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Cellular Respiration
Hay is produced from living plants. Even after cutting, the cells in hay continue to respire -- burning plant sugars to produce energy, according to the Virgina Cooperative Extension Service. The respiration of plant cells produces small amounts of heat. If the hay is loose, the heat is released into the atmosphere. In baled hay, the heat is released into the bale and can raise the internal temperature of the hay bale.
Bacterial Growth
If hay is sufficiently dry, cellular respiration eventually ceases and the bales reach ambient temperature. However, moisture concentrations over 16 to 20 percent, depending on bale size, produces an environment suitable for the growth of bacteria. Mesophilic, or warm-temperature, bacteria are naturally present on the hay crops and can begin to grow at warm temperatures and elevated moisture contents. The respiration of the mesophilic bacteria releases additional heat into bales, raising internal bale temperature to up to 140° Fahrenheit.
Heat-Loving Bacteria
If interior bale temperatures remain high, they can produce a favorable environment for the growth of thermophilic, or heat-loving, bacteria. The respiration of which can raise bale temperatures to as much as 170° Fahrenheit, according to the Virgina Cooperative Extension Service. The actions of the thermophilic bacteria alter the physical makeup of the hay, creating microscopic pores so it combines easily with oxygen.
Combustion
Combustion requires heat, fuel and oxygen. The heat produced by bacterial respiration inside wet hay bales can eventually connect with air and ignite. Once started, the fire is fueled by the hay itself and possibly eventually the barn in which it is stored. If interior hay bale temperatures reach 150° Fahrenheit, take precautions by moving the bales away from buildings and separating them to provide increased air circulation. Temperatures of over 175° Fahrenheit indicate a real fire danger; call the fire department.