Hay moisture content is the amount of water remaining in the forage after drying. This measurement is the most important factor determining how well hay preserves once baled. The moisture level must be low enough to halt biological processes that cause spoilage, heating, and dry matter loss. Hay moisture can definitively increase after baling, initiating a chain of events that severely impacts quality and storage safety.
Understanding Post-Baling Moisture Fluctuations
Baled hay remains susceptible to post-harvest changes. Even when packaged at a safe moisture level, it can undergo “sweating” in the first few days of storage. This phenomenon results from ongoing metabolic activity within the compressed plant material.
The plant cells, along with naturally occurring microorganisms like bacteria and fungi, continue to respire after baling, especially if the internal moisture content is above ideal thresholds. This respiration process consumes sugars and starches in the hay, converting them into carbon dioxide, heat, and water. Because the hay is tightly compressed, this newly generated moisture and heat cannot escape easily, causing the internal moisture reading to temporarily spike.
Hay is naturally hygroscopic and readily absorbs moisture from the surrounding environment. If bales are stored in areas with high ambient humidity, fog, or dew, the hay will wick this moisture into its structure. Placing bales directly on concrete floors or the ground also facilitates moisture wicking, drawing water up into the bottom layers of the stack.
The Dangers of High Moisture Hay
When internal moisture rises above the safe threshold (typically 15% to 20% depending on bale size), it creates an optimal environment for microbial growth. Fungi, such as Aspergillus and Fusarium, consume digestible nutrients like carbohydrates. This microbial activity reduces the hay’s nutritional value, resulting in dry matter loss and a decrease in Total Digestible Nutrients (TDN) for livestock.
The excessive growth of these microorganisms also results in the formation of mold and the release of dusty spores, which can lead to respiratory issues in animals and humans. Furthermore, some fungi produce mycotoxins, potent toxic compounds that pose serious health risks to livestock that consume the spoiled hay. The heat generated by this heightened microbial activity is the first step toward the most concerning danger: spontaneous combustion.
Microbial respiration can increase the internal temperature of a bale to approximately 140°F (60°C). If this heat cannot dissipate due to poor ventilation or tight stacking, a more dangerous phase begins. Above this temperature, the process shifts to chemical oxidation, where the hay reacts with oxygen, generating heat faster. Once the internal temperature reaches approximately 160°F (71°C), the hay is in danger of igniting without any external spark, usually within the first two to six weeks after baling.
Practical Steps for Monitoring and Safe Storage
Producers can mitigate the risks of post-baling moisture increase through practical monitoring and storage strategies. The initial step involves using a hay moisture probe during baling to ensure the forage is within the safe range (typically below 18% for small square bales and 15% for large bales). After baling, monitoring the bales for the first few weeks is critical, as this is the period of highest risk.
A temperature probe, such as a long-stemmed hay thermometer, should be used to check the core temperature of suspect bales daily. If an internal temperature exceeds 130°F (54°C), the bale should be isolated to allow for increased air circulation and cooling. If the temperature continues to climb toward the 160°F (71°C) threshold, fire suppression measures must be put into place before moving the bale.
Proper storage is the best defense against external moisture absorption and heat buildup. Bales should be stored under cover and kept off the ground or concrete floor by using pallets, gravel, or a plastic sheeting barrier to prevent wicking. Stacking bales with air space between them, rather than tightly packing them, promotes ventilation and allows the heat and moisture from the “sweat” period to escape. Finally, a chemical preservative like propionic acid can be applied during baling, allowing hay to be safely packaged at a slightly higher moisture content by temporarily inhibiting mold and bacterial growth.