Hay fire risk

Spontaneous combustion occurs about 4 – 14 weeks after baling normally but can occur earlier or up to four months later on some occasions. If the stack heats to excessive levels action is necessary to prevent the possibility of spontaneous combustion. Haystacks should be monitored for heating within days of stacking. If it does heat then monitoring should continue until heating subsides.

There has already been some hay fires this season. So why have these occurred, and how we can manage this risk in the future? An understanding of the causes of the problem will allow us to be more proactive in addressing the risks.

Currently the simple measurement of moisture is virtually the sole assessment of suitability to bale, and likelihood of risk to follow. This can take the form of probes, visual examination or microwave technology to mention a few means.

(Source: ABC)

Never the less, even an accurate moisture reading will not be a great guide to all risk of hay heating.

Historically this is a (overly) simple view of how baling was assessed

 Moisture level   




Do I bale?




As in some previous years, this year moisture alone may not be an accurate means of judging risk. Why is that?

The real reason that hay heats and burns is microbial action.

Moisture measures are just indicators of likely microbial action. The moisture itself is not causing heat. The heat is a result of oxidation of plant nutrients by fungi, mould and yeast.

Baled Hay naturally contains millions of bacteria and mould fungi that produce heat as they consume hay nutrients....usually hay gets dry enough that microbes die or become dormant, but when moisture is present they persist, heating occurs , visual mould may be evident , or both.”
Bruce Anderson – University of Nebraska- Extension Forage specialist- ehay newsletter May 2008.

What has been apparent in similar dry years is a massive water soluble carbohydrate (WSC or plant sugars) level in drought affected crops. This is especially so in cereal crops. Levels in excess of 25% WSC may be common, and levels in excess of 30% are not unheard of. This is probably twice the normal level of WSC that we would see in cereal crops. Nutrient transfer from root mass to seed head is a natural process that occurs every year. In dry years crops may senesce before proper seed head development occurs, with the WSC level maintained in the stem (and leaves to a lesser degree).

The reason this is crucial in the context of hay fires is the nutrient source that this provides to the microbial population that is causing our heating. They are able to feast on the WSC, and even at quite low moisture levels are able to survive in a favourable micro environment and cause hay heating.

So if we accept that WSC are (along with moisture) another important driver of bug growth and hay heating, what would our risk matrix look like?


Do I bale?

 15% moisture

20% moisture 

25% moisture 

10% WSC



Some risk 

20% WSC 

Some risk 

 High risk

No baling

   30% WSC

High risk 

 No baling

No baling


Look for signs of heating and monitor temperature

If the hay moisture level is known to be high due to poor wilting conditions, or rain or heavy dew after baling, the shed should be monitored. Signs of possible heating include steam or condensation under the roof. Check for a caramel or burnt tobacco smell. Smoke indicates the problem is serious!

If in doubt about how whether WSC and moisture were OK at baling, monitor the temperature of a hay stack by driving a metal rod into the stack. Leav the rod in it for several hours and then check for warming (see the table below for more details). The metal rod can be left in place for ongoing monitoring or reinserted in other parts of the stack.

Note: One hot spot can start a fire and heat development is likely to be uneven across a stack, so the metal rod method can only be used as an indicator.

Some people have used a thermometer inserted down a pipe to measure temperature within the stack. This can be effective but risks introducing oxygen into a hot spot, which could then cause ignition. Specialist thermometers with a long probe can be custom-made or thermocouple wire placed in the stack as it is built.

Due to the many variables involved in spontaneous combustion it is not possible to be definitive about the temperature at which a haystack is definitely going to burn and therefore should be pulled apart to prevent a fire.


Temperature   How does the rod feel?   What does it mean?  

 50 C

 Warm to touch but doesn't burn   Feed quality is being damaged 

 60 C

 Uncomfortable to hold  Hotter than normal caramelising reaction, needs monitorinh

 70 C

 Touch only, too hot to hold

 Monitor regularly (several times a day), consider safety precautions.

Move hay. Most microbes are killed by extreme temperature.

 80 C

 Too hot to touch. Dangerous

Get help! Have water supply on hand before moving bales.

Bales could ignite if moved and exposed to oxygen

 100 C

  Critical. hay is likely to ignite

 240 - 280 C

   Ignition point, depending on hay type and oxygen availability

Information for this article was sourced from Ian Sawyer, Feedworks and Nsw Industry & Investment (Primefact 716).