Farmers have been advised to be wary of building straw stacks near buildings, crops and trees after hundreds of bales were destroyed by four separate fires in 24 hours.

About 60 firefighters were called into action in the early hours of this morning to deal with more than 400 tonnes of straw alight at Massingham Heath, and 600 bales at Etling Green, near Dereham.

The Massingham fire had six pumps in attendance at one point, while Etling Green involved a dozen crews over a period of almost eight hours to bring the blaze under control.

And later this afternoon, there were two more, with appliances from Dereham and Fakenham called to extinguish 600 tonnes of burning straw on High house Road in Beetley, and crews from Brandon and Thetford able to allow another stack to safely burn itself out in Methwold.

Stuart Horth, head of community safety for Norfolk Fire and Rescue Service, said all four incidents were still under investigation, and their cause could not yet be confirmed.

But he said farmers needed to take precautions against all the possible causes of such fires.

'There are several things which could happen,' he said. 'It can be a piece of glass in the sun which acts like a magnifying glass and sets fire to things. It could be self-combustion – inside older bales if they are slightly damp the biological action can release heat and make them more combustible. Then there is the possibility of accidental ignition, maybe from a cigarette butt, or it could be arson.

'There is a range of things that farmers could do. Firstly they need to make sure these stacks are built safely so they are not going to fall over – that's more about safety than about fire. They should be kept away from buildings and anything else at risk. With Massingham, the fire was near a row of trees, which then becomes a concern that it could have gone into a field of standing corn.'

Mr Horth said the firefighters would weigh up the risks of allowing straw fires to burn out, against whether to extinguish them with water.

'We can choose to put it out, which takes a lot of water and causes run-off which can cause environmental issues, so unless it is really close to something we need to protect, we often let it burn, as we did with Massingham,' he said. 'Or if it is burning more slowly we can ask the farmer to use his machinery to make breaks in the straw, while we protect him.

'Fire can always carry a risk. We try to manage that risk, and when we are comfortable that a stack is relatively safe, we often hand it to back to the farmer while it is still burning.'

Have you witnessed or photographed a straw stack fire? Email newsdesk@archant.co.uk.