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)
Moisture level |
15% |
20% |
25% |
Do I bale? |
Yes |
Maybe |
No |
Do I bale? |
15% moisture |
20% moisture |
25% moisture |
10% WSC |
Yes |
Yes |
Some risk |
20% WSC |
Some risk |
High risk |
No baling |
30% WSC |
High risk |
No baling |
No baling |
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 |