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Hay Fires

Hay fires can be caused by lightening striking nearby trees or fences, arson, contact with electric fences, sparks from cigarettes or welding equipment and spontaneous ignition.  Fires caused by spontaneous ignition can be prevented by good storage practices and checking the stored hay’s temperature. 

 Hay fires usually occur within six weeks of baling but they can also occur in hay that is several years old.  Fire can occur in loose hay, small bales, and large bales or in stacks.  Hay fires can start in hay that is stored inside as well as hay that is stored outside.  The most common cause of hay fires is excess moisture.

High moisture in 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.  When the internal temperature of hay rises above 130 degree F, a chemical reaction begins to produce flammable gas that can ignite the hay if the temperature continues to increase.

Freshly cut forage material is not dead.  Some respiration continues and a very small amount of heat is produced.  This heat, generated by respiration is probably of little consequence except that it may help provide proper conditions for growth of bacteria.  As the moisture content of the crop decreases during the curing process, respiration will slow and eventually cease.

When the temperature reaches about 130 degrees F and if there is some thermophilic (heat loving) bacteria present, that can lead to a second heating phase. This heating phase could generate enough heat to raise the temperature as high as 160 to 170 degrees F before bacterial activity ceases.  The bacteria and the heat they generate convert the hay to a form similar to a carbon sponge with microscopic pores.  In this form and at the high temperature, the material combines readily with oxygen and it can self-ignite in the presence of air.

To determine how hot it is inside a stack of hay, you should use a probe and thermometer.  A probe can be made from a 10-foot piece of pipe or electrical tubing.  Sharpen the pipe or screw a pointed dowel to one end, then drill several ¼-inch diameter holes in the tube just above the dowel. The probe can be pushed into the stack and then the thermometer can be lowered to the end of the probe on a light wire.  If the probe is horizontal, use a heavier wire to push the thermometer into the probe.   Leave the thermometer for 10 to 15 minutes, then remove the thermometer and read the temperature. 

If you detect temperatures above 175 degrees F, a fire is imminent.  If your hay stack has reached 175 degree F or higher or should you see fire or smell smoke, call the fire department immediately. The experts advise not to move any of the hay at this point since this would expose the over heated or smoldering hay to oxygen and may result in a fire that can rage out of control.

A caution, the experts say that when you’re checking the hay stack, you should put boards or plywood on top of the hay before walking on it.  This will help prevent you from falling into burned out pockets beneath the hay surface should there be a fire that hasn’t been detected yet.  It is also advised to tie a life line/rope around your waist and to have a second person on the other end in a safe location to pull you to safety should the surface collapse. 

Temperature Interpretations

Below 130 degrees F - No problem

 130-140 degrees F -   No problem, but check for the temperature to go up or down. t

150 degree F - Temperatures will most likely continue to increase. Move the hay to provide air circulation and cooling.  Monitor temperature often.                                                                                             

175 -190 Degrees F - Fire is imminent or may be present a short distance from the probe. Call the Fire Department.

200 Degrees or above - Fire is present at or near the probe. Call the Fire Department. Inject water to cool hot spots if you are able to do so safely.