High-moisture haystacks and bales can catch on fire because they have chemical reactions that build heat. ... When hay's internal temperature rises above 130 degrees Fahrenheit (55 degrees Celsius), a chemical reaction begins to produce flammable gas that can ignite if the temperature goes high enough.

Can hay just catch on fire by itself?

Plant reactions and bacterial growth combine to raise temperatures in even the driest hay bales, although most never catch fire on their own. However, when hay is packed tightly, and enough moisture gets in, the conditions may be right for spontaneous hay combustion.

Can hay bales explode?

STILLWATER — It might sound strange, but with no apparent ignition source, hay bales can burst into flames and cause much damage to storage facilities or spark a raging wildfire. ... At 20 percent moisture, the hay is considered dry. However, mold will grow through respiration and produce heat until that point.