Fresh cut forage material continues to respire (a process which breaks down sugars and releases water) as it decreases in moisture content from around 80 percent to the less than 20 percent needed for storage as baled hay. During this respiration process some heat is generated and released from the forage. It is best for hay quality and safety if most of this heat can dissipate prior to the hay being baled. Being too quick to bale hay and store it in the barn can result in loss of hay quality and possibly result in a barn fire as well.
Hay baled at above 15 percent moisture will undergo some temperature elevation during the first couple of weeks in storage. This rise in temperature is caused by continued plant respiration and microbes feeding on the newly cut forage. Heating that stays below 130 degrees F does not result in decreased hay quality and helps to dry the hay out.
According to information from Virginia Cooperative Extension, when hay undergoes significant heating during storage, hay color can change dramatically, turning from green to various shades of brown. The degree of color change can be indicative of the severity of heat damage to the hay. Some people refer to this change in hay color as caramelization. Whatever it is called, it is the result of a process called the Maillard reaction which fuses plant sugar and amino acids into an indigestible compound. The severity of nutrient loss can only be quantified by conducting a specific chemical analysis of the hay to determine the level of acid detergent insoluble nitrogen.
Newly baled hay that is stored in a barn should be checked frequently with a thermometer and probe for the first few weeks to make sure that temperatures do not climb to levels at which a barn fire could occur. Symptoms and signs of hay heating include a slight caramel odor in the barn, visible vapor, strong musty smell, strong burning odor, or hay that feels hot to the hands.
A probe can be made from a ten foot section of electrical conduit that can be driven into the hay to help check temperatures. Crimp one end of the conduit to make a point and drill six to ten one-half inch diameter holes in the tube just above the crimp. Lower the thermometer into the probe and read after 10 to 15 minutes.
The following is a range of temperatures (in degrees Fahrenheit) and what you should do with your hay: below 130 - no problem; 130 to 140 - no problem, temperature may go up or down so recheck in a few hours; 150 - temperature will likely climb, move hay out of the barn to allow air circulation and cooling; 175 to 190 - fire is imminent or may be present a short distance from the probe, call the fire department and continue monitoring the temperature; 200 or above - fire is present at or near the probe, call the fire department and start injecting water to cool hay.
Do not move hay above 175 degrees as this will expose the overheated or smoldering hay to more oxygen which may result in a raging fire. Do not attempt to climb up on a stack of hay that is smoky or might be on fire, as you might fall into a burned out cavity and become trapped.
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