Hay barn fires are a real hazard

By Jason Hartschuh, CCA, Allen Gahler, Mark Sulc, Ohio State University Extension

Mother Nature has been at it again, hardly giving us enough days to make dry hay with a risk of pop-up showers every afternoon. These conditions are very dangerous for hay producers. Since wet hay does just rot it may also burn. Hay fires are caused when bacteria in wet hay create so much heat that the hay spontaneously combusts in the presence of oxygen. At over 20% moisture, mesophilic bacteria release heat-causing temperature to rise between 130 degrees F to 140 degrees F with the temperature staying high for up to 40 days. As temperatures rise thermophilic bacteria can take off in your hay and raise the temperature into the fire danger zone of over 175 degrees F.

Assessing risk

If the hay was baled between 15% and 20% moisture and acid preservatives were used, there is still potential for a hay fire but not as great as on non-treated hay. Having a moisture tester on your baler can help you know the variability across your field in moisture and when to use hay preservatives. Without a moisture tester, if you find darker green damp spots occasionally or humidity is high, be sure to monitor for heating. Most propionic acid-based products are effective if applied at the correct rates at inhibiting bacteria growth in hay up to 25% moisture, with variable effectiveness at 25-30% moisture.

Temperature assessment

Temperature (°F/ °C)



No Action needed


Hay is entering the danger zone, check temperatures twice per day. Disassemble haystacks moving bales outside to allow air circulation to cool the hay.


Hay has reached the danger zone. Carefully check the hay temperature every few hours. Disassemble stacked hay to promote air circulation to cool hay be very careful of even hotter spots. Have a tank of water present while unstacking.


Hot spots or fire pockets are likely. Alert fire service to the possible hay fire incident. Close barns to minimize air movement around the hay. With the assistance of the fire service, remove hot hay. Be aware that bales may burst into flames, and keep tractors wet so the tractor does not catch fire.


Fire is present within the haystack near the temperature probe. With the assistance of the fire service, remove hot hay. If possible, inject water into the hot spot to cool the hay before moving. Most likely a fire will occur, keep tractors wet and fire hose lines charged in the barn and along the route of where bales will be stacked.


Monitoring the haystack

There are a couple of options available to monitor hay temperature. One of these is high technology, like the cables that can be used to monitor the temperature in stored grain. There are a couple of companies that produce cables that would be placed between bales in a stack or monitoring probes that are placed in bales and use radio frequency to transmit signals.

If you believe that you may be at risk for hay heating, monitoring temperature is critical.  It should be done daily until temperatures stabilize in the safe zone or reach 150°F when monitoring needs to be increased too twice daily. This can be done with technology or manual temperature probes. When monitoring hay temperature, be very cautious, hot hay can burn within the stack and cause cavities underneath that you can fall into. Use planks to spread out your weight while walking on the stack and have a harness system attached to the ceiling in case you fall into a burned-out cavity. Also, work in pairs with someone on the ground within voice range to assist you if you find yourself in a bad situation. Temperature monitoring should continue for possibly six weeks until values stabilize in the safe zone.

Temperature monitoring depends on the stack size but should be taken close to the center of the stack. In larger stacks ideally, this is 8 feet down in the stack. This can be done by purchasing a long probe thermometer or building your own. Building your own can be done with a 3/8 to 3/4 piece of pipe or electrical conduit cut into a closed point. The pipe size will depend on the thermometer probe size you will put in the pipe. A larger pipe can be used and a thermometer on a string is lowered into the pipe. Drill 3/16-inch holes in the bottom four feet of the pipe. Leave the thermometer in the stack for about 10 minutes to get an accurate reading. A less accurate method is to leave a pipe in the stack all day, and if a section is too hot to hold in your hand when removed you are at risk for fire. Or even better, use an infrared thermometer to measure the temperature of the pipe. Any time temperatures are above 175 degrees F hay should not be removed from the barn until the local fire department is present. You are at risk for fire. Once the fire department is present, hay should be carefully removed from the barn with charged fire hoses ready if spontaneous combustion occurs. Have a safe a well-drying hay season this year!   

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