Nearly every well-equipped farm has at least one silo—a tall cylindrical structure in which slightly fermented fodder is stored in a controlled environment for use as animal feed. This stored fodder, called silage, or ensilage, rivals fresh feed in providing the nutrients necessary for livestock.
To prepare silage, forage plants, such as corn, legumes, and grasses, are harvested in early maturity, before they have begun to dry out and before their protein content has greatly decreased. The plants are chopped into lengths fine enough to ensure good packing and to prevent the formation of large air pockets, which would cause the fodder to spoil.
Inside the silo, the green, moist fodder turns mildly sour. It is still edible by farm animals, but the acids produced by this fermentation process check the growth of microorganisms that would otherwise cause the silage to decay. Under proper storage conditions, the silage keeps for several months.
A common type of silo is a cylindrical tower 30 to 50 feet (9 to 15 meters) in height. It is often made of steel with a fused-glass or porcelain inner surface for corrosion resistance, though silos may also be made of other metals, masonry, or wood. A silo may be a horizontal pit, a trench, or a stack above or below the ground. To keep air out, the structure may be sealed with earth, canvas, heavy paper, or plastic. The silage must be removed from the silo at a certain rate per day to prevent spoilage.