Hay is cured forage used as feed for livestock. When grasses or legumes are baled, the ideal moisture level is generally less than 20%, often aiming for 15-18% for denser bales, to ensure stability during storage. Exceeding this threshold accelerates biological and chemical processes, compromising the feed’s value and storage integrity. These reactions lead to the two main dangers of wet hay: fire and spoilage.
Internal Heating and Spontaneous Combustion
The primary risk associated with overly wet hay is spontaneous combustion. This process begins with microbial respiration within the dense bale structure, where bacteria and fungi consume plant sugars and release heat. High moisture content, particularly above 20%, creates optimal conditions for this activity, rapidly raising the hay’s internal temperature to 130–150°F.
This initial microbial heating is significant because the heat does not dissipate due to the hay’s insulating properties. At temperatures exceeding 130°F, the process transitions from biological to chemical, where oxidation reactions take over. These chemical reactions are exothermic, meaning they generate their own heat and accelerate rapidly in a self-sustaining cycle.
As the temperature continues to climb, the hay mass can reach the ignition point, typically around 440°F for forage. This thermal runaway often smolders deep inside the bale before erupting into flames upon exposure to fresh air. Warning signs include a musty or caramel-like odor, visible steam rising from the stack, or a slumping appearance.
Nutritional Loss and Mold Development
Excess moisture immediately begins to degrade the hay’s quality as feed before the temperature reaches dangerous levels. The same fungi and bacteria that cause heating actively consume the most digestible components. This microbial activity depletes the hay’s non-structural carbohydrates, the main source of energy, resulting in lower nutritional value.
Moisture encourages rapid mold proliferation, often visible as white, green, or black patches. This fungal growth reduces palatability and leads to the production of mycotoxins by certain species. Ingesting mycotoxin-contaminated hay can cause serious health issues in livestock, including respiratory distress, liver damage, and digestive problems.
High heat, even if it does not lead to fire, causes a chemical change known as the Maillard reaction, or heat damage. This reaction binds amino acids (protein) with sugars, forming compounds poorly digestible by animals. This effectively reduces the available protein and energy content. The combined effect of microbial consumption and heat damage substantially reduces the hay’s feeding value, resulting in dry matter loss.
Mitigation Strategies for Wet Hay
Immediate action is necessary if wet hay is suspected of heating. Owners should monitor the internal temperature using a dedicated hay probe or by inserting a metal rod deep into the stack. If the rod is too hot to comfortably hold after ten minutes, or if the temperature reaches 150°F, the hay is in the danger zone and must be monitored multiple times daily.
If temperatures approach 175°F, fire department notification is recommended. The hay should be carefully separated and moved to an isolated area away from buildings and other dry forage. For long-term prevention, proper curing is necessary, ensuring the forage dries to a safe moisture level before baling. Bales should be stored off the ground, often on pallets, and kept under cover to prevent re-wetting.
Adequate air circulation is also necessary, requiring space between rows of stacked bales and barn walls to allow heat and moisture to escape. Slightly damp hay may be salvaged using chemical preservatives like propionic acid at the time of baling, which inhibits mold and microbial growth. Hay that is heavily molded or heat-damaged should have limited use, potentially repurposed as bedding rather than a primary feed source.