Hay is one of the more studied materials in spontaneous combustion. As hay varies by the type of grass and location grown utilized in its preparation, it is very hard to establish a unified theory of what occurs in hay self-heating. It is anticipated that dangerous heating will occur in hay that contains more than 25% moisture. The largest number of fires occurs within 2 to 6 weeks of storage, with the majority occurring at 4 to 5 weeks.
The process may begin with microbiological activity (bacteria or mold), but at some point, the process has to become chemical. Microbiological activity will also limit the amount of oxygen available in the hay. Moisture appears to be quite important, no matter what process. At 100 °C, wet hay absorbed twice the amount of oxygen of dry hay. There has been conjecture that the complex carbohydrates present in hay break down to simpler sugars, which are more readily oxidized.