Keeping an Eye on Stored Hay Temperatures

As hay producers, our goal is to always put the best quality product into the bale as possible.  But sometimes weather conditions do not cooperate, and hay is baled at moisture levels higher than we would like. As a general rule, hay that is baled without using a preservative at moisture greater than 20% may be at risk. With large bales, the critical moisture content is even less.

As hay dries, it continues to respire. When hay is baled, the respiration continues, giving off heat. There are microorganisms in the baled hay, and with excessive moisture these microorganisms will multiply, and generate more heat. The cycle can continue until the hay reaches a temperature where combustion will occur.

This process doesn’t necessarily happen quickly; it can take a few weeks (3-4 weeks) for hay to reach these critical temperatures. So, it’s important that producers take time to check the status of hay that was put away wetter than normal. Here are recommended temperatures and corresponding actions that should occur:

• Temperature 125^oF -- No action needed.
• Temperature 150^oF -- Entering the danger zone. Temperatures should be checked twice daily. If possible, stacked hay should be disassembled to allow more air to move around heated bales for cooling.
• Temperature 160^oF -- Reaching the danger zone. Temperature should be checked every two hours. If possible, stacked hay should be disassembled to allow more air to move around heated bales for cooling.
• Temperature 175^oF -- Hot spots or fire pockets are likely. If possible, stop all air movement around hay. Alert fire service of a possible hay fire incident.
• Temperature 190^oF -- Remove hot hay. This should be done with the assistance of the fire service. The fire service should be prepared for hay to burst into flames as it contacts fresh air.
• Temperature 200^oF or higher – Most likely a fire will occur. Remove hot hay only with assistance from the fire service.
  (Source: National Resource, Agriculture, and Engineering Service [NRAES])

Monitoring the temperature of the hay can be done with various tools. Commercial thermal cameras or guns may be available through your local fire department.  Temperature and moisture probes are often purchased for this purpose.  If using one of these probes, it should be left in the bale for 10-15 minutes to obtain an accurate reading. Keep in mind, these probes may only be about 20” long, and the highest temperatures may be deep within the stack of hay. For this reason, you can use a long piece of pipe with some small holes drilled in the bottom.  Lower a thermometer on a string and leave in the pipe for 15 minutes to get an accurate reading. If no thermometers are available, a 3/8” piece of iron pipe can also be driven into the hay stack. Let the pipe in the stack for 20 minutes and pull it out. The general rule of thumb is that if it’s too hot to hold in your hand, you have a problem. If accessing the hay stack from the top, be sure to place planks or plywood across the top of the bales so that the weight of the person is distributed. A cavity can be formed from burned-out hay and risk of falling into that cavity can occur.

As a side note, hay that has heated during storage will often appear brown in color, with a caramelized appearance. While livestock often prefer this flavor, nutrients have been lost due to excessive heating. A forage analysis is recommended prior to feeding to determine the accurate quality of the hay.