Silage is fermented, high-moisture forage to be fed to ruminants, cud-chewing animals like cattle and sheep. It is fermented and stored in a structure called a silo. Silage is most often made from grass crops, including corn (maize) or sorghum. Silage is made from the entire plant, not just the grain.

Silage can also be made from many other field crops, and other names (oatlage for oats, haylage for alfalfa) are sometimes used when this is done. Sometimes a mixture is used, such as oats and peas.

Making Silage

Silage must be made from plant material with a suitable moisture content, which ranges from about 55% to 70% depending on the construction of the storage structure and hence the degree of compression and the amount of water that will be lost during 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 drops to an acceptable level. In New Zealand and Northern Europe it is harvested directly from the paddock by a tractor-mounted silage chopper, blown into a trailer-mounted cage and transported to the silo or heap.

In the US, the plant material is collected, chopped into pieces about 1/2" (13 mm) long and packed into the storage. In the early days of mechanized agriculture, cornstalks 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 units that are pulled by a tractor, or they can be self-propelled machines. Harvesters blow the silage into the wagon via a chute at the rear or side of the machine.

Once a wagon is full, the wagon is taken back to the farm. For tower silos, the silage is emptied into a stationary blower which blows the silage up into the silo. 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 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 fed out from the bottom in winter. The disadvantage is that it requires considerable effort to compress the stack in the silo to cure properly. So much of the silage made there is stacked in a heap on the ground called a clamp and compressed with a tractor rolling over the top as it is stacked. The result is a fat pancake-like stack. It is much easier to get the cured silage from such stacks in winter when the ground is very wet because the tractor can access all sides of the heap.

Fermentation

Silage undergoes anaerobic fermentation, typically beginning about 48 hours after the silo is filled. Traditionally, the fermentation is caused by indigenous microorganisms; today, some silage is innoculated with specific microorganisms to speed the fermentation or to improve the resulting silage. The process converts sugars to acids and exhausts any oxygen present in the crop material. The fermentation is essentially complete in about two weeks.

Silo effluent

The fermentation process releases a liquid. The amount of liquid can be excessive if there is too much moisture in the crop when it is ensiled. Silo effluent contains nitric acid (HNO₃), making it corrosive. It also can be a contaminant of lakes and streams, since the high nutrient content can lead to algae blooms.

Storing silage

Silage must be protected from oxygen or it will spoil. Silage must be firmly packed to minimize the amount of air present.

Safety

Silos are hazardous, and people die every year in the process of filling and maintaining them. The machinery used is dangerous, and with tower silos workers can fall from the silo's ladder or work platform.

There are also respiratory hazards from the fermentation process itself. Nitrous oxide gas is released in the early stages of fermentation, and can kill. The reduced oxygen environment inside the silo can cause asphyxiation, and molds formed when air is allowed to reach cured silage can cause toxic organic dust syndrome.

Nutrition

The ensiled product retains a great deal of the nutrients present in the plant, much more so than if the crop were dried and stored as hay or stover. Silage is most often fed to dairy cattle, because they respond well to highly nutritious diets.