1. Forage moisture content
2. Fineness of chop
3. Exclusion of air
4. Forage carbohydrate (sugar) content
5. Bacterial populations, both naturally-occurring and supplemental

Phases of normal fermentation
The conversion of fresh forage to silage progresses through four phases of fermentation that are normally completed within 21 days of ensiling (see Figure 1*). A fifth phase may occur if improper silage production practices cause undesirable or abnormal silage fermentation.

Phase 1 – Plant respiration
The respiration phase begins as soon as forage is mown. This phase is also called the aerobic phase because it can only occur in the presence of oxygen. When cut, green plants continue to live and respire for several hours (or longer if packed poorly in storage). The plant cells within the chopped forage mass continue to take in oxygen because many cell walls are still intact, and plant enzymes that break down proteins (proteases) continue to function. At the same time, aerobic bacteria naturally present on the stems and leaves of plants begin to grow. These processes consume readily available carbohydrates stored in the plant and produce carbon dioxide, water and heat.

Sugar + oxygen = carbon dioxide + water + heat

The heat produced by aerobic bacteria causes an initial rise in silage temperature; normal fermentation results in initial temperatures that are no more than 20?F greater than the ambient temperature at ensiling.

The respiration phase usually lasts three to five hours, depending on the oxygen supply present. From a management standpoint, the primary goal is to eliminate oxygen as soon as possible and keep it out for the duration of the storage period.

Practices that help rapidly exclude air from the silage mass include chopping forage at proper particle length (about .375 to .75 inch long), harvesting at proper moisture for the crop and the storage structure, packing adequately by distributing silage evenly, compacting silage well and sealing the storage structure immediately.

Phase 2 – Acetic acid production
This phase begins as the supply of oxygen is depleted and anaerobic bacteria that grow without oxygen begin to multiply. The acetic acid bacteria begin the silage “pickling” process by converting plant carbohydrates to acetic acid. This acidifies the forage mass, lowering the pH from about 6.0 in green forage to a pH of about 5.0. The lower pH causes the acetic acid bacteria to decline in numbers, as they cannot tolerate an acidic environment. The early drop in pH also limits the activity of plant enzymes that break down proteins. This phase of the fermentation process continues for one to two days and merges into Phase 3.

Phase 3 – Initiation of lactic acid production
The third phase of the fermentation process begins as the acetic acid-producing bacteria begin to decline in numbers. The increased acidity of the forage mass enhances the growth and development of lactic acid-producing bacteria that convert plant carbohydrates to lactic acid, acetic acid, ethanol, mannitol and carbon dioxide. Homolactic bacteria are preferred because they can convert plant sugars to lactic acid exclusively. Bacterial strains within this group grow in anaerobic conditions, and they require low pH.

Phase 4 – Peak lactic acid production and storage
The fourth and longest stage of the fermentation process is a continuation of Phase 3; lactic acid production continues and peaks during this time. Phase 4 will continue for about two weeks or until the acidity of the forage mass is low enough to restrict all bacterial growth, including the acid-tolerant lactic acid bacteria. The silage mass is stable in about 21 days, and fermentation ceases if outside air is excluded from the silage. However, improper ensiling practices will result in an undesirable continuation of the process, as discussed below.

If silage has undergone proper fermentation, the expected pH will range from 3.5 to 4.5 for corn silage and 4.0 to 5.5 for haylage, depending on forage moisture content. The remainder of Phase 4 is the material storage phase. Generally, lack of oxygen prevents the growth of yeast and molds and low pH limits the growth of bacteria during storage.

Undesirable fermentation
Remember that silage is part of a dynamic biosystem where proper fermentation is delicately balanced based on the exclusion of oxygen, the availability of water-soluble carbohydrates, the moisture content of the crop mass and the microbial and fungal populations present on the crop. These factors affect the rate or extent of fermentation and the nutritional value of silage.

Excessive oxygen – The presence of oxygen in the forage mass increases the rate at which plant carbohydrates are converted to heat and carbon dioxide. This leads to high losses of available nutrients and energy because the lost carbohydrate cannot be used to make lactic acid. Respiration typically increases neutral detergent fiber (NDF) and acid detergent fiber (ADF) and decreases net energy for lactation (NEL) of silage. These changes reduce forage quality.

Respiration not only depletes plant sugars, but the heat produced can limit the activity of lactic acid bacteria and cause protein to bind to lignin. The ideal temperature for acid-producing bacteria activity is about 80?F to 100?F. Excessive oxygen trapped in the forage mass will cause initial temperatures to rise well above 100?F and limit lactic acid production. In addition, excessive heating encourages the growth of undesirable fermentation bacteria, yeasts and molds.

Heating soon after ensiling also can lead to Maillard browning, which lowers protein quality and digestibility. During browning, proteins combine with plant sugars to form a brown lignin-like compound. This increases the level of bound protein and ADF in the silage. Forages with moisture contents of 20 to 50 percent are most susceptible to browning. Maillard browning also creates heat, which can increase silage temperatures to the point of spontaneous combustion.

Finally, excessive oxygen and the resulting high silage temperatures increase the rate at which proteases convert crude protein to soluble protein (ammonia, nitrates, nitrites, free amino acids, amines, amides and peptides). High levels of soluble protein in forages can create imbalances in the rumen if the ration is not properly balanced for degradable and undegradable protein.

Low plant sugar levels – The production of acid, especially lactic acid, is the most important change in the fermentation process. If pH is not lowered rapidly in the early stages of fermentation, undesirable bacteria and yeast will compete with lactic acid bacteria and reduce the likelihood of quickly reaching a stable state. For this reason, many aspects of silage management focus on lowering pH rapidly to encourage the proliferation of lactic acid bacteria.

To produce lactic acid, bacteria must have sugar available, and if sugars are depleted during fermentation, lactic acid production stops. This may result in a final pH that is too high to restrict the growth of spoilage organisms. Two factors dictate the amount of sugar required for maximum fermentation: water and crop species.

In wet forage, a lower pH is needed to prevent undesirable bacteria growth. This means more sugar must be available for conversion to acid. Legumes have a natural buffering capacity and require more acid to reach a low pH than grasses or corn. The combination of low sugar content at harvest and high buffering capacity means alfalfa is especially prone to incomplete fermentation. Plant sugar levels required for maximum fermentation of various crops are presented in Table 1*.

Phase 5 – Butyric acid production
Provided the ensiled forage contains an adequate supply of readily available carbohydrates, fermentation in the silo will not progress to Phase 5 when proper production practices are followed. This phase involves the production of butyric acid and other undesirable products such as ammonia and small proteins called amines. Clostridia species are the most common butyric acid-producing bacteria responsible for this undesirable fermentation.

Clostridia are spore-forming bacteria that normally live in manure and soil and can grow in silage when oxygen is absent. They typically multiply in silage after most of the acetic and lactic acid bacteria stop growing. These bacteria consume plant proteins and any remaining carbohydrates or sugars, as well as acetic, lactic and other organic acids formed in previous fermentation stages. Butyric acid is a sour-smelling, low-energy acid that tends to decrease feed intake. Therefore, growth of clostridia increases losses of digestible dry matter and produces sour-smelling silage with low nutritional value and limited palatability.

Different species of clostridia have varying effects on fermentation (see Table 2*). Some species ferment lactic acid and sugars to produce butyric acid, gaseous carbon dioxide and hydrogen, while others can ferment free amino acids to acetic acid and ammonia. These compounds raise silage pH.

Sugar = butyric acid + carbon dioxide + hydrogen gas

Lactic acid = butyric acid + carbon dioxide + hydrogen gas

Amino acids (alanine + glycine) + water = acetic acid + ammonia

A variety of other nonprotein nitrogen compounds are created when clostridia break down plant proteins, and some, including putrescine and cadavarine, have especially unpleasant odors. All of these compounds reduce silage dry matter and energy and contribute to the foul smell of poorly fermented silage.

Moisture content greater than 70 to 72 percent and low initial carbohydrate levels set the stage for Phase 5 of the fermentation process. Legume crops (such as alfalfa) contain relatively low levels of carbohydrates compared to corn silage and require field wilting to increase the concentration of carbohydrates and reduce moisture content in the forage mass. High water-soluble carbohydrate levels in corn silage generally result in a rapid decline in pH that inhibits growth of clostridia. The best preventative actions to avoid clostridial fermentation are drying forage to at least 30 percent dry matter or using silage additives if forage dry matter is below 30 percent. Also, allowing 21 to 28 days between spreading manure and harvesting silage can help reduce the number of clostridia present on the forage at the time of ensiling.

Characteristics of silages that have undergone clostridial fermentation include pH above 5, high ammonia-nitrogen levels, more butyric acid than lactic acid and a strong, unpleasant odor. Some clostridia may produce toxins, including those that cause enterotoxemia. Cows fed this silage typically eat less or go off feed completely, produce less milk and have increased incidence of metabolic diseases such as ketosis or displaced abomasum.  PD

References omitted due to space but are available upon request.

—From Penn State University Extension

Figure and Tables omitted but are available upon request to editor@progessivedairy.com

C. M. Jones, Research Associate; A. J. Heinrichs, Professor, Department of Dairy and Animal Science; G. W. Roth, Professor, Department of Crop and Soil Sciences, Penn State University

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