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Silage
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Silage is fermented, high-moisture fodder that can be fed to ruminants or used as a biofuel feedstock for anaerobic digesters. It is fermented and stored in a process called ensilage or silaging, and usually made from grass crops, including maize or sorghum, using the entire plant, not just the grain. Silage can be made from many other field crops, and other terms (oatlage for oats, haylage for alfalfa) can be used.
It is sometimes a mix of two crops, such as oats and peas. Haylage means ensiled forages, made up of grass, alfalfa and alfalfa/grass mixes. This is used extensively in the Midwest and Northeastern areas of the United States. It is also used widely in Europe for dairy cattle diets.
Balage is another form of stored forage. In this case hay, alfalfa or grass is cut and baled while still fairly wet. That is, it is too wet to be baled and stored as hay. In this case the dry matter is around 60 to 70%. The bales are wrapped tightly in plastic wrappers. The material then goes through a limited fermentation in which short chain fatty acids are produced which protect and preserve the forage. This method has become popular on smaller farms.
Making silage
Silage must be made from plant material with a suitable moisture content, about 55% to 70%, depending on the means of storage the degree of compression and the amount of water that will be lost in storage. For corn, harvest begins when the whole-plant moisture is at a suitable level. For pasture-type crops the grass is mowed and allowed to wilt for a day or so until the moisture content drops to a suitable level.
The plant material is collected, chopped into pieces about 1/2" (14 mm) long and packed. In the early days of mechanized agriculture, stalks were cut and collected manually using a knife and horsedrawn wagon, and fed into a stationary machine called a "silo filler" that would chop the stalks and blow them up a narrow tube to the top of a tower silo. Current technology uses mechanical forage harvesters that collect and chop the plant material, and deposit it in trucks or wagons. These forage harvesters can either be tractor-drawn or self-propelled. Harvesters blow the silage into the wagon via a chute at the rear or side of the machine. Silage may also be emptied into a bagger, which puts the silage into a large plastic bag that is laid out on the ground.
In California, Australia, and frequently in New Zealand, silage is placed in large heaps on the ground and rolled by tractor to push out all the air, then wrapped in plastic covers held tight by recycled tyres.
In New Zealand and Northern Europe the silo or "pit" is often a concrete bunker built on the side of a bank so that chopped grass can be dumped in at the top and drawn from the bottom in winter. This requires considerable effort to compress the stack in the silo to cure properly.
Fermentation Silage undergoes anaerobic fermentation, which starts about 48 hours after the silo is filled. Traditionally, the fermentation is caused by indigenous microorganisms, but today, some silage is inoculated with specific microorganisms to speed fermentation or improve the resulting silage. The process converts sugars to acids and exhausts any oxygen present in the crop material. The fermentation is essentially complete after about two weeks. Silage inoculants contain one or more strains of lactic acid bacteria, and the most common is Lactobacillus plantarum. Other bacteria used in inoculants include Lactobacillus buchneri, Enterococcus faecium and Pediococcus species.
Silo effluent The fermentation process releases liquid. Silo effluent contains nitric acid (HNO3), which is corrosive. It can also contaminate water courses unless precautions are taken. The high nutrient content can lead to eutrophication (growth of algae blooms).
Storing silage Silage must be firmly packed to minimize the oxygen content, or it will spoil.
Anaerobic digestion
Silage is a useful feedstock for anaerobic digestion. Here silage can be fed into anaerobic digesters to produce biogas that in turn can be used to generate electricity and heat.
Safety Silos are hazardous, and people die every year in the process of filling and maintaining them. There is a risk of injury by machinery or from falls. When a silo is filled, fine dust particles in the air can become explosive because of their large aggregate surface area. Also, fermentation presents respiratory hazards. Nitrogen dioxide gas is released in the early stages of fermentation, and can kill. Lack of oxygen inside the silo can cause asphyxiation. Molds that grow when air is allowed to reach cured silage can cause toxic organic dust syndrome. The silage itself poses no special danger.
Nutrition The ensiled product retains a much larger proportion of its nutrients than if the crop had been dried and stored as hay or stover. Silage is most often fed to dairy cattle, because they respond well to highly nutritious diets.
Since silage goes through a fermentation process, energy is used by fermentative bacteria to produce volatile fatty acids (VFA), such as acetate, propionate, lactate, and butyrate, which preserve the forage. The result is that the silage is lower in energy than the original forage, since the fermentative bacteria use some of the carbohydrates to produce VFA. Thus, the ensiling process preserves forages, but does not improve the quality or the nutrient value.
See also
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