Silage dates back to about 2000 B.C. However, the modern era did not begin until 1877, when a farmer in France, A. Goffart, published a book based upon his own experiences with corn silage.

Since the 1950s, the amount of silage made in most developed countries has increased steadily and often at the expense of hay. Silage-making is much less weather-dependent than hay-making, and silage is mechanized more easily, is suited better to large-scale livestock production and is adapted to a wider range of crops, i.e., corn, sorghums, and winter or spring cereals.

A well-preserved silage of high nutritional value is achieved by harvesting the crop at the proper stage of maturity; minimizing the activities of plant enzymes and undesirable, epiphytic microorganisms (i.e., those naturally present on the plant); and encouraging the dominance of lactic acid bacteria (LAB). Two dominant features must be considered for every silage:

1. the crop and its stage of maturity
2. the management and know-how imposed by the silage-maker

The key “ensileability” criteria for a crop are:

•dry matter (DM) content
•sugar content
•buffering capacity (resistance to acidification)

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 acidification 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
4. feedout

Each phase has distinctive characteristics that must be controlled in order to maintain forage (silage) 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
•proteolysis

Respiration is the complete breakdown of plant sugars to carbon dioxide and water, using oxygen and releasing heat. Simultaneously, plant proteases degrade proteins to primarily amino acids and ammonia and, to a lesser extent, peptides and amides (i.e., asparagine and glutamine).

The loss of sugar is crucial from the standpoint of silage preservation. Sugars are the principal substrate for the LAB to produce the acids to preserve the crop. Excessive heat production (i.e., temperatures above 42 to 44°C) can result in Maillard or browning reactions, which reduce the digestibility of both protein and fiber constituents. 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 impermeable cover (i.e., polyethylene sheeting) is applied.

Fermentation phase
Once anaerobic conditions are reached in the ensiled material, anaerobic microorganisms begin to grow. The LAB are the most important microflora because forages 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, and many of their end products have no preservative action.

The enterobacteria have an optimum pH of 6 to 7, and most strains will not grow below pH 5.0. Consequently, the population of enterobacteria, which is usually high in the pre-ensiled forage, is active only during the first 12 to 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 to 4.8, at which point clostridia cannot 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 to 21 days. 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 to 75 percent), active fermentation is completed in seven to 14 days. At this point, fermentation of sugars by LAB has ceased, either because the low pH (below 4.0 to 4.2) stopped their growth or there was a lack of sugars for fermentation.

The populations of epiphytic microorganisms 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 the chopping process tends to increase the microflora numbers compared with those on the standing crops, and the LAB population is most enhanced.

This phenomenon was explained earlier as inoculation from the harvesting machine and microbial multiplication in the plant juices liberated during harvest. However, findings have demonstrated these large increases in microflora numbers were impossible to achieve by microbial proliferation and growth, because the time involved was too short or by contamination from harvesting equipment, which could occur in the first load but not in later loads.

A new “somnicell” hypothesis proposes bacteria assume a viable but unculturable stage on the surface of intact plants. The chopping process activates the previously dormant population by releasing plant enzymes and manganese compounds. The LAB ferment to primarily 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.

Stable phase
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, 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 (via microbial respiration), 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 proliferate 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.

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 respired 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. Besides the loss of highly digestible nutrients in the silage, some species of molds can produce mycotoxins 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. Once yeasts or bacteria reach a population of 107 to 108 colony-forming units (cfu) per gram of silage or molds reach 106 to 107 cfu per gram, the silage will begin to heat, and digestible components, like sugars and fermentation products, will be lost quickly.

The time required for heating to occur is dependent of several factors:

1. numbers of aerobic microbes in the silage
2. time exposed to oxygen prior to feeding
3. silage fermentation traits
4. ambient temperature

Under farm conditions, DM losses in the feedout phase are largely a function of silage management practices. Few data are available to quantitate feedout losses in farm-scale silos, but laboratory studies indicate DM losses are about 1.5 to 3.0 percent per day for each 8 to 12ºC rise in the silage temperature above ambient.

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 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.  FG

References omitted but are available upon request at editor@progressivedairy.com.

—From Kansas State University Silage Team website

Keith K. Bolsen, Ben E. Brent and Ron V. Pope
Department of Animal Sciences and Industry
Kansas State University