Hay is dried forage, such as grasses or legumes like alfalfa, harvested and preserved for animal feed. The process of making and baling hay aims to reduce the plant’s moisture content to a low level. This reduction halts natural biological processes and prevents the growth of spoilage organisms. Moisture is the greatest threat to stored hay, determining its nutritional value, safety, and lifespan.
Moisture Tolerance and Safe Levels
The amount of moisture tolerated depends on the hay’s density and bale size. Hay must be “cured” or field-dried before baling to remove unbound water. This curing reduces moisture content enough to suppress microbial activity, but not so low that the hay becomes brittle and loses leaves during handling.
The safe range for baling hay is 15% to 20% moisture content. Less dense, small square bales can tolerate moisture closer to the 20% maximum. Large, dense round or rectangular bales retain internal heat and moisture more effectively and should be baled at or below 18%; 15% is recommended for long-term storage. Hay baled above these thresholds is considered “wet” and poses significant risks.
Spoilage, Mold, and Nutritional Loss
Excessive moisture activates the growth of microorganisms, primarily fungi (mold) and bacteria, which break down the hay’s organic matter. This microbial activity consumes digestible nutrients, such as sugars and carbohydrates, leading to a loss of total digestible nutrients (TDN). The process also generates heat and moisture, which fuels the growth of spoilage organisms.
Mold growth is visible as white, gray, or black patches, making the hay less palatable and causing livestock to refuse the feed. Beyond nutritional depletion, certain molds like Aspergillus and Fusarium produce mycotoxins. These toxic compounds cause serious health issues for livestock, including reproductive problems, liver damage, and immune system suppression. Horses are sensitive to respiratory issues caused by inhaling mold spores, which can lead to chronic conditions.
The Risk of Spontaneous Combustion
The most destructive consequence of baling hay with high moisture is spontaneous combustion—a fire that starts without an external spark. This process begins when hay is baled above 20% moisture, creating an ideal environment for mesophilic bacteria. As these bacteria rapidly consume starches and sugars, they generate heat, causing the internal bale temperature to rise to 130°F to 140°F.
If the heat cannot escape, the temperature continues to climb, and the initial mesophilic bacteria die off. The sustained heat allows thermophilic, or heat-loving, bacteria to take over, raising the internal temperature above 175°F. At this high temperature, a chemical oxidation reaction begins, which is an exothermic process that generates its own heat without further microbial activity. This chemical reaction causes the temperature to rapidly escalate, leading to ignition, often between 448°F and 527°F.
Most hay fires occur within the first six weeks after baling, making early monitoring necessary. Warning signs of dangerous heating include a musty or “caramel” odor, the presence of steam, or the bale feeling excessively hot. Farmers monitor this risk using a hay probe to check the internal temperature; a reading above 150°F signals immediate intervention is required.
Protecting Hay Through Proper Storage
Preventing moisture damage relies on optimizing storage conditions to maintain the low moisture content achieved during baling. Stored hay must be protected from external water sources, including rain, snow, and moisture wicking up from the ground. Storing hay indoors in a well-ventilated structure offers the best protection and reduces losses.
When indoor storage is not available, bales stored outside must be elevated off the ground using pallets, tires, or a thick layer of gravel. This elevation prevents soil moisture from being drawn into the bottom layers, where the greatest spoilage occurs. Proper stacking is necessary, allowing air circulation around the bales to dissipate residual moisture and heat. For outdoor stacks, a protective cover like a tarp is required, which should be secured to shed water while still allowing for air movement underneath.