Understanding Hay Fires

Courtesy of PennState Extension

Hay fires are unique to the horse and agricultural industry. Baled hay can be its own fuel and ignition source. The majority of hay fires occur within 6 weeks of baling, usually caused by excessive moisture in the bale. Ideal moisture range for hay at baling is 15 to 18 percent. Even after grass and legume forages are harvested, plant respiration continues and generates a small amount of heat. In properly harvested forages, respiration decreases and will eventually cease during drying and curing.

The heat of respiration is normal and under appropriate curing is inconsequential. However, if moisture levels are too high, the respiratory heat will provide an environment suitable for the already-present mesophilic microorganisms (that require moderately warm temperatures) to grow and multiply. As these microorganisms grow, heat is produced as a byproduct of their respiration and reproduction.

Once the bale interior reaches temperatures of 130 to 140oF, the environment becomes unsuitable for these organisms and most die. If microorganism activity declines, the interior bale temperature also declines. This cycle may be repeated several times, but the maximum temperature will be lower each time. Hay that has sustained these heat cycles has lost its quality as a feeding source, but poses no threat as an ignition source.

Baled hay becomes a potential fire hazard when the interior bale temperature does not cool after the first heating cycle. If conditions are favorable, the heat created by the mesophilic organisms provides an environment for thermophilic, or heat-loving, microorganisms to take over. When the thermophilic micro-organisms begin to multiply, their heat of respiration can raise the interior bale temperature to 170oF before they die from the heat. This is an extremely high temperature and can cause the bale to ignite if oxygen is present. The growth of microorganisms within the hay bale creates a microscopic cavernous environment, similar to a sponge. The damaged material in the bale combines readily with oxygen and, in its already-heated state, can self-ignite quickly. A burning bale of hay may be difficult to detect because the inside of the bale burns first. Hay fires are very difficult to extinguish completely. The tightly laced forages prevent water from penetrating to the core. Only a forceful blast of water can penetrate deep enough to extinguish the fire.

Hay temperature monitoring can be done to ensure that bale temperatures never reach critical levels. Under less-than-ideal field curing conditions, hay may have been baled above the recommended 15 to 18 percent moisture level. Check newly baled hay twice a day for heat buildup.

A temperature probe is available at most farm supply companies (e.g., Nasco, Gemplers) and stores (Agway) from $12 to $20. If bale temperatures have reached 150oF, monitor the interior bale temperature frequently, as the temperature is most likely to climb. By the time the interior bale temperature reaches 175 to 190oF, a fire is about to occur, and at 200oF, a fire has already erupted.

An alternative to purchasing a temperature probe is to make one, using a metal rod 3/8 to 1/2 inches in diameter (Ogburn 1995). Drive the rod into the hay and let it stand for at least 15 to 20 minutes before removing it. If the temperature within the bale is less than 130oF, you should be able to hold the metal comfortably in your bare hand. If the bale has reached a temperature of 160oF or greater, the rod will be too hot to hold comfortably in your bare hands. If the rod is too hot, let it cool for a few minutes and then reconfirm by taking another sample. When hot hay bales are found, summon the fire department. Be sure to tell the dispatcher that you have hot hay bales that may ignite instead of saying that you have a hay fire. This will help the fire company in planning on how to deal with your situation.

Hay Storage Recommendations
There are plenty of theories about how to stack bales in a storage or mow. It is a good idea to stack bales on their sides with the stems of the cut hay running up and down. This allows convection ventilation of warm, moist air up and out of the bale. The greener or moister the hay, the looser it should be packed to allow cooling and curing without danger of mildew formation or combustion. Realize though that loosely packed bales are more prone to tumbling out of their stacked formation. Using pallets, or at least a layer of dry straw, under the bottom row will reduce storage losses from ground moisture. One strong recommendation to reduce fire hazard (with an added benefit of decreasing dust levels in the barn) is to store hay and bedding in a separate building from the horse stable.