Hay readily attracts and holds moisture from its environment. This moisture content is the single most important factor determining the quality, nutrient retention, and safety of stored forage. Understanding how hay interacts with water is foundational to preventing spoilage and fire risks associated with improper storage.
The Science of Hygroscopy and Hay Structure
Hay’s ability to absorb water is rooted in the physical structure and composition of its plant fibers. These fibers, consisting primarily of cellulose, hemicellulose, and lignin, are hydrophilic (water-attracting). This attraction is known as hygroscopy, where the material passively draws in and holds water molecules from the surrounding air. The plant cell walls contain polar groups, such as hydroxyl groups, which establish hydrogen bonds with atmospheric water molecules. This process is dynamic, meaning the hay’s moisture level fluctuates based on the relative humidity of the storage environment. Therefore, hay stored in a humid location will naturally absorb water, even without direct exposure to rain.
The Critical Moisture Level and Microbial Spoilage
When hay is baled, the goal is to reduce moisture to a level that halts biological activity and preserves nutrients. The threshold for safe storage is below 20% moisture content. Baling hay above this level provides an environment for microbial organisms, specifically mold and bacteria, to thrive. The rapid growth of these microorganisms leads to fermentation and the degradation of valuable nutrients, such as carbohydrates and proteins. This activity causes a loss of dry matter and a decline in feeding quality. Furthermore, mold proliferation can produce harmful mycotoxins, posing a health risk to livestock.
Spontaneous Combustion: The Danger of Internal Heat
Excessively high moisture content, typically above 20% to 25%, creates the danger of spontaneous combustion. The microbial processes responsible for spoilage are exothermic, generating heat as they break down the plant material. Because hay bales, especially large, dense ones, are excellent insulators, this heat becomes trapped. As microbial activity continues, the internal temperature rises rapidly. Once the temperature reaches approximately 150°F (65°C), the process shifts from biological to chemical. Chemical oxidation reactions take over, which are self-sustaining and cause the temperature to spike higher. Fire danger becomes a concern when the internal temperature reaches 170°F to 175°F (77°C to 79°C). The bale reaches its ignition point, typically between 448°F and 527°F (231°C and 275°C), often within the first two to six weeks after baling.
Managing Moisture Content for Safe Storage
Preventing the risks associated with moisture begins with ensuring the forage is properly dried before baling. Producers should use a hay moisture meter to confirm content is within the safe range, ideally 14% to 18%. This pre-storage measurement reduces the risk of excessive heating.
After baling, the storage environment must limit further moisture absorption. Hay should be stored indoors or covered, placed on a well-drained surface (gravel, pallets, or tires) to prevent moisture wicking from the ground. Adequate ventilation is necessary to allow residual moisture and heat to escape, achieved by maintaining space between rows of stacked bales. Regular temperature monitoring using a long-stem thermometer in the first few weeks provides an early warning of dangerous internal heat buildup.