Hay and silage are the forms in which harvested forage is most often stored. The two differ in terms of how they are stored and their moisture content. While hay is stored at a moisture level below 20 percent, silage is stored at 40 to 60 percent moisture level. When hay is stored at moisture levels greater than 20 percent, the forage becomes moldy and may suddenly catch fire.
Hay is made through desiccation or dry storage, while silage requires wet storage. Storage of forage as hay uses the acidifying power of lactic bacteria, which reduces the pH to around 4, below which all chemical reaction and fermentation stops. When making hay, green forage is cut and dried as quickly as possible. Drying can be natural, such as with exposure to the sun on the ground. Artificial circulation can also be used to dry the hay. When there are no rains, sun-drying requires 2 to 3 days.
Silage is preserved in an anaerobic environment with a pH of 3.6 to 5.0. This can be done in specialized structures or wrappings. Silage is made through a fermentation process at moisture levels between 40 and 85 percent, which preserves forage in its wet state away from air. Wilted silage has a moisture level of 60 to 75 percent, while unwilted silage has a moisture level of 70 to 85 percent.