Released July 27, 2007
HOPE, Ark. - Every once in a while, the tabloids will tell the mysterious and gruesome story of a human combusting spontaneously. While the scientific community can’t agree on the subject of humans bursting into flames, researchers agree that spontaneous combustion occurs in nature – especially when it comes to hay.
The weather pattern experienced in southwest Arkansas this year has been good for forage production, but harvesting the forage has been difficult because of the frequency of the rainfall. Producers have found it difficult at times to find enough dry weather to properly cure their hay for baling before the next rain shower.
Spontaneous combustion is always a possibility with stored hay, particularly if the hay was baled with high moisture content. Producers don’t routinely bale hay until the moisture level is below 20 percent, but if a rain shower is imminent, hay can be baled with higher than normal moisture content.
All hays go through a heat, or curing process, when they’re baled. The heat produced is caused by live plant tissue respiration coupled with bacteria and mold activity.
High moisture hays heat rapidly. If the heat produced by plant tissue and mold respiration is insulated by the hay and not allowed to escape, it continues to build, sometimes to dangerous levels.
If you suspect that excess heat is building in your hay, the interior temperature of the hay should be checked with a thermometer. A probe can be made from a 10-foot piece of conduit with a piece of sharpened dowel rod screwed into the tip. Drill several ¼-inch holes into the conduit behind the tip. Attach a string to a thermometer and insert the thermometer into the conduit after the conduit is placed into the interior of the bale.
Allow the thermometer to remain for 10 minutes before removing. Record the temperature and take more readings until the temperature stabilizes and drops. If the temperatures continue to rise, more frequent readings may be needed.
As the temperature in a bale rises above 130 degrees, a chemical reaction occurs that doesn’t require oxygen and produces flammable gases that exist at temperatures above their ignition point. If oxygen reaches these gases, they ignite spontaneously and fire results.
At 130-140 degrees, the plant sugars begin to caramelize and the hay becomes palatable to cattle. Unfortunately, the nutritional quality of the hay begins to decrease. At 150 degrees, the hay reaches the danger zone for spontaneous combustion to occur. At 160 degrees, the temperature should be checked every four to six hours. At 175 degrees, call the fire department, and if stored in a barn, begin to carefully remove the bales. At 185 degrees, hot spots and pockets are expected and flames will occur as the heating hay comes into contact with the air. At 212 degrees, fire is almost a certainty.
A word of caution: As the temperature probe is inserted into a heating bale, enough oxygen may be admitted to cause fire to erupt. For this reason, all bales checked should be outside storage.
Another word of caution: Never walk on bales to check temperatures because if pockets of hay have already burned out inside the bale, you could fall into the bale and cause the bale to erupt into flames. Also, hays treated with certain preservatives may produce toxic gases when heated.
The relative humidity and the temperature at baling can affect the moisture content of the baled hay. For example, at an air temperature of 85 degrees and relative humidity at 70 degrees, the moisture content of the baled hay is at 18 degrees; at 90 degrees relative humidity, the bale moisture content is 37 degrees.
Spontaneous combustion can and should be avoided by following good storage and baling practices.
For more information about hay, contact your county extension agent. The Cooperative Extension Service is part of the U of A Division of Agriculture.
Contact: Lamar James, (501) 671-2187, firstname.lastname@example.org