What is Silage?
Silage is the feedstuff produced by the fermentation of a crop, forage, or agricultural byproduct of generally greater than 50 percent moisture content. Hay normally contains less than 15 percent moisture. Ensiling is the name given to the process, and the container (if used) is called a silo. Silage dates back to about 2000 B.C. The current practice of ensiling these forage materials did not begin until the late 1800's. Since the 1950s, the amount of silage made in most developed countries has increased steadily and most often in place of hay. Silage-making is much less weather-dependent than hay-making, and silage is mechanized more easily, better suited to large-scale livestock production, and is adapted to a wider range of crops such as corn, sorghums, and winter or spring cereals.

A well-preserved silage of high nutritional value is achieved through several steps:
1) harvesting the crop at the proper stage of maturity.
2) minimizing the activities of plant enzymes and undesirable.
3) taking advantage of �epiphytic� microorganisms (i.e., those naturally present on the plant).
4) encouraging the dominance of lactic acid bacteria (LAB).

Two dominant features must be considered in making every silage: a) the crop and its stage of maturity and b) the management and know-how of the silage-maker.

The key to a crop being a good candidate for ensilability include:
*dry matter (DM) content of the crop
*sugar content
*buffering capacity (resistance to change in pH).

In these respects, corn is the "nearly perfect" crop, whereas alfalfa is at the other extreme and is the most difficult crop to preserve as silage. Grasses usually contain more water-soluble carbohydrates (WSC) and have less resistance to reduction of pH than legumes.

The Ensiling Process
When making decisions about silage management techniques, it is important to have a good understanding of the events that occur during silage preservation. The major processes involved can be divided into four phases: 1) aerobic, 2) fermentation, 3) stable, and 4) feedout. Each phase has distinctive characteristics that must be controlled in order to maintain product quality throughout the periods of harvesting, silo filling, and silage storing and feeding.

Aerobic Phase
As the chopped forage enters the silo, two important plant enzyme activities occur: respiration and proteolysis (breakdown of proteins). Respiration is the complete breakdown of plant sugars to carbon dioxide and water, using oxygen and releasing heat. Simultaneously, plant proteases (protein-affecting enzymes) for the most part degrade proteins to primarily amino acids and ammonia.

The loss or breakdown of the sugars in the plant is crucial from the standpoint of silage preservation. Sugars are the principal substrate or food for the LAB to produce the acids to preserve the crop. Excessive heat production can result in heat damage, which reduces the digestibility of both protein and fiber. The main aerobic phase losses occur during exposure to air before a given layer of forage is covered by a sufficient quantity of additional forage to separate it from the atmosphere or before an air impermeable cover (i.e., plastic sheeting) is applied.

Fermentation Phase
Once anaerobic conditions are reached in the ensiled material, anaerobic microorganisms begin to grow. The lactic acid bacteria, LAB's, are the most important microbes, because forages stored as silage are preserved by lactic acid. The other microorganisms, primarily members of the family Enterobacteriaceae, clostridial spores, and yeast and molds, have negative impacts on silage. They compete with the LAB for fermentable carbohydrates (starches and sugars used as food), and many of their end products do not contribute to preservation.

The enterobacteria have an optimum pH of 6-7, and most strains will not grow below pH 5.0 (more acidic). Consequently, the population of enterobacteria, which is usually high in the pre-ensiled forage, is active only during the first 12-36 hours of ensiling. Then their numbers decline rapidly, so they are not a factor after the first few days of the fermentation phase.

Growth of clostridial spores can have a pronounced effect on silage quality. Clostridia can cause secondary fermentation, which converts sugars and organic acids to butyric acid and results in significant losses of DM and digestible energy. Proteolytic clostridia ferment amino acids to a variety of products, including ammonia, amines, and volatile organic acids. Like the enterobacteria, clostridial spores are sensitive to low pH, and clostridia require wet conditions for active development. Clostridial growth is rare in crops ensiled with less than 65 percent moisture, because sufficient sugars usually are present to reduce the pH quickly to a level below 4.6 - 4.8, at which point clostridia can't grow. For wetter forages (70 percent moisture or more), reducing the pH to less than 4.6 either by the production of lactic acid or by direct acidification with the addition of acids or acid salts is the only practical means of preventing the growth of these bacteria with today's technology.

The period of active fermentation lasts from 7-21 days. In other words it normally takes 7 to 21 days before the ensiling process has advanced enough that good preservation will take place. It is normally recommended that a silage be left a minimum of 21 days after putting up before feeding. Forages ensiled wetter than 65 percent moisture usually ferment rapidly, whereas fermentation is quite slow when the moisture content is below 50 percent. For forages ensiled in the normal moisture range (55-75 percent), active fermentation is completed in 7-14 days. At this point, fermentation of sugars by LAB has ceased, either because the low pH (below 4.0-4.2) stopped their growth or there was a lack of sugars for fermentation.

The populations of epiphytic microorganisms (normally existing on the plant) on silage crops are quite variable and are affected by forage specie, stage of maturity, weather, mowing, field-wilting, and chopping. Numerous studies have shown that the chopping process tends to increase the microbial numbers compared with those on the standing crops, and the LAB population is most enhanced.

As mentioned earlier the LAB ferment water soluble carbohydrates (WSC) mainly to lactic acid, but also produce some acetic acid, ethanol, carbon dioxide, and other minor products. This is a rather large group of bacteria, which includes species in six genera (Table 1 - see below). They are divided into two categories; the homofermentative LAB produce only lactic acid from fermenting glucose and other six-carbon sugars, whereas heterofermentative LAB produce acetic acid, ethanol, and carbon dioxide in addition to lactic acid. In the fermentation phase, competition between strains of LAB determine how homofermentative the ensiling process will be. 

Don't get caught up in the names of the bacterial strains. This is primarily to help you understand how the fermentation process works and that there are many types of organisms involved in the process.

Following the active growth of LAB, the ensiled material enters the stable phase. If the silo is properly sealed and the pH has been reduced to a low level, little biological activity occurs in this phase. However, very slow rates of chemical breakdown of hemicellulose can occur, releasing some sugars. If active fermentation ceased because of a lack of WSC, the LAB might ferment the sugars released by hemicellulose breakdown, causing a further slow rate of pH decline.

Another major factor affecting silage quality during the stable phase is the permeability of the silo to air (i.e., oxygen). Oxygen entering the silo is used by aerobic microorganisms, causing increases in yeast and mold populations, losses of silage DM, and heating of the ensiled mass. Pathogens, such as Listeria monocytogenes, have been found to grow rapidly in silages exposed to oxygen infiltration at low levels. The risk of L. monocytogenes is greater in low-DM silages and at high levels of oxygen ingress into the silo. The amount of aerobic loss in this phase is related not only to the permeability of the silo but also to the density of the silage. If the silage is left unsealed, substantial DM losses can occur at the exposed surface. These losses can be reduced by covering the surface of the ensiled material with polyethylene sheeting, whether in vertical tower or horizontal bunker, trench, or stack silos. Oxygen can pass through polyethylene, but at a very slow rate. Cracks in the silo wall or holes in the polyethylene seal obviously increase the rate at which oxygen can penetrate the silage mass.

The Feedout Phase
When the silo is opened, oxygen usually has unrestricted access to the silage at the face. During this phase, the largest losses of DM and nutrients can occur because of aerobic microorganisms consuming sugars; fermentation products (i.e., lactic and acetic acids); and other soluble nutrients in the silage. These soluble components are broken down to carbon dioxide and water, producing heat. Yeasts and molds are the most common microorganisms involved in the aerobic deterioration of the silage, but bacteria, such as Enterobacteriaceae and Bacillus spp., also have been shown to be important in some circumstances and create some problems. Besides the loss of highly digestible nutrients in the silage, some species of molds can produce mycotoxins and/or other toxic compounds that can affect livestock and human health.

The microbial activity in the feedout phase is the same as that occurring because of oxygen infiltration during the stable phase. The major difference is the amount of oxygen available to the microorganisms. At feedout, the microorganisms at the silage face have unlimited quantities of oxygen, allowing them to grow rapidly. Under farm conditions, DM losses in the feedout phase are largely a function of silage management practices. A fast filling rate and tight sealing of the silo minimize the build up of aerobic microorganisms in the silage and maximize the production of fermentation products that will inhibit their growth. Adequate packing of the ensiled material reduces the distance that oxygen can penetrate the exposed silage face. Finally, feeding rate and silage density determine the length of time the silage is exposed to oxygen prior to feedout, and the shorter the exposure time, the less likely a silage is to heat during the feedout phase.

This gives you some of the basics to understanding how silage is put up and how the mechanics work. In the next issue we'll discuss some of the practical applications and how this forage preservation system can work on a typical cattle operation.