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The fermentation process: A closer look

Anna E. Foley Published on 30 May 2014
Silage pit

It takes 0.34 mega-calories (Mcal) to make 1 pound of milk. One of the least expensive feeds that can be used to obtain these calories is silage.

The production of high-quality silage results in a greater amount of nutrients, hence calories, available to your cow.

From the time forage is cut in the field until it is consumed by your cow, biological degradation processes occur that decrease the nutrients available.

The goal of silage production is to minimize degradation so as to conserve the greatest amount of digestible protein and energy for your cow.

Because good silage management practices can improve quality and help prevent or at least minimize losses in forage dry matter, a general understanding of what occurs during the fermentation process of your silage is critical.

Before discussing the process, it is important to point out the three important events that must occur in order to make high-quality silage:

1. Rapid removal of air (oxygen)

2. Rapid production of lactic acid that results in a quick drop in pH

3. Continued exclusion of air from the silage mass during both storage and feedout

The process
Fermentation involves both aerobic (oxygen-loving) and anaerobic (oxygen-hating) bacteria, and is generally divided into five phases.

Aerobic fermentation occurs when the silo, pit or bag is being filled (phase 1) and at feedout (phase 5). The remaining phases (2 through 4) occur under anaerobic conditions.

Phase 1 (aerobic, oxygen-loving)

During this phase both the plant and the micro-organisms present on the plant are undergoing respiration. They do this by using water-soluble carbohydrates – WSC (e.g., glucose, fructose, sucrose) – and oxygen to form carbon dioxide, water and energy:

C₆H₁₂O₆ + 6 O₂  6 CO₂ + 6 H₂O = energy

glucose + oxygen  carbon dioxide + water = heat

The energy is in the form of heat and lends itself to the most notable aspect of this phase, increased temperature.

Under ideal conditions, the temperature of your ensiled material will peak at 15 to 20°F above ambient temperature at the time of ensiling.

If temperature exceeds this level, extensive respiration (heat) has occurred. This excess heat changes the structure of the protein, and it now becomes heat-denatured protein, measured as acid-detergent insoluble nitrogen (ADIN). This is protein your cow cannot utilize.

In addition to damaging available protein, extended respiration continues to use, and to waste, the highly digestible sugars in your forage.

Both of these processes result in dry matter (DM) loss. Phase 1 usually lasts no more than 24 hours; under ideal conditions, this phase should last only a few hours.

When either all of the oxygen is consumed or excluded, or supplies of water-soluble carbohydrates are exhausted, respiration stops.

Phase 2 (anaerobic, oxygen-hating)

Anaerobic bacteria, primarily the enterobacteria, take over now. They can tolerate the heat produced during the aerobic phase and are viable within a pH range of 7 to 5. These bacteria ferment soluble carbohydrates into acetic acid.

This acid serves to decrease pH, to set up the next fermentation phase. When pH drops below 5, these bacteria die off.

Phase 3 (anaerobic, oxygen-hating)

This is the transition phase, during which the lowered pH favors the growth of an anaerobic group of bacteria that produce lactic acid, replacing those that produce acetic acid.

Lactic acid is the strongest of the silage acids (acetic, butyric, lactic) and is therefore responsible for the greatest drop in silage pH. This acid should comprise 65 to 70 percent of total silage acids, or 3 to 6 percent on a DM basis.

There are two types of lactic acid bacteria (LAB): heterofermenters and homofermenters. Heterofermenters use one molecule of glucose to make one molecule of lactic acid, plus one molecule of acetic acid, plus various other products such as ethanol and carbon dioxide.

Heterofermenters produce significantly less lactic acid resulting in a slower drop in pH and with the production of CO2, there is a loss of both DM and energy, creating a less efficient fermentation.

Homofermenters use one molecule of glucose to make two molecules of lactic acid, resulting in little or no DM loss. These efficient homofermenters are more desirable because they work faster, retain more nutrients and preserve the silage better.

Phase 4 (anaerobic, oxygen-hating)

This is the longest phase in the ensiling process. Lactic acid formation continues until the pH of the forage is low enough (usually less than 4.5 pH) to inhibit the growth and metabolism of the lactic acid bacteria.

When this pH is reached, the forage is in a stable state and will remain so until oxygen is re-introduced.

The final pH of an ensiled crop depends on its buffering capacity. Buffering capacity measures the degree to which a forage sample will resist a change in pH.

In general, cereals are easier to ensile than grasses or legumes because their buffering capacity is lower and their WSC content is higher.

Consequently, pH decline of cereals such as corn silage is faster and final pH levels are usually lower than that of grass or alfalfa silages. Corn silage reaches a final pH at or below 4, while the final pH of haylage is around 4 to 4.5.

It is important to point out here that pH does not determine quality of silage; instead, pH indicates whether or not the ensiled product is in a preserved state.

Phase 5 (aerobic, oxygen-loving)

This phase refers to silage as it is being fed from the storage structure. This phase is important because up to 30 percent of the silage DM can be lost from secondary decomposition, which can occur on any surface of the silage exposed to oxygen.

All phases of fermentation are necessary; however, the faster the phases proceed, the greater the quality of your silage. Oxygen should be eliminated as quickly as possible at harvest.

Proper packing and sealing will speed the ensiling process, improving the opportunity for quality fermented forage.

Once oxygen is removed, acetic acid-producing and especially lactic acid-producing bacteria can grow and begin to lower the pH of your silage.

Exclusion of air must continue in order to maintain the quality of your silage. Adhering to these requirements as closely as possible will ensure a high-quality silage.  FG

References omitted due to space but are available upon request. Click here to email an editor.

Proper face management will minimize exposure to oxygen and decrease the opportunity for secondary decomposition. Photo courtesy of Keith Bolsen.