Persistent showers in May and June made it next to impossible to bale dry hay. With the high humidity, forage dry-down in the field was slow. In some cases, to beat the rain, hay was baled and stored at higher than ideal moisture levels.
Normal respiration by fungi and bacteria in wet hay causes the temperature to rise up to 100 to 120oF (38 to 48oC) immediately after baling. This level of heating occurs in hay baled at 15-20 percent moisture and is often called normal sweating during hay curing. Most people can hold their hand or arm in the hay for 30 seconds under such conditions. However, excess moisture in hay can cause heating to levels capable of spontaneous combustion.
A good practice is to check baled hay four or five days after baling for its initial temperature rise level. A steel rod can be used initially to check the temperature in the mow by inserting it into the mow, waiting fifteen to twenty minutes and then pulling it out. If the rod is hot, check the actual temperature in the mow by placing a thermometer in a pipe and inserting the pipe into the mow for fifteen to twenty minutes.
A safety warning: It can be dangerous to walk on mows that have heated since one can drop into a burning pocket of hay. To improve safety you can use a long plank to walk on and have someone assist you.
Temperatures to keep in mind when checking a hay mow:
• 65oC (150oF) Up to this point temperatures are not considered critical but should be monitored whenever the mow temperature is over 52oC (125oF).
• 70oC (160oF) Time to take action. Monitor the temperature and make a plan to remove the hay.
• 77oC (170oF) Keep checking the temperature, looking for hot spots.
• 80oC (175oF) Hot spots should be anticipated. Above this temperature, keep all doors and other openings closed to prevent drafts.
• 82oC (180oF) Call your fire department and make immediate provisions for standby services. Make arrangements for additional protective services and notify your insurance agent.
• 88oC (190oF) Remove the hay rapidly to a safe distance away from the barn. While hay may heat above 93oC (200oF) without ignition, it is best to remove it with the help of the fire department.
Finally, hay that is borderline in moisture content should be stacked so air and moisture can escape. When possible, stack bales on edge with a minimum number of layers. Also note that weeds can increase the moisture content of otherwise dry hay.
From the Archives:
OMAFRA Update 3, July 2000
