Crop and Soil Environmental News, May 2002

Hay Fires: Should I Be Concerned?

Each year a small number of Virginia producers lose hay and barns to hay fires. A much more common occurrence though is quality losses that occur due to excessive heating of freshly harvested hay. How do heating and quality losses occur and what can you do to monitor and prevent hay fires? These questions are addressed in the following Forage Tip from Virginia Cooperative Extension.

Producers are challenged each year with small windows of opportunity in their efforts to bale and store hay at the proper moisture level while avoiding the risk of rain damage. Forage cut for hay must go from approximately 80% moisture to 20% moisture or less in order to be stable in storage as baled hay. As the cut forage dries down, both plant and microbial respiration (burning of oxygen) continues in the field.

All hay baled above 15% moisture will undergo some elevation in temperature the first couple of weeks in storage. Many producers refer to this elevation in hay temperature following baling as "sweating" or "going through a heat". This rise in temperature is caused by both plant and microbial respiration. Collins and other researchers from Kentucky report that a small amount of heating (130°F) does not decrease hay quality and actually serves to dry down the hay by evaporating some of the moisture content.

Baling and storing hay high in moisture content (>20%) without taking steps to reduce or control microbial activity responsible for heat of respiration may reduce nutritional quality. Soluble carbohydrates are the principal group of compounds utilized "burned off" during respiration. The decrease of soluble carbohydrates and other chemical components during microbial respiration results in an increase in acid detergent fiber (ADF) which lowers digestibility.

When hay undergoes significant heating during storage, hay color can change dramatically, for example, green to various shades of brown. The degree of color change (e.g. light brown to dark brown) is indicative of the severity of heat damage to the hay. This type of heat damage represents a chemical reaction that fuses plant sugar and amino acids into an indigestible compound and is called the Maillard reaction. This compound is also referred to as bound protein even though the sugars are rendered indigestible. The degree of heat damage can be quantified by conducting a chemical analysis for acid detergent insoluble nitrogen (ADIN).

Baling and storing hay to high in moisture content can result in spontaneous combustion or a hay fire. Hay stored at moisture levels sufficient to maintain high relative humidity of the air in the hay mass allows plant and microbial respiration to generate heat and elevates hay temperatures to 158°F. The 158°F temperature may be reached within a few days or it may take several weeks if the air is drier. Above 158°F heat continues to be generated by oxidative chemical reactions.

When the temperature exceeds 175°F, the thermal death of microbes takes place. The increase in temperatures due to the oxidative chemical reactions is basically responsible for greatly increasing the potential for a rapid increase in heat to combustion temperatures of 448 to 527°F. The amount of time required for heating up to combustion may vary from four to ten weeks; however, it could be earlier or later. The moisture content of the forage, bale density, climatic and storage conditions (e.g. size of stack, high moisture hay covered with dry hay) are all factors that influence the time until combustion.

Important Points and Recommendations

• Small rectangular bales should be baled at 20% moisture or less to keep molding and heating to a minimum.
• Since large round or rectangular bales retain internal heat much longer than conventional bales, they should be baled less than 18% moisture.
• When baling above 20% moisture propionic acid can be applied to reduce microbial activity and subsequent heating. Check for recommended application rates on the label.
• Round bales should usually be left in the field for a few days/weeks (depending on moisture at baling) to allow heat to dissipate. When moist hay is stacked immediately after baling, the stack concentrates the heat, temperatures rise, quality losses occur, and the stage is set for a hay fire.
• Check your hay regularly. Symptoms of heating include: slight caramel odor, strong burning odor, visible vapor, strong musty smell, and hay that feels hot to the hands.
• Make a probe that can be driven or inserted into the hay mass to check the temperature. For example: take a 10 foot piece of pipe or electrical tubing. Attach a pointed dowel to one end and drill 6 to 10 1/2 inch diameter holes in the tube just above the dowel. Drive the prove into the hay stack and lower a thermometer on a string into the probe. The thermometer should be left for 10-15 minutes in several areas of the stack to ensure an accurate reading.
• Watch for the following temperatures:
  • 150°F (65° C) * Beginning of the danger zone. Check temperature daily.
  • 160°F (70°C)* Dangerous. Measure temperature every four hours and inspect the mow.
  • At 175°F (80°C)* Call the Fire Department. Wet hay down and remove it from the barn away from buildings and other dry hay.
  • At 185°F (85° C)* Hot spots and pockets may be expected. Flames will likely develop when heating hay comes in contact with the air. Be extremely careful at this stage when moving hay.
  • At 212° (100°C)*Critical. Temperature rises rapidly above this point. Hay will almost certainly ignite.

Take precautions and be extremely careful upon entering the barn. Pockets may have already burned out under the hay surface. Before entering a barn, place long planks on top of the hay. Do not attempt to walk on the hay mass itself. Always tie a rope around your waist and have a second person on the other end in a safe location to pull you out should the surface of the hay collapse into a fire pocket. This last recommendation may seem extreme, but precautions are essential when hay temperatures exceed 160°F.

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