It might sound strange, but with no apparent ignition source, hay bales can burst into flames and cause much damage to storage facilities or spark a raging wildfire.
 
The combination of low wind speeds, high humidity, moist hay and hot temperatures is a recipe for disaster. With this set of circumstances, tightly stacked hay bales have been known to combust. Without proper preventative measures, hay barns or any other structures close to the bales, would be lost.

Hailin Zhang, director of the Oklahoma State University Soil, Water and Forage Analytical Laboratory with the department of plant and soil sciences, has some advice for producers and buyers.

“When you go in and immediately harvest these forages, they have a high water content,” said Zhang. “One of the things that does continue is the process of respiration.”

Respiration is a normal plant process to produce food for itself, which in turn also produces heat. This process will continue to generate heat until the moisture content of the hay drops below 40 percent.

At 20 percent moisture, the hay is considered dry. However, mold will grow through respiration and produce heat until that point. This heat, along with the hay itself, mixed with oxygen causes the combustion.

Zhang said combustion typically takes place along the surface of the bales because the oxygen has trouble penetrating into the middle. Before this happens, there are some preventative measures that should be taken.

“Make hay while the sun is shining,” Zhang said referring to the old adage that has a lot of truth to it. “We need to get this hay dried as quickly as possible.”

The summer months in Oklahoma provide adequate drying conditions with high temperatures, a slight breeze and low humidity.

If there is any question as to whether the hay was baled at the correct moisture, the temperature of the bales should be closely monitored. A bale that measures less than 120 F is in no danger.

Bales between 120 F and 140 F need some attention. Bales should be removed from a barn or structure and separated so they can cool off. Once a bale reaches more than 140 F, it is generally too late, Zhang said.

“Once you start moving them at that temperature, that’s when you really get into danger,” he said. “That’s when you are putting yourself in danger.”

These types of issues will generally occur within five to seven days of baling, so it is best to leave bales in the field for several days before storing.