A lot of hay has been baled recently. So, hay growers are reminded of a few things about hay fires and how they can be prevented. When hay is baled too wet, hay or barn fires can be a result. However, hay fires can generally be prevented if hay is baled at appropriate moisture (not always easy to do) and the temperature of recently baled hay monitored.
Most of the time, 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 (fire).
It is not unusual for the temperature within a bale to reach 100 degrees, and may go as high as 130 degrees before beginning to drop. 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. A good practice is to leave bales scattered in the barn for three to four weeks before placing them in a stack.
If the bales are wetter than they should be, the temperatures can easily rise above 130 degrees. At 140 to 150 degrees microbial growth and chemical reactions generate heat at an increasingly rapid rate.
If hay temperatures reach 150 degrees, bales need to be moved 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 called and you should wait for them before removing the hay in case of a flame up.
To check the temperature of hay, several types of thermometers can be used. Find one that is durable and easy to use and will measure up to 200 degrees.
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. Do not use mercury thermometers.
Various electronic thermometers with remote sensors and compost thermometers can be used, but have limitations. It is best and necessary in most cases to use some kind of hay probe. One can be made using steel pipe or electrical conduit. 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. 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.