Agrical warns of potential increase in farm fires this summer due to spontaneous combustion

In the farming media this week we’ve been warning of a potential increase in barn fires this summer caused by spontaneous combusting hay bales because of the recent wet weather and we’ve been urging farmers to take practical preventative steps.

Nigel Collinson, our managing director said: “The chance of hay bales spontaneously combusting is higher when we’ve had a lot of rain – which we certainly have had over the last month.  If farmers bale the hay before it’s dried out enough, the risk of spontaneous combustion increases as it’s the moisture in the grass which causes the heat to rise inside the bale.”

“We know from having worked in rural loss adjusting for nearly 20 years that it’s a hard call for the farmer regarding the timing of baling as it’s not an exact science; it’s down to the farmer’s experience and knowledge.  There are so many variables and the timing of baling may be out of the farmer’s control to a certain extent, as it may rely on the availability of contractors.  We’re not trying to teach farmers to suck eggs but we’re urging them to think about the storage of bales which may pose a risk,” said Nigel Collinson.

Hay is not of universal quality because there are numerous types of grass, grown in various locations and subjected to different weather conditions – meadow grass retains more moisture than rye grass for example. Scientists have found it hard to establish a unified theory of why hay can spontaneously combust.  Dangerous heating can happen in hay that contains more than 20-25% moisture content and the largest number of fires happen within four to five weeks of storage.

The process begins with anaerobic microbiological activity within the baled hay which produces heat. If the activity continues through on-going chemical reactions fuelled by higher levels of moisture then the risk of combustion will increase markedly.

We’ve been urging farmers to take practical steps to prevent spontaneous combustion fires.  These are:


  • Try not to bale grass that is too wet or too green.
  • Stack the bales considered at risk outside and away from farm buildings.  Over time the risk decreases and bales can be moved into a barn.
  • Allow plenty of ventilation in the stack.
  • Keep checking the stack and test the temperature using an electronic probe.
  • If the stack has a musty smell or smells of caramel this is a sign it’s producing heat and the temperature should be tested.

Nigel Collinson continued: “It’s a balancing act for farmers and somehow they have to weigh up the risk of a potentially spoilt crop against baling the hay too early and having an increased risk of combustion within the stack.  In our experience many of the farm fires we’ll be called out to from early July to the beginning of August will be down to spontaneous combustion.  Bizarrely when we have a short spell of good weather the race is on to get the hay in and we experience more claims, conversely when the weather is unsettled, farmers have no option but to leave the hay out longer and we see less claims.”