Hot hay, high risk

23 October 2026

Michele Jolliffe, Dairy Extension Officer, Hamilton

Harvest has started and so has the risk of heating hay. Every year, Victorian farmers lose hay, machinery and sheds to hay fires caused by heating bales. A few simple checks in the weeks after baling can help prevent a costly and potentially dangerous fire.

The good news is that with regular monitoring and early action, many hay fires can be prevented.

Don't assume the job is finished

Once hay is stacked, it's important to keep checking it over the following weeks. If hay is baled before all the forage has fully cured, a series of biological and chemical processes may cause the hay to heat up once it's baled.

Damp or green hay creates the perfect conditions for bacteria and fungi to grow. As these micro-organisms break down plant material, heat builds up within the stack. If temperatures continue to rise unchecked, the hay can eventually ignite.

In addition to fire risk, this process also reduces the nutritional value and quality of the hay.

If heating goes unnoticed, temperatures can continue to rise. Once hay reaches around 70°C, the risk of spontaneous combustion increases significantly.

Know what to look for

A heating stack will often show warning signs before it becomes a fire risk. Keep an eye out for:

• condensation on the shed roof
• mould developing on bales
• a strong, acrid smell
• steam rising from the stack
• hot, humid air around the top of the stack.

If some hay was baled a little wetter than ideal it should be stacked loosely to improve airflow. Spreading them over a wider area, rather than building 1 large dense stack, can also help reduce heat build-up.

Most heating occurs within 2 to 3 weeks of baling. However, some stacks continue heating and have reached ignition point 3 months or more after baling. Regular checks during this period are well worth the effort.

Check stack temperatures with a simple crowbar test

One of the simplest ways to check a stack’s temperature is with a steel crowbar. Push a steel crowbar as deep as possible into the stack and leave it there for around 2 hours. When you pull it out, carefully feel the temperature of the metal.

As a guide:

• comfortably warm to hold stack temperature is likely below 50°C
• too hot to hold for long: temperature may be approaching 60°C
• can only be touched briefly: temperatures could be around 70°C
• too hot to touch temperatures are likely above 70°C and there is a serious risk of fire.

If temperatures are reaching this level, act immediately and use extreme caution.

If the stack is getting too hot

If stack temperatures reach 60–70°C, pull the stack apart as soon as possible. This improves air flow and helps cool the bales. Have water, firefighting equipment, and experienced people on hand in case a fire starts.

If the stack is hotter than 70°C, or if you see or smell smoke, don’t pull the stack apart. Call Triple Zero (000) immediately.

Put safety first

One of the most important safety messages is simple: never walk on a heating haystack. The centre of the stack may have become charred and unstable. What looks solid from the outside can collapse without warning, causing someone to fall into extremely hot material and suffer serious injuries.

If there is any concern about heating hay:

• keep machinery well away from the stack
• don't store valuable equipment in the same shed
• restrict access to the area
• never allow children to play in or around haystacks.

After all the effort of making and storing hay, don't leave things to chance. A quick check each week could prevent the loss of valuable fodder, machinery, or sheds. If something doesn't seem right, act early.

For more information about hay fire risks, visit the CFA website and search ’hay fire prevention.’ For further information on hay making, visit agriculture.vic.gov.au or call 136 186.

Media contact: media@deeca.vic.gov.au