What is a spontaneous combustion hay fire?

There is a tragedy that strikes farmers across the United States. It seemingly comes out of nowhere, but once it starts it is impossible to stop. This phenomenon is the spontaneous combustion of hay fire. It occurs when wet hay is put up and is often compounded by stacking. The damp hay generates bacteria and molds. This chemical reaction produces heat raising the temperature of the bale. As this chemical reaction continues and the heat increases, the hay becomes susceptible to inflammation. With the wrong amount of oxygen and high temperatures, the hay will ignite spontaneously combusting. 

If a farmer is unsure about the temperature of a hay bale this can be checked with a simple candy thermometer. If the temperature of the bale is above 150 degrees Fahrenheit it is hitting dangerous levels. You may be wondering how common these types of fires are. In the United States in 2021 there were 113 recorded barn fires. Of these fires, less than 14% were categorized as an obscure cause and only a small percentage of those were spontaneous combustion. Given these low numbers nationwide, it is a strange occurrence to see three spontaneous combustion fires on Utah farms as we did in the fall of 2021. It is even more rare to have seen two hay fires ignite in the same week as we did last Thanksgiving. 

These occurrences can likely be traced back to a Utah summer storm and humidity. On August 18-19 of 2021, there was a rainstorm that hit most of Utah. This storm was bookended with a month of sunshine and likely seemed like a drop in the bucket of an otherwise dry summer. An event such as this occurring on cut hay could cause the kind of moisture that leaves a crop susceptible to mold and eventually spontaneous combustion. Hay can also absorb moisture from the water around the bales after it is stacked and put up for the winter.

If wet hay is baled and stacked this can generate heat quicker, but can also delay a fire due to a lack of oxygen. The typical hay fire occurs within 3-7 days of the hay being put up. If the hay is disturbed as bales are removed, and the stack exposed to an infusion of oxygen, this can result in an unexpected fire. After 15 days the temperature of the hay begins to decrease and over 60 days the hay will cool to safe levels. These two Thanksgiving fires are certainly unusual but could be the result of something as simple as an ignored August rain.

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