Every hay season several hay stacks spontaneously combust across Victoria, in single moist bales or in a stack of any size.

Once the hay has been stacked it's important to regularly monitor it for heating and act quickly.
Unfortunately, even with technological advances in machinery for harvesting conserved fodder, there is still nothing cost effective to help monitor the internal temperatures of haystacks.
So, it's back to some very simple techniques to keep an eye on your hay stacks.
Watch for signs of heating
Keep an eye out for signs that haystack is heating. Steam condensation on the shed roof, mould growth, acrid fumes and hot, humid air at the top of the stack are all good indicators.
When building stacks with suspect wet hay, stack the bales loosely to allow air movement and spread the stack over a wider area, for example, over several bays, thereby reducing heat build-up.
Most heating will occur over the first one or two weeks but has been known to reach 'flash point' up to eight weeks after baling. Monitor the stack temperature regularly.
Determine the stack temperature?
Stack temperature can be measured using a crowbar inserted as deep as possible into the stack and left for two hours. After two hours, the crowbar can be removed and felt by hand to give you a rough guide of the internal stack temperatures.
The temperature of the bale is up to about 50 degrees Celsius if you can hold the crowbar without discomfort. It is up to 60 degrees Celsius if the bar can be held for a short time only and up to 70 degrees Celsius if the bar can only be touched briefly.
If you can't hold the bar, temperature is above 70 degrees Celsius and fire is a real possibility.
Treat the stack with caution
In the past it has been suggested to push a small pipe into the stack and lower a small thermometer to the end of the pipe to measure the temperature of warming hay.
However, this technique should not be used for severely heating hay as It may oxygenate the inner section of the stack and cause a fire to start.
What can be done if the stack heats to danger levels?
If a stack starts to get 'dangerously hot' pull it apart as quickly as possible.
Be aware that as the bales are pulled out of the stack, especially in a tightly built stack, oxygen will now get access to the hot spot and potentially cause ignition.
Make sure that water is readily available and suitably qualified people able to assist in fire suppression if required.
Safety warning
Avoid walking on the stack as the extremely high temperatures may have charred the centre of the stack. Any extra weight on the stack may cause the stack to collapse into the dangerously hot centre and cause serious injury.
If there is the slightest risk of a fire starting due to wet hay, never keep machinery in the hayshed and don't allow children to play in or near heating stacks.
The Country Fire Authority website has good information about hay fire risk. Search 'hay fires' at www.cfa.vic.gov.au/
For information about dry seasonal conditions visit agriculture.vic.gov.au