Fermentation is inevitable for fresh hay, and spontaneous combustion in hay bales is its unpredictable consequence. Hay moisture and harvesting weather change from year to year, even from field to field. It is a real risk for even the most experienced and most cautious farmer.

Fermentation causes the hay bales heat up and its intensity depends on, for example, the moisture content or the species of the hay. Normally the temperature rises to around 40°C, but temperatures above 50°C are not rare either. In some cases, the temperature can continue to rise up to 70°C and above. At such temperatures the hay has already entered a danger zone and spontaneous combustion becomes more and more likely.

Spontaneous combustion (auto-combustion, self-combustion) is a natural phenomenon in fresh hay. This chemical process occurs when damp hay heats up and ignites. If the hay moisture content is greater than 13-15 % at the time of baling, the damped compressed hay begins to ferment. This fermentation process results into production of heat and flammable gases that eventually climb to a temperature higher than their ignition point. As the fermentation process continues, the hay bale temperature can reach up to 80°C. Past this threshold, any presence of oxygen (e.g. air draft) triggers spontaneous combustion and the hay bale is set on fire, resulting in a damaging fire in the storage facility.

A single overheated bale can spontaneously combust and burn down the entire hay storage, along with any nearby property: buildings, vehicles, machinery, etc. The value of the material damages of a hay fire is typically at least €200,00 to 300,000. Fire insurance is essential, but it can never compensate all the losses, particularly immaterial ones. It can take years to recover, both financially and psychologically.

The temperature can rise from a safe to a dangerous level overnight. Occasional manual measurements can easily miss these rising temperatures, whereas continuous monitoring detects the hot bales in their early stages, and precautions can be taken long in advance. Monitoring of even a part of the haystack will increase the situation awareness compared to manual measurements.

 When overheating is detected in time, it is possible to take preventive actions: moving the heated bale off the stack and promoting air circulation to cool the hay. However, for temperatures exceeding 80°C, the bale must not be disturbed any further, as movement may cause air to enter into the bale, setting fire to the hay.

Temperature measurements during the fermentation period are necessary to identify potential danger. By daily sampling with a manual thermometer you can already get an indication of the phase of the fermentation process. Manual temperature measurements, however, are laborious and they can only reach the outermost hay bales in the stack. With wireless sensors the temperature measurements reach from the outermost to the innermost hay bales in the stack.