Spontaneous combustion in hay is almost always due to uneven curing in the hay before it is stacked. Hay with a moisture content of less than 25% will not spontaneously ignite. However, a fire hazard is created more often, by small pockets of wet material within a bale.
It is important to ensure that all hay is well cured and dry before stacking. If the hay is from areas of lush growth, or (after rain) from low lying ground, extra diligence is required.
If you suspect a haystack is heating, an iron rod or crow bar can be thrust into the stack at intervals and allowed to remain undisturbed for about two hours. This will give an indication of the heat within the haystack.
Checking temperatures should be undertaken regularly. If the temperature is above 75° C, precautions should be taken in case the stack spontaneously ignites.
If temperature is high, the only solution is to pull the stack apart. Water and firefighting equipment should be on hard to extinguish a possible fire. Hot hay should be placed clear of other flammable material because fires can still occur up to 24 hours or longer from the time of removal from the stack.
As a guideline, the following guidelines in relation to the heat of the iron rod or crow bar are provided:
| 25° - 35° C | Comfortable |
| 40° - 50° C | Tolerable |
| 55° - 65° C | Tolerable to touch only |
| 70° and above | Too hot to touch |