Monitor Hay Temperature to Reduce Fires

Hay bale or barn fires can be prevented if farmers bale hay at appropriate moistures and monitor the temperature of recently baled hay.

Generally, hay will go through a heating phase within one to two weeks after baling. During this time, hay should be monitored to ensure it does not reach temperatures that can damage the hay or lead to spontaneous combustion, said Doug Overhults, an agricultural engineer with the University of Kentucky Cooperative Extension Service.

It is not unusual for the temperature within a bale of hay to reach 100 degrees Fahrenheit, and may go as high as 130 degrees before beginning to decline. If the temperature peaks below 130 degrees, there may be some loss of quality but no danger of fire. With free air circulation around a bale, both heat and moisture can dissipate. A single bale rarely heats enough to catch on fire, but when they are placed close together or stacked with other bales that are also heating, it is much more difficult for the heat to leave the bales, he said.

If the bales are wetter than they should be, the temperatures can easily rise above 130 degrees. At 140 to 150 degrees more microbial growth and chemical reactions within the hay cause it to generate heat at an increasingly rapid rate.

If hay temperatures reach 150 degrees, bales need to be moved to allow for better air circulation and the temperature should be checked frequently. At 180 degrees fire is imminent, and at 200 degrees it is likely to be present. In either case, the fire department should be notified. It is best to wait for them to arrive before removing the hay from the stack in case of a flame up, Overhults said.

Smoke from hay that has been treated with an acid preservative may contain toxic fumes, so keep people away from the smoke and inform the firefighters of the treatment that was applied.

To check the temperature of hay, several types of thermometers can be used, Overhults said.

"There is really a multitude of ways to do this," he said. "There are no stringent requirements. We'd like something that's easy to use, something that's durable. W'd like to have something that would measure up to 200 degrees. It does not
have to be real accurate. If it is within 5 degrees, you have enough information to make a decision."

Attaching a string or thin wire and lowering or pushing it into a probe that has been inserted into the hay is one way to use a simple glass thermometer. Do not insert them directly into
the hay because they break very easily. It is best to use only spirit-filled glass thermometers. That prevents any risk of accidentally contaminating hay with mercury from a broken thermometer.

Electronic thermometers with remote sensors and a digital readout can be used. Avoid LED displays because they are often hard to read in bright light. An LCD is a better choice.
For farmers who are also moisture testing their hay, some of the electronic moisture meters also measures temperature. Long stem thermometers, commonly called compost
thermometers, are probably the most rugged and reliable. With these types, the price increases with the dial size and length of the stem. It may be tempting to stick these directly into a hay bale, but the stem can be easily bent and the accuracy or
operation of the thermometer could be destroyed, Overhults said.

It is best and necessary in most cases to use some kind of hay probe. A farmer can easily make one for himself using steel pipe or electrical conduit, he said. Probes can also be
purchased commercially.

When using a thermometer, measure the wettest hay first. Probe square bales from the side and round bales from the end. The probe should be inserted near the center of the bale.
In round bales, if the core is loosely formed, probe six to 12 inches away from the center where the hay will be more tightly packed.

In large stacks, it may be difficult to reach the center, but it is important to get at least five to 10 feet down from the top or in from the side, Overhults said. The most critical factor is to
reach where the wettest hay is stored. It is best to probe at several locations and at different depths within a stack to locate the warmest spot. For more information on temperature checking and other aspects of hay production contact a local office of the Cooperative Extension Service.