Baled hay can catch fire if stored between 30-40 percent moisture due to heat produced by microorganisms and plant respiration.
Hay baled at safe moisture levels of 20 percent or less may rise to a temperature of 130-140 degrees for a few days before gradually cooling off, notes the University of Arizona Cooperative Extension Service. If hay temperature rises above 140 degrees, temperature should be monitored every few hours. At temperatures between 150-160 degrees, it is time to prepare to remove hot hay from the stack or secure a source of water in case temperature continues to rise.
The Extension Service says it would be wise to call the fire department when hay temperature exceeds 180 degrees. When temperature reaches 200 degrees, bales may burst into flames when removed from the stack if not wetted.