It defies what we know of fire… something is wet enough to catch on fire? Doesn’t make sense? But that’s just the case for a hay bale.
We need three things for a fire to start… heat, fuel and oxygen. We meet all these requirements here.
Let’s talk about the heat process… of course the sun heats the hay bale but there is another process that helps it get even hotter… and that’s moisture.
For a bit after it’s cut down, hay is still going through it’s respiration process. This is when a plant is producing it’s own energy by intaking oxygen and releasing carbon dioxide. All plants go through this process. A byproduct is moisture. Dew and fog can also increase the moisture content.
Water has a high heat capacity, meaning it can aid the hay bale into heating up to well over 140 degrees inside. A moist hay bale will be hotter than a dry hay bale.
Of course the fuel is the actual hay itself and the fire typically starts on the outside of the hay bale because that’s where the oxygen is.
This is all a recipe for a combustable hay bale. The danger zone for a fire to occur is around 140 degrees. The best way to prevent a fire is to monitor the internal temperature. You should take action at around 120 degrees and move the bales away from stuctures and out of barns. According to the National Ag Safety database, hay fires usually occer within 6 weeks of baling but fires have been known to start in several year old bales.