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 corn (maize). 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.

Table of contents
1 About silos
2 Making Silage
3 Fermentation
4 Silo effluent
5 Safety
6 Nutrition

About silos

Silos must be airtight or the silage will spoil. They must provide a means for the silage to be firmly packed to minimize the amount of air present.

Three types of silos are in widespread use today:

1. Tower silos
2. Bunker silos
3. Bag silos.

A concrete stave silo, a type of tower silo

Tower silos are cylindrical structures, typically 12-24' (4-8m) in diameter and 30-80' (10-25m) in height. They can be made of many materials. Wood staves, concrete staves, cast concrete, and steel panels have all been used, and have varying cost, durability, and airtightness tradeoffs. Tower silos are usually unloaded from the top of the pile, originally by hand using a pitchfork, and most often today using mechanical unloaders. Bottom unloaders have been used at times, but have problems with difficulty of repair, and will not work when the silage sticks to the walls of the structure and will not fall down.

An advantage of tower silos is that the silage tends to pack well due to its own weight, except in the top few feet.

Bunker silos are trenches, usually with concrete walls, that are filled and packed with tractors and loaders. The filled trench is covered with a plastic tarp to make it airtight. These silos are usually unloaded with a tractor and loader. They are inexpensive and especially well suited to very large operations.

Bag silos are heavyweight plastic bags, usually around 6-8' (2-3m) in diameter, and of variable length as required for the amount of material to be stored. They are packed using a machine made for the purpose, and sealed on both ends. They are unloaded using a tractor and loader or skid-steer loader. The bag is discarded in sections as it is torn off. Bag silos require little capital investment. They can be used as a temporary measure when growth or harvest conditions require more space, though some farms use them every year. Their distinctive shape and normally white color has led them to be called "tractor eggs" in Germany and Scandinavia.

Making Silage

Silage must be made from plant material with a suitable moisture content, which ranges from about 55%-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 hay-type crops, the hay is mowed and allowed to wilt for a day or so until the moisture drops to an acceptable level.

Then, the plant material is collected, chopped into pieces about 1/2" (1 cm) long, and packed into the storage structure. 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 choppers that collect and chop the plant material, and deposit it in trucks or wagons. For tower silos, a stationary silo blower is used to blow the chopped plant material into the silo.


Silage undergoes fermentation, typically beginning about 48 hours after the silo is filled. The fermentation is essentially complete in about two weeks. It is an anerobic process that converts sugars to acids and exhausts any oxygen present in the crop material.

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.


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.


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.