Dorset & Wiltshire Fire and Rescue has issued a reminder to farmers to “keep an eye” on freshly baled hay over the coming month due to fire risk.

The fire service said wet hay is more likely to lead to spontaneous combustion than dry hay and warned that if hay is put into a barn or stack when it has more than 22% moisture, it loses forage quality and has an increased risk of catching fire.

“High moisture hay stacks can have chemical reactions that build heat. Hay insulates, so the larger the haystack, the less cooling there is to offset the heat,” Dorset & Wiltshire Fire and Rescue said.

“When the internal temperature of hay rises above 130°F (55°C), a chemical reaction begins to produce flammable gas – and this can ignite if the temperature goes high enough.

“Hay fires generally occur within six weeks of baling. Heating occurs in all hay above 15% moisture, but generally it peaks at 125-130°F, within three to seven days, with minimal risk of combustion or forage quality losses.”

Temperature within a stack then declines to safe levels in the next 15 to 60 days, depending on bale and stack density, ambient temperature and humidity, and rainfall absorbed by the hay.

Hay fires

To avoid hay fires, the fire service said small, rectangular bales should not exceed 18-22% moisture, and large round or rectangular bales should not exceed 16-18% moisture for safe storage.

In addition, farmers are urged to check their hay regularly.

“If you detect a slight caramel odour or a distinct musty smell, chances are your hay is heating. At this point, checking the moisture is too late, and you’ll need to keep monitoring the hay’s temperature,” Dorset & Wiltshire Fire and Rescue said.

“A simple probe inserted into the haystack can accurately monitor temperature. You can make a probe from a 10ft piece of pipe or electrical tubing.

“Sharpen the pipe or screw a pointed dowel to one end, then drill several 1/4-inch diameter holes in the tube just above the dowel.”

“Drive the probe into the hay stack and lower a thermometer on a string into the probe. The thermometer should be left for 10 minutes in several areas of the stack to ensure an accurate reading.”

Watch for the following temperatures:

  • 65°C – this is the beginning of the danger zone. After this point, check temperature daily.
  • 70°C – the temperature is getting more dangerous. Measure temperature every four hours and inspect the stack.
  • 80°C – hay temperature is becoming critical. Start to make the hay wet and remove it from the barn, or dismantle the stack away from buildings and other dry hay.
  • 85°C – you are likely to find hot spots. Flames will likely develop when heating hay comes in contact with the air.
  • 100°C – critical temperature, where hay is very likely to ignite. Call 999 immediately.

To reduce the risk of barn fires, farmers are urged to remove hay from fields as soon as possible after harvesting and ensure it is dry before storing to prevent spontaneous combustion.

Fires can spread easily, Dorset & Wiltshire Fire and Rescue said, so farmers are urged to store hay and straw away from other buildings, especially those containing fuels or chemicals and separate from livestock or equipment.

“Store hay and straw in stacks at least 10 metres apart and ensure there is sufficient space between the top of the stack and any electrical roof lighting.

“Replace halogen lights with LED ones.”