Jason Hartschuh, OSU Extension Educator ANR, Crawford County
(Previously published in the C.O.R.N. Newsletter 15-2021)
(Image Source: Alabama Cooperative Extension System)
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°F – 140ºF with temperature staying high for up to 40 days. As temperatures rise, thermophilic bacteria can take off in your hay and raise temperature into the fire danger zone of over 175°F.
Assessing Your Risk
If hay was baled between 15% – 20% moisture and acid preservatives were used, there is still potential for a hay fire but not as great as on non-treated hay. A moisture tester on your baler can help you know how moisture varies across your field and when to use hay preservative. Without a moisture tester, if you occasionally find darker green damp spots or humidity is high, be sure to monitor for heating. Most propionic acid-based products are effective at inhibiting bacteria growth in hay up to 25% moisture, with variable effectiveness at 25% – 30% moisture, if applied at the correct rates.
Temperature Assessment
| Temperature (°F/ °C) | Action |
| 125°F/51.6°C | No Action needed |
| 150°F/65.6°C | Hay is entering the danger zone, check temperatures twice per day. Disassemble haystacks moving bales outside to allow air circulation to cool the hay. |
| 160°F/71.1°C | Hay has reached the danger zone. Carefully check 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. |
| 175-190°F/79.4-87.8°C | 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 fire service, remove hot hay. Be aware that bales may burst into flames, keep tractors wet so the tractor does not catch fire. |
| 200°F+/93.3°C+ | Fire is present with in the haystack near the temperature probe. With the assistance of fire service, remove hot hay. If possible, inject water into the hot spot to cool 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 options available to monitor hay temperature. One of these is high technology, like the cables that can be used to monitor temperature in stored grain. There are a couple 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 signal.
If you believe that you maybe 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 increased to 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 3/8-3/4 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 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 175ºF 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 and well drying hay season this year!