Hay fires usually occur within six weeks of baling, but they may occur in hay several years old. Fire can occur in loose hay, small bales, large bales or in stacks. The fires can occur in hay stored inside as well as in hay stored outside. Regardless of when or where the fires occur, the most common cause is excessive moisture. Freshly cut forage materials are not dead.
Some respiration continues and a very small amount of heat is produced.
This heat generated by respiration is probably of little consequence except that it may help provide proper conditions for growth of bacteria. As the moisture content of the crop decreases during the curing process, respiration will slow and eventually cease. Forage crops are always contaminated with countless microorganisms. These microorganisms are no problem when the hay is harvested and cured to the proper moisture content before baling and storage. After baling, however, a small supply of air and a favorable moisture level cause the microorganisms to begin to feed and multiply, generating heat in the process.