There are two basic causes of hayshed fires.
The first is due to accident and poor preparation. In hot weather dry hay is easily set alight by sparks from machinery, hot exhausts or cigarettes.
Hay is also vulnerable to bushfires or grassfires unless care is taken to prepare a firebreak before hot weather.
The second cause is spontaneous combustion, where moist hay naturally generates enough heat to burn.
Spontaneous combustion occurs when moist hay heats up due to a combination of natural microbial and chemical reactions.
If enough air (oxygen) is present the moist hay may get hot enough to ignite, with fire then spreading through the stack or shed.
Factors which affect heating in hay include moisture, air temperature, humidity, bale size, density and stack arrangement and crop type, which influences water-soluble carbohydrate (simple sugar) levels.
As a general guide, hay which has more than 16 to 18 per cent moisture is at risk of heating or going mouldy, which will affect nutritional quality of the feed.
With more than 20 per cent moisture (80 per cent dry matter) the risk of heating increases, and if hay is baled in the range 25 to 35 per cent moisture (75 to 65 per cent dry matter), then spontaneous combustion is likely.
Risk of heating is also affected by bale size, density and how hay is stored.
Smaller bales have a relatively large surface area, so can dissipate heat.