Rainy weather leading up to a hay harvest is more than just inconvenient — it could be downright tragic. That’s because in storage, wet hay can be a breeding ground for bacteria that release heat and cause mold formation. According to Purdue Extension, this kind of situation increases the risk of spontaneous combustion and, ultimately, barn fires.
In an article posted on Purdue’s website, Agronomy Professor Keith Johnson said that a moisture content of more than 20 percent without using a preservative is the start of the dangerous recipe, creating conditions with enough heat to where spontaneous combustion is possible (the red-alert temperature is around 175 degrees). If you do cut and bale under wet conditions, it could take as long as three or four weeks before spontaneous combustion occurs. In the meantime, touch the bales to check for heat, pay attention to steam rising from them, check for condensation on surfaces in the barn, and use a temperature probe on the hay if necessary. Air circulation is important to help keep your cut-crop cool.
In the article, Johnson offered a few recommendations to help speed drying after hay is cut. You can lay the hay in a wide swath with a mower-conditioner, exposing the hay to more sunlight. If drying continues to be a problem, consider trying tedding or windrow inversion.
Purdue has a Forage Field Guide, found here, that delves even deeper into hay combustion.