Hay may look innocuous, but beneath the surface it can become a ticking time-bomb if conditions allow.
In a recent article for Agriculture Victoria, Andrea Henry explains how spontaneous combustion in hay occurs when internal heat builds up through microbial and chemical processes. Unlike visible spark-ignited fires, these fires start deep within a haystack and often smoulder undetected until they burst into flame.
The risk factors are well defined. Hay baled with more than 20% moisture sparks mould growth and fermentation. Dense packing traps heat and moisture; limited airflow prevents heat dissipation; warm, humid conditions and poor storage practices tip the balance. When high-moisture hay is stored in un-ventilated barns or sheds, these systems combine to create the perfect environment for internal ignition.
Early detection is critical to avoid catastrophe.
The Agriculture Victoria article lists key warning signs:
- steam rising from the stack,
- a musty or caramel-like smell,
- visible mould growth,
- condensation under roofing, or
- changes in the shape of the stack.
Farmers are advised to use a temperature probe or even insert a metal rod through the stack: if the rod feels hot afterwards, that’s a red flag.
Prevention is where the real control lies.
Thorough curing before baling is emphasised: ideal moisture levels differ by bale type (small square bales 18-20%, large square 12-15%, round 15-18%).
Storage tactics are also key: monitor temperatures and humidity (with probes or sensors), ensure good ventilation, separate suspect bales, limit stack size, leave space for airflow, protect hay from moisture ingress. Modern sensor-systems that provide real-time hay temperature and humidity data are highlighted as effective tools.
If you detect heating hay, swift action matters.
The advice: pull the stack apart to release heat and moisture, move bales outdoors if feasible, and call 000 if smoke or fire is visible.
The article cautions strongly: never attempt to extinguish a hay fire alone – these fires can reignite and spread rapidly.
Ultimately, spontaneous combustion of hay is preventable; understanding the underlying biological and chemical processes and adopting sound curing, monitoring and storage practices can protect property, livestock and livelihoods.
Henry, A. (2025, 10 November). The science behind hay fires. Agriculture Victoria. https://agriculture.vic.gov.au/about/media-centre/media-releases/2025-releases/the-science-behind-hay-fires
