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 acids produced.

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 ensiling.

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.

AEROBIC PHASE

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 energy.

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 cattle.

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.

STORAGE PHASE

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.

FEEDOUT PHASE

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 acid.

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 feedout.

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 weather.

Table 10. Typical fermentation profile for well-fermented whole plant corn silage.

Profile	Analysis
Silage pH	3.6-4.0
Fermentation end-products	4-6%
Lactic acid	<2%
Acetic acid	<0.1%
Propionic acid	<0.5%
Ethanol	<0.5%
Nitrogen fractions
Ammonia nitrogen	<5% of total N
ADIN (bound N)	<12% of total N
Microbial assay
Yeast	<100,000 CFU¹/g of silage
Molds	<100,000 CFU/g of silage
Total aerobes	<100,000 CFU/g of silage
¹CFU = colony forming units.

Ensiling

Fermentation process

AEROBIC PHASE

ANAEROBIC FERMENTATION PHASE

STORAGE PHASE

FEEDOUT PHASE

Further Reading

Wisconsin