Each year hay fires damage or destroy buildings and equipment as well as consume valuable feed supplies. Electrical problems, over heated exhaust pipes or just a thoughtless passerby are often the cause. But spontaneous combustion causes a lot more fires than you might think.

It may seem ironic, but wet hay is more likely to lead to a spontaneous combustion fire than dry hay. If hay is put into a barn or stack when it has more than 22 percent moisture, not only does the hay lose forage quality, but it has an increased risk of spontaneous combustion. High moisture haystacks can have chemical reactions that build heat. Hay insulates, so the larger the hay stack the less cooling there is to offset the heat.

When the internal temperature of hay rises above 130 degrees Fahrenheit a chemical reaction produces a flammable gas that can ignite if it gets hot enough.

To avoid hay fires, small rectangular bales should not exceed 18 percent to 22 percent moisture when stacked. Large round or rectangular bales should not exceed 16 percent to 18 percent moisture for safe storage.
Check your hay regularly. If you detect a slight caramel odor or a distinct musty smell, chances are your hay is heating. At this point, checking moisture is too late, and you'll need to keep monitoring the hay's temperature.