Making Silage: The Fermentation Process

Harvesting forages as silage is a compromise between minimizing field and fermentation losses. Efficient fermentation ensures a more palatable and digestible feed, encouraging optimal dry matter intake that translates into improved animal performance. The primary management factors that are under the control of the producer are:

1. Stage of maturity of the forage at harvest.

2. Type of fermentation that occurs in the silo or bunker.

3. Method of harvesting, type of storage structure, silo management and method of feeding.

Attention to details such as speed of harvesting, moisture content, length of chop, silage distribution and compaction can improve the fermentation process and reduce storage losses.

Six Phases of the Ensiling Process

PHASE 1 Aerobic microorganisms are present on the forage surface at the time of harvesting. Aerobic respiration by freshly cut plant material and aerobic bacteria begins at harvesting and continues after the forage is piled and packed. Aerobic respiration consumes the oxygen contained within and between the forage particles creating the desired anaerobic conditions. Aerobic respiration also consumes the soluble carbohydrates needed by the beneficial lactic acid bacteria and by rumen microbes.

Phase 1 ends when all the oxygen has been consumed. Under ideal conditions, Phase 1 lasts only a few hours; with improper management, this phase may continue for several weeks and result in significant reduction in feed quality.

The respiration process produces water and heat in the silage mass. Excessive heat build-up during Phase 1 can greatly reduce the digestibility of proteins. Plant enzymes break down proteins during Phase 1. Proteins are first reduced to amino acids and then to amines and finally to ammonia. Up to half of all the plant protein may be broken down during this process. As the silage becomes more acidic, the activity of these enzymes declines.

Good silage-making technique minimizes air infiltration to shorten the time required to achieve an anaerobic environment. Key management factors are crop choice, content of soluble carbohydrate, crop maturity, moisture content, chop length, rate of filling and packing, and proper sealing of the storage structure.

Table 3. Average nutrient composition of farm-grown forages from the South-Coastal Forage Competition in BC in the years 1993-97. (Courtesy of D. Bates, BC Ministry of Agriculture and Food.)
Nutrient Grass Hay Grass Silage
Low Avg High Low Avg High
Dry matter (%) 80.6 88.1 91.8 19.5 40.2 75.1
Acid Detergent Fiber (%) 23.8 29.8 38.1 23.4 31.2 41.7
Neutral Detergent Fibre (%) 43.9 56.3 65.9 33.3 48.9 65.3
Total Digestible Nutrients (%) 55.8 66.8 74.6 50.8 65.0 75.4
Crude Protein (%) 9.1 17.6 24.4 8.8 17.7 26.1
Nitrogen (%) 1.5 2.8 3.9 1.4 2.8 4.2
Nitrate-N (%) 0.01 0.11 0.40 0.0 0.05 0.26
Ammonium-N as % of total N 3.6 17.5 47.5
Heat Damaged Protein (%) 2.4 4.4 11.2
pH 3.9 4.9 6.4
Phosphorus (%) 0.19 0.33 0.48 0.22 0.37 0.55
Potassium (%) 1.17 3.13 4.99 1.30 3.05 4.50
Magnesium (%) 0.13 0.23 0.38 0.11 0.24 0.53
Calcium (%) 0.26 0.47 0.85 0.28 0.56 1.39

PHASE 2 Phase 2 begins when anaerobic bacteria take over. These bacteria ferment soluble carbohydrates into acetic acid. Acetic acid production is desirable because it reduces pH to set up the succeeding fermentation phases. Also, acetic acid can be used as an energy source by rumen microbes. Phase 2 usually lasts no longer then 24 - 72 hours, ending when the pH of the ensiled mass falls below 5.0, killing the acetic acid-producing bacteria.

PHASE 3 This is a transition phase in which the lower pH favours the growth of an anaerobic group of bacteria that produce lactic acid, replacing those that produce acetic acid.

PHASE 4 In this Phase the lactic acid bacteria predominate. Lactic acid is the most desirable of the fermentation acids. In well-preserved silage, lactic acid should comprise more than 60% of the total silage organic acids and the silage should contain up to 6% lactic acid on a dry matter basis. Lactic acid can be utilized by cattle as an energy source. Phase 4 is the longest phase in the ensiling process as it continues until the pH of the forage is low enough to inhibit the growth of all bacteria. When this pH is reached, the forage is in a stable state so long as oxygen is excluded.

Table 4. Effect of adding absorbent feedstuffs to direct-cut grass silage on effluent and content of water-soluble carbohydrates and crude protein, relative to wilted silage.
Additives Effluent Water soluble carbohydrates Crude protein
% % %
Wilted silage (28% dry matter) 4.5 13.9 22.1
Direct cut silage (16% dry matter) 18.0 12.8 22.5
+ 10% Barley 11.8 16.8 20.7
+ 10% Beet pulp 9.2 23.0 20.5
+ 10% Alfalfa cubes 6.3 12.2 22.3
+ 20% Alfalfa cubes 1.9 14.9 21.3
+ 30% Alfalfa cubes 0.6 16.0 22.3
(based on S. Fransen and F. Strubi. 1998. J. Dairy Sci. 81: 2633-2644)

PHASE 5 The final pH of the ensiled forage depends largely on the type of forage being ensiled and the condition, especially moisture content, at the time of ensiling. Haylage should reach a final pH of around 4.5 and corn silage near 4.0. Drier silage generally has higher stable pH than wet silage. The pH alone is not a good indicator of the quality of silage or of the type of fermentation that occurred.

Forages ensiled at moisture levels greater than 70% may undergo a different version of Phase 4 where undesirable Clostridia bacteria proliferate instead of lactic acid bacteria. Clostridia bacteria produce butyric acid rather that lactic acid, which results in sour silage. With this type of fermentation the pH may stabilize at 5.0 or above.

PHASE 6 This phase refers to the silage as it is being fed out from the storage structure. This phase is important because up to 50% of the silage dry matter losses occur from secondary aerobic decomposition. Phase 6 occurs on any surface of the silage that is exposed to oxygen while in storage and in the feed-bunk. High populations of yeast and mould can lead to significant losses due to aerobic deterioration of the silage. Proper management is vital to reduce these losses and improve the bunk-life (aerobic stability) of the silage.