Growers baling and storing hay that’s 20% or higher in moisture should diligently check its temperature to avoid possible hay fires. As stacked hay’s temperatures rise, the risk of spontaneous combustion also increases, warns Penn State Extension forage specialist Marvin Hall.

The slightest odor of smoldering hay, or a haystack that’s warm or hot to the touch, signals that a fire may already be burning, he says. The only way to determine the severity of the problem is to take temperature readings of the stack. Hall suggests the following guidelines:

As stack temperatures reach 150°F, check them twice daily. If possible, disassemble the stack to improve air circulation and cooling. Use caution if moving heated bales away from buildings or combustible material, as they can burst into flames when exposed to air. He suggests first wetting those bales down.

If a stack averages 160°, check its temperature every two hours. At 175°, a stack could contain hot spots or fire pockets. At this point, stop all air movement around the hay, if possible, and alert the local fire service.

At 190°, remove hot hay with the assistance of the fire service.