensiling process is illustrated in Figure 1.
FIGURE 1. Short-term changes during ensiling of forage
Source: Nutritional Ecology of the Ruminant, P. Van Soest, Cornell University Press, 1994, p. 217.
There are several ensiling/storage methods that will accomplish the ensiling process. All
methods have advantages and disadvantages, and have widely ranging capital costs.
Some methods of storing silage include trench, bunker, concrete silos, oxygen-limiting
silos, heap silage, and bale silage.
Whatever the system, the ensiling and storage system’s main functions are to exclude air
during the ensiling process and to prevent air from entering the silage during storage.
Limiting air present in the silage will enhance feed quality and reduce spoilage. Slow silo
filling, forage that is not chopped finely enough, inadequate packing and allowing air to
enter stored silage all will reduce quality.
Efficiencies of some silage and other feed storage methods are outlined in Table 1. and
Figure 2. Much higher losses can occur if good production practices are not followed,
e.g., mending tears in plastic covers, using concrete floors, etc.
Horizontal Silos
There are two types of horizontal silos – below ground level (i.e., pit or trench) and above
ground (i.e. bunker and stack). The main advantage of horizontal silos is their low capital
cost and suitability to feeding livestock in widely separated pens.
Trench silos are usually dug into a slope with the “downhill” end open for drainage and
access. It is preferable to have the open end facing south. Earth walls should be sloped at
a ratio of 1:2 to prevent caving in and to enable adequate silage packing. Where soil is
unstable, it is necessary to line the walls with concrete or untreated wood. A convenient
width for loading with a tractor and front-end loader is 15 m (50 ft). A reinforced
concrete floor sloped one to two per cent toward the open end will drain and stand up
under high moisture from silage, rainfall and spring thaw.