Plant sugars are fermented by anaerobic bacteria to organic acids which
reduce the pH of the plant material. This process preserves the crop
during long-term storage. The efficiency of fermentation and amount of
fermentation loss is influenced by a number of factors; the ability to
achieve and maintain anaerobic (without oxygen) condition in the silo,
the amount of fermentable sugars in the crop, the quantity and type of
bacteria present on the crop, and the quantity and type of fermentation
High quality corn silage results when lactic acid is the predominant
acid produced during fermentation. Lactic acid is the most efficient
fermentation acid and will drop the pH of the silage the fastest. Under
proper ensiling condition corn silage will normally ferment rapidly and
achieve a stable pH of 4.) or below within the first week after
The major chemical and microbiological changes that occur during the
fermentation process can be divided into four distinct phases: aerobic,
anaerobic fermentation, storage, and feedout.
The aerobic phase of fermentation begins at harvest and continues until
the oxygen is depleted, shortly after ensiling. During this stage, plant
sugars in the freshly chopped plant material are broken down to carbon
dioxide, water, and heat in a process known as respiration. Aerobic
microorganisms (yeast, molds, and aerobic bacteria) present on the
chopped plant material also use plant sugars during this initial phase
and are a significant source of respiration. Increased growth of yeasts
and molds during this phase can predispose the silage to heating and
spoilage during the feedout phase.
Respiration hurts silage quality because it uses highly digestible
energy, reduces the amount of material available for the beneficial
lactic acid bacteria, and produces heat. Temperatures above 100 oF can
produce heat-damaged protein (ADIN) which is unavailable to the animal.
Under normal ensiling conditions the temperature of the ensiled material
will peak at 15 oF to 20 oF above the ambient temperature at the time of
ensiling. If the temperature of the silage exceeds this level, extensive
respiration has occurred.
Another important chemical change that occurs during the aerobic phase
is the degradation of plant proteins to nonprotein nitrogen (NPN),
peptides, amino acids, and ammonia by plant cell proteases. The extent
of proteolysis will depend on the rate of pH decline, temperature and
moisture content of the ensiled crop. In corn silage, the NP level can
increase from 20% of total nitrogen in the pre-ensiled forage to over
50% within 24 hours post-ensiling. Proteolysis is not desirable,
particularly for high-producing dairy cow, because excess soluble
nonprotein nitrogen results in poorer efficiency of nitrogen utilization
and lower milk production. Likewise, elevated levels of ammonia nitrogen
in silages have been associated with lower dry matter intake.
The aerobic phase reduces silage quality and should be minimized. Under
good management practices the aerobic phase will last only a few hours.
With improper managementâ€”i.e., harvesting the crop too dry, poor
compaction, poor chop length, slow filling, and/or not covering the
siloâ€”this phase may continue for several weeks.
ANAEROBIC FERMENTATION PHASE
Once the oxygen has been depleted the anaerobic fermentation phase
begins. During this phase a succession of different populations of
anaerobic bacteria ferment sugars. The sugars are converted primarily
into lactic acid, but also acetic acid, ethanol, carbon dioxide, and a
few other minor products. The production of acid lowers the pH of the
ensiled crop which inhibits the growth of other microbes.
The principal bacteria for ensiling are the lactic acid bacteria (LAB).
LAB are divided into two broad categories. The homofermentative LAB
produce acetic acid and carbon dioxide as well as lactic acid.
Homofermenters are more desirable than herofermenters because their
fermentation is more efficient, resulting less loss of dry matter and
Initially, the heterofermentative LAB are predominant. These organisms
remain active until the pH of the ensiled material drops below 5. As the
pH of the ensiled forage reaches 5, the homofermentative LAB become
predominant. These bacteria are extremely acid tolerant and grow
quickly. Since they produce only lactic acid, the silage pH drops more
rapidly. The bacteria remain active until the silage reaches a stable pH
of 4 or below, or until the fermentation sugars are depleted.
When the natural population of LAB is very low, acetic acid bacteria may
proliferate. These bacteria are less desirable than LAB since they
produce mainly acetic acid which slows the drop in pH, increases dry
matter losses, and can reduce dry matter intake in beef and dairy
In corn silage the active anaerobic fermentation process generally lasts
less than a week. The rate of fermentation depends on the quantity and
type of LAB present on the crop at ensiling and the moisture content of
the silage. Wetter forages ferment faster than drier ones.
During the storage phase the pH of the ensiled material remains
relatively stable and there is minimal microbial and enzymatic activity
if the ensiled crop is kept anaerobic.
The major factor affecting silage quality during the storage phase is
entry of oxygen into the silo. Oxygen increases yeast and mold growth,
which results in dry matter loss and heating in the ensiled forage.
The amount of top silage is directly related to the density of the
silage and the amount of exposed surface area. The worst-case scenario
would be an uncovered silage pile put up too dry and poorly packed.
Aerobic losses under these circumstances can approach 20%. Other causes
of excessive storage loss are cracks in silo walls, poorly sealed doors
in upright silos and rips in plastic covers or bags.
The feedout phase begins once the silo is opened and continues until the
silage is consumed. Once silage is re-exposed to oxygen, yeasts and
molds become active again. They convert residual sugars, fermentations
acids, and other soluble nutrients into carbon dioxide, water, and heat.
Feedout losses can represent up to 30% of the total dry matter loss in
the ensiling process.
Generally, the first signs of aerobic deterioration are heating and an
off odor, followed by fungal growth on the surface of the silage and/or
in the feedbunk. By the time fungal growth appears, substantial amounts
of dry matter and nutrients have already been lost. Besides the loss of
highly digestible nutrients, some molds can produce mycotoxins which can
cause illness or reduced performance in livestock.
Higher levels of aerobic microorganisms present in the silage will cause
the silage to deteriorate faster when re-exposed to oxygen on feedout.
The level of aerobic microorganisms present in the silage is largely
determined by their presence on the crop before harvest and their level
of growth during the initial aerobic phase. Although many yeasts and
molds can survive the low pH levels typically achieved in silage, the
acidic environment restricts their growth. Thus, a pH of 4 or less helps
make the silage aerobically stable during feedout.
The type and amount of fermentation acids produced during the
fermentation will also affect the degree of aerobic stability of the
silage. A typical fermentation profile for well-fermented corn silage is
listed in table 10. Some acids produced during fermentation are more
toxic to yeasts and molds than others. Butyric acid is the most toxic
followed by propionic and acetic acid. Lactic acid is the least
effective at suppressing the growth of yeasts and molds. Thus, the
aerobic stability or bunk life of silages produced by the most efficient
homofermentative lactic acid fermentation is often poorer than
malfermented silage containing elevated levels of butyric and/or acetic
The level of residual sugar remaining in the silage after fermentation
can also influence aerobic stability. Yeasts and molds grow
approximately twice as fast on sugars as they do on fermentation acids.
Silage produced from immature corn silage will generally have higher
levels of residual sugars and are more prone to aerobic deterioration on
The ambient temperature has a major influence on the aerobic stability
of silage. Microbial growth rates increase exponentially with
temperature up to approximately 130áµ’F. This means silage fed out during
warm weather deteriorates faster than silage fed out during cooler
Table 10. Typical fermentation profile for well-fermented whole plant
| Ammonia nitrogen
||<5% of total N
| ADIN (bound N)
||<12% of total N
||<100,000 CFU1/g of silage
||<100,000 CFU/g of silage
| Total aerobes
||<100,000 CFU/g of silage
|1CFU = colony forming units.
Note: Web resources for Wisconsin are maintained by
Mike Rankin and
Forage. Please see
for an up-to-date listing.
Inoculants for Silage
by Francisco Contreras-Govea, UW Agronomy Research Associate, and Dr. Richard Muck,
USDA Dairy-Forage Research Center. A "Focus on Forage" fact sheet.
Inoculantes Microbiales para ensilaje
of Corn Silage Inoculants on Aerobic Stability
Written by Dr. Richard Muck, USDA Dairy Forage Research Center
Corn Silage Inoculants
A MS PowerPoint presentation given by Dr. Richard Muck, USDA Dairy-Forage Research
Center, at the 2001 Forage Teaching and Technology Conference.
for Silage Aerobic Stability
by David Combs and Pat Hoffman, UW Dairy Scientists. A "Focus on
Forage" fact sheet.
Adding Anhydrous Ammonia
to Corn Silage
by Dr. Ron Schuler, UW Biological Systems Engineering Dept. A "Focus
on Forage" fact sheet.
UWEX Bulletin A3511
Prevent Hay Mow and Silo Fires
UWEX Bulletin A2805
Factors Affecting Bunker
by Dr. Brian Holmes, UW Biological Systems Engineering Dept., and
Dr. Richard Muck, USDA Dairy-Forage Research Center