Silage is the feedstuff resulting from the preservation of green forage crops by acidification. Acidification is the result of the fermentation of the forage in the absence of oxygen.

There are two main phases in the ensiling process. The first is the aerobic phase which occurs in the presence of oxygen (air). Oxygen is present in the forage as it is placed in the silo. This oxygen is consumed by the living plant material through the process of respiration. Under aerobic conditions plant enzymes and microorganisms consume oxygen and bum up the plant water-soluble carbohydrates (sugars) producing carbon dioxide and heat. The length of this phase is variable depending on ensiling conditions; it could last for a few hours or for as long as several days. It is good silage making practice to limit this phase as much as possible since water-soluble carbohydrates are being consumed and other nutrients are being destroyed. The heat generated by an extended aerobic phase can raise the temperature of the ensiling forage material sufficiently to cause heat damage. Good silage making practices reduce the amount of time that aerobic microorganisms and oxidizing plant enzymes are able to function. This is accomplished by chopping the silage to a short length, packing it thoroughly, and sealing the silo effectively.

The second or anaerobic phase begins when the available oxygen is used up through plant respiration and aerobic bacteria cease to function. Anaerobic bacteria (bacteria that grow in the absence of oxygen) then begin to multiply rapidly and the fermentation process begins. Ideally the microorganisms which grow most rapidly will be predominately lacto~ bacilli species which produce lactic acid from the fermented plant material. The lactic acid which is produced will lower the pH of the silage. Fermentation completely ceases after 3 to 4 weeks when the pH becomes so low that all microbial growth is inhibited.

If ensiling procedures are such that lactic acid producing bacteria aren't favored, clostridial type microorganisms will grow. These organisms utilize plant water-soluble carbohydrates, lactic acid and protein for growth and produce butyric acid. The quality of silage is greatly reduced if a clostridial type of fermentation predominates.

The foregoing is a very simplistic summary of the ensiling process. In addition to lactobacilli and clostridial microorganisms, silage also contains yeasts, molds, coliforms, bacilli and propionic acid producing bacteria. In addition to utilization of plant sugars as energy sources, silage microorganisms degrade protein to amino acids, amines and ammonia during fermentation. Literally hundreds of fermentation products are formed in addition to lactic and butyric acids.


Factors Affecting Silage Fermentation

The primary factors affecting the success of silage fermentation are:

     * water-soluble carbohydrate content
     * buffering capacity of the forage
     * moisture content of the forage
     * type of bacteria which predominate
     * speed of fermentation.


Water-soluble carbohydrates

Microorganisms use water-soluble carbohydrates as the main energy source for growth. The main sugars present in plants are fructose, glucose, sucrose, and fructosans. There is only limited fermentation of other carbohydrates in plants such as starch, cellulose and hemicellulose.

Under some conditions fermentation may limit low WSC content of the forages. Under these conditions pH is not lowered enough to achieve safe preservation. Normally a minimum of 6 to 12 per cent WSC are required for proper silage fermentation. It is therefore important to stand the factors which will influence the WSC content forages.


Water Soluble Carbohydrate Levels in Dry Matter of Different Crops

Forage		% Water soluble carbohydrates
Alfalfa	Vegetative Early bloom Full bloom	9 7 7
Clovers	Cut 1 Cut 2	9 5
Grasses		8 – 25
Timothy		10
Orchard grass		6
Quack grass		13
Brome grass		9
Reed canary grass		7
Legume grass mixture (50:50)		13
Corn silage	Early harvested Medium Late harvested	31 14 8
Barley	Heading completed Flowering Milk stage Soft dough stage Ripe	17 18 32 24 5
Sunflowers	Flowering Seed stage Dough stage	19 19 12

Examples of water soluble carbohydrates in experimental forage. Adapted from information in McDonald, P. 1981. The Biochemistry of Silage. John Wiley & Sons, Toronto and Smith, D. 1973. The nonstructural carbohydrates Page 105. In Chemistry and Biochemistry of Herbage. G.W Butler and R.W. Bailey, Ed. Vol. 1. Academic Press, New York.


Type of plant
Legumes, in particular contain low amounts of WSC and this is one reason why they are quite difficult to ensile. There are differences between grass species in the amount of sugars which they contain with timothy and orchardgrass having a lower amount than other grasses. Forage corn contains enough WSC to ensure successful ensiling and barley forage contains adequate amounts for preservation at the mature stage when silage is normally made.

Growing conditions
Plants contain much greater amounts of WSC when growing conditions have been cool with plenty of sunshine. Heavy rainfall during growth can reduce WSC up to 50 per cent.

Stage of growth
The WSC content of grasses increases as the plants matures. In whole plant barley forage WSC increases until about the milk stage.

Management conditions
There is a decrease of WSC during wilting. If wilting takes longer than normal, this reduction in sugars can have a very detrimental effect on the ensiling process.

Drought
Drought will reduce the WSC content of forages.

Daily variations
Total WSC concentrations appear to increase in the morning and start to decrease in the afternoon.

Fertilization
High rates of nitrogen fertilizer can influence the nitrate concentrations in forages. High nitrate levels which are undesirable in the ensiling process are generally associated with lower levels of WSC. Some of the nitrates in forage are ultimately degraded to ammonia which tends to raise the pH of the silage.

Planting density
WSC decrease if plant densities are high.

Buffering capacity
The buffering capacity of forages has an influence on the ease with which the forage can be ensiled. Buffering capacity of forages can be defined as the degree to which forage material resists changes in pH. Forages with a high buffering capacity will be highly resistant to a reduction in pH which is necessary for good preservation. Therefore more acid must be produced to reduce the pH to desired levels. This is undesirable in silage because more WSC must be used to produce the additional acid. Where the buffering capacity is high, it has been estimated that twice the amount of WSC is required to give good fermentation, compared with forages with a low buffering capacity.

The organic acids (malic, succinic, malonic and glyceric acid) in forages are mainly responsible for buffering capacity. In the ensiling process these organic acids are degraded by bacteria and are replaced by acids with stronger buffering properties. These replacement acids cause the buffering capacity of the forage to increase two to fourfold. Plant proteins also increase the buffering capacity of silage.

Legumes are generally well buffered which means that more acid is required to cause changes in the pH of the fermenting material. As a general rule about 10- 1 2 per cent WSC in legume dry matter will be sufficient for ensiling to occur whereas with grasses a minimum of only 6-8 per cent is required. Beet pulp, which has a low buffering capacity, only requires about 4-6 per cent WSC for good preservation. As can be seen from the previous table, legumes often contain less than the minimum amount of WSC allowing an undesirable type of fermentation to occur.


Buffering Capacities of Forages

Crop	Buffering capacity2
Forage corn	200
Orchard grass	300
Ryegrass	250 – 400
Alfalfa	400 – 600
Clover	500 - 600

¹ Adapted from McDonald, Volume 3, P. 1973. The ensilage process. Page 33. In Chemistry and Biochemistry of Herbage. G.W. Butler and R.W Bailey, Ed. Academic Press, New York and other sources.
² Buffering capacity is expressed as milliequivalents of NAOH required to change the pH from 4.0 to 6.0 in 1 kg of forage dry matter.


Environmental conditions also influence the buffering capacity of forages. First cut alfalfa has more buffering capacity than second or third-cut alfalfa, whereas the reverse is true for some grasses. The buffering capacity of ryegrass has been shown to decrease with maturity.

Moisture content
In general, the lower the moisture content in the crop, the higher will be the pH at which anaerobic stability is reached (see following table). Organic acids are lost in the wilting process and this reduces the buffering capacity of the plants which improves the ensiling process. This factor is one of the reasons why field wilting is beneficial with crops which have low WSC contents and high buffering capacities.

Another aspect which makes wilting beneficial for ensiling is that lactic acid bacteria are more tolerant to lower moisture concentrations than are the undesirable clostridial organisms. Very wet forage (greater than 70 per cent water) is therefore undesirable since clostridial growth may not be inhibited even when the pH drops to 4. Also with wet forages, the nutritional value and voluntary intakes of the silage produced are often low.

Wilting is usually restricted to perennial grasses and legume crops. Cereals usually have moisture levels suitable for rapid ensiling.


Effects of Wilting on the Composition of Grass Silage

	Unwilted	Wilted
pH	4.0	5.1
Dry matter (%)	15.8	35.0
Lactic acid (%)	16.5	3.4
Acetic acid (% of DM)	1.9	0.6
Butyric acid (% of DM)	0.5	0.1
Ammonia N (% of total N)	10.9	8.0
Water-soluble carbohydrates	1.1	18.5

From Morgan, Edwards and MeDonald. 1980. J. Agric. Sci. Camb. 94:287


Several methods are commonly used to estimate moisture content of forages prior to being placed in the silo. Several different types of moisture testers are commercially available. The hand or squeeze method of estimating moisture content in chopped forage is described in table I I in the section on silage harvesting. Microwave ovens can be used to analyse a forage sample for moisture content. The method is described in table 12 in the section on forage harvesting.

Type of bacteria which predominate
The most desirable fermentation will occur where lactic acid producing bacteria predominate. Although it is frequently assumed that fresh forage is adequately supplied with lactic acid producing bacteria, the numbers may be low under some circumstances. Many of the silage inoculants on the market are formulated to increase the numbers of these bacteria.

Following good silage making procedures will facilitate development of anaerobic conditions that will aid in the rapid growth of lactobacilli in the silage.

Speed of fermentation
Since the primary aim of storing forages as silage is to preserve the material with a minimum of nutrient loss, it is desirable to limit the nutrient consuming activity of aerobic micro-organisms and to inhibit the breakdown of protein by clostridia micro-organisms under anaerobic conditions.

The preceding discussion has covered the main factors influencing how rapidly fermentation will progress. Managing silage making to ensure rapid fermentation will help minimize nutrient loss during the ensiling process.


Characteristics of Good Ensiling Procedures

Exclusion of air
It is extremely important that air he excluded to optimize fermentation. There are several reasons for this:

     * Plant respiration results in the loss of nutrients which would otherwise be available for the anaerobic
        bacteria to use for lactic acid production.
     * Temperature of the ensiling forage will increase when oxygen is present. This is generally undesirable
        since high temperatures lead to heat damage of the silage.
     * Air in the silo delays the breakdown of the plant cells and the release of plant juices. This in turn delays
        the onset of rapid fermentation since the nutrients in the juices are not immediately available to the
        microorganisms.
     * Exposure to excessive oxygen during silo filling encourages growth of fungi which cause greater
        instability and susceptibility to aerobic deterioration when silage is being fed.

All of these factors tend to delay the development of the lactic acid producing bacteria, encourages the proliferation of undesirable clostridia bacteria, and promotes protein breakdown.
The actual amount of atmospheric oxygen which is initially trapped in the forage is very small. Measurements have shown that if sealing is adequate, 99.5 percent of the oxygen can be used up in 30 minutes by respiring plant tissues. This amount of combustion causes very little temperature rise (less than 3'C). However, considerable air can enter the forage if silage is not covered with plastic or sealed in some other manner. Slow silo filling, forage that is not chopped finely enough, and inadequate packing are other causes of excessive air in silage.

Low temperature
When microbial growth occurs in silage, there is a rise in temperature. In general, the greater the growth rate of micro-organisms, the higher the temperature. It is known that the rate of acidification is greater when silage temperatures are higher and that the onset of fermentation is earlier. As a result, people have incorrectly concluded that hot silage is good silage. Higher temperatures encourage the growth of undesirable clostridia which result in increased butyric acid and ammonia formation which is detrimental to quality. Temperatures in the 15 to 25°C range have been shown to allow growth of the more important lactic acid producing species of bacteria while inhibiting the undesirable clostridial species. Silage temperatures should thus not exceed 30°C.

High temperatures will cause heat damage in silage. When the temperature of the silage exceeds 40°C in the presence of oxygen, a chemical reaction between plant WSC and protein occurs. This reaction produces brown products which give heat damaged silage its characteristic brown color with a tobacco or carmelized odor. The protein that is bound in this 'browning reaction' is largely indigestible to the rumen microorganisms and to the animal. As can be seen in the following table, the higher the temperature in the silage, the greater the reduction in protein digestibility. Fresh forage protein typically has a true digestibility of approximately 90 per cent, regardless of the forage source. Excessive heating can reduce this digestibility to 30 per cent or less. It should also be noted that the longer the time that the high temperature persists, the greater will be the damage to the forage.


Protein Digestibility (%) as Affected by Temperature and Length of Storage

Heating period (days)	43ºC (110ºF)	57ºC (135ºC)	71ºC (160ºF)
0	69.7	69.7	69.7
3	68.7	65.8	60.2
9	68.4	64.4	50.0
18	65.2	58.6	35.8
30	65.4	49.0	30.1

From Gailagher, D. W and K. R. Stevenson. 1976. Heat Damage in Hay-Crop Silage. Ontario Ministry of Agriculture and Food. Factsheet No. 76-007.


Routine feed analysis does not detect heat damage. A special analysis to determine acid detergent insoluble nitrogen, more commonly referred to as ADIN, is required to determine the extent of heat damage to forages.

A limited amount of heat damage in silage may be beneficial in that heating produces protein that is resistant to digestion in the rumen but can be digested in the small intestine. This "bypass or escape protein" often results in improved performance of the animal, particularly those at high production levels. Controlled application of heat to produce bypass protein in some feed ingredients is used by the feed industry. However, this requires good control of temperature and duration of heating to be successful. In practice there is little control of either in silage making, therefore excessive heating in the silo should be avoided.

Rapid acidification
Since it is ultimately the pH of the silage which stops the fermentation, pH is of great importance. It is the rate of acidification rather than the ultimate pH achieved that is of the greatest importance, since rapid acidification reduces the risk of early growth of clostridial organisms. In direct cut, high moisture situations, ensiling procedures should be such that rapid acidification is achieved. However, with high dry matter crops and if sterilants have been used, the rapid decrease in pH is not so important.


Methods of Controlling the Ensiling Process

Moisture content
This is the most important means of controlling the ensiling process. Wilting in the field is the practical way to obtain the correct moisture for ensiling. A desirable moisture level is 65 per cent, but slightly higher moistures may be desirable when long chop lengths are used, when packing is minimal, or when the silage is not well sealed. Less moisture (40 to 50 per cent) is required in some oxygen limiting silos. The recommended moisture content of forage stored in different types of structures is given in the following table.

At moisture contents greater than 65 per cent, seepage and loss of nutrients will occur in tower silos more than 30 feet high. Such high moisture levels will also encourage growth of undesirable clostridial organisms.

At moisture levels lower than 60 per cent, excessive heating is likely to occur unless an oxygen limiting silo is used.


Silo Type and Recommended Moisture Levels

Silo type	Recommended moisture level %