Hay Bale Fires

Why does wet hay catch on fire?

When harvested at a higher moisture level, a forage crop sometimes stays damp and respires well after baling. That continued respiration in the presence of oxygen after it’s baled creates conditions that can cause bales to spontaneously combust. Wet hay that continues to respire can generate heat and eventually spontaneously combust.

After hay is baled and stored at higher moisture levels, the fire risk from spontaneous combustion is greatest in the first two to six weeks. And that risk continues if hay bales are stored where moisture can linger, like a barn with a leaky roof or high-humidity area.

“Hay placed in storage should have a moisture content under 25%,” according to The Pennsylvania State University Agriculture and Biological Engineering Department. “Higher levels of moisture require an oxygen-limiting storage system. The heat generated by the crop plus the presence of oxygen increases the risk of a fire.”

Key times to think about hay bale fire prevention

Hommer recommends the following best practices throughout the growing season to minimize hay bale fire risk, especially from spontaneous combustion once bales are stored:

  • Harvesting. Harvest forage within the optimal moisture range and allow adequate drying time in the field before baling. 
  • Conditioning. Mower conditioners, tedders and rakes can speed hay drying but can also harm leaf retention and crop quality. 
  • Baling. Bale hay up to 20% moisture to reduce the potential for hay bales spontaneously combusting or spoiling. 
  • Storing. Since moisture contributes to bale heating and fires, store hay under a roof to keep it as dry as possible. Hay probes and sensors can help monitor temperatures in stored hay and can help prevent hay fires or spontaneously combusting bales.