STILLWATER, Okla. Spring and early summer cuttings often present the greatest risks for hay fires because of the difficulties of drying hay before baling.
If rain is in the forecast, producers often are tempted to bale at a little higher moisture content than recommended to avoid weather-related damage, said Ray Huhnke, Oklahoma State University Cooperative Extension agricultural engineer.
"Unfortunately, severe heating can occur when hay is baled too wet and then is packed too tightly into a storage facility," Huhnke said. "This severe heating can cause significant dry matter and quality loss or, worst of all, a hay fire."
Heating results from plant respiration and microbial activity. It can occur in baled hay at moisture contents as low as about 13%.
"Therefore, heating is a natural occurrence with temperatures reaching more than 125 degrees Fahrenheit even in hay baled at safe moisture contents," Huhnke said.
If excess moisture is present, heat resistant fungi become active which can drive hay temperature to more than 150 degrees Fahrenheit.
At about 170 degrees Fahrenheit, the microorganisms die. However, heat producing chemical reactions can continue to increase hay temperature.
"Spontaneous combustion can occur if hay temperature reaches 450 degrees Fahrenheit and the material is exposed to air," Huhnke said. "Furthermore, hay fires can occur more than two weeks after the hay is placed into storage."
Producers, therefore, should monitor the temperature of stored hay. The hay should be checked daily to see if there has been any rise in temperature.
Temperatures below 140 degrees Fahrenheit generally indicate no particular heating problem.
"Temperature readings between 140 and 170 degrees Fahrenheit provide no clear indication of pending problems," Huhnke said. "If the temperature is above 180 degrees Fahrenheit, call the fire department."
Huhnke said it is imperative the fire department be present before a producer tries to move any hay that he or she suspects has dangerously high temperatures.
"Remember, spontaneous combustion can occur when the materials are exposed to air," he said.
If it is not possible to measure temperature with an instrument, use a long steel rod as a probe.
"Drive the rod into the inner stack and leave it for at least 15 minutes," Huhnke said. "If the rod is too hot to handle, the temperature inside the stack is probably above 120 degrees Fahrenheit and caution is warranted."
Producers should never stand on top of a stack they suspect may be heating because smoldering hay can create a cavity or pocket that often cannot be detected from the top of the stack.
Preventing hay fires starts at the time the hay is baled. Optimum moisture content for baling depends on bale size.
"For small, square bales, the moisture content should be no more than about 20% without preservatives," Huhnke said. "During warm, moist conditions, reduce the moisture content when baling small squares to 18%."
The upper limit for large packages, including round bales, is about 16% to avoid taking special precautions to prevent excessive heating.
If round bales are stored outside and unprotected, moisture content at baling can be increased to about 20%.
For centuries, producers have relied on experience and the "feel" of hay to estimate moisture content. Today, there are a variety of tools available to more accurately determine the moisture content before and after hay is baled.
"Microwave ovens, forced heated-air drying units and electronic moisture testers are some of the tools available today," Huhnke said.
"Still, it is vital a hay producer follows proven procedures and manufacturer recommendations on how to use any device and how to interpret the results."