Hay is dried grass, legumes, or other herbaceous plants used primarily as feed for livestock or as a garden mulch. When this dried forage material gets wet, either from rain during the curing process or from improper storage, excess moisture initiates a cascade of biological and chemical reactions that degrade the material. This leads to spoilage, loss of nutritional value, and in severe cases, a fire hazard. The consequences of wet hay are significant for both animal health and property safety.
The Immediate Problem: Mold and Spoilage
The rapid growth of mold and bacteria is the primary consequence of wet hay, as these microorganisms thrive in moist, warm environments. Hay stored with a moisture content above the safe threshold, typically between 15% and 20%, provides ideal conditions for fungal spores to germinate and multiply. This microbial activity begins quickly, often within 24 to 72 hours, leading to spoilage.
Spoiled hay often displays patches of white, black, or green mold and emits a distinct, musty odor. Microorganisms consume the hay’s digestible nutrients, reducing its feed value and palatability for livestock. Certain molds can produce mycotoxins, which are toxic compounds. Ingesting contaminated hay can cause serious health issues in animals, including liver and kidney damage, neurological disorders, and reproductive problems.
Horses are particularly susceptible to respiratory risks, as inhaling mold spores can lead to conditions like Recurrent Airway Obstruction (“heaves”). The mold itself can cause respiratory distress and colic, making proper ventilation necessary when feeding hay exposed to moisture. As a mulch, wet hay breaks down too rapidly, losing its effectiveness as a weed barrier and potentially introducing pathogens to the garden.
The Hidden Danger: Spontaneous Combustion
Spontaneous combustion is a major risk of wet hay and is the most common cause of fire in hay storage. This process is initiated by the same microbial activity that causes spoilage, particularly in a tightly packed, insulated environment like a large bale or stack. As bacteria and fungi break down the plant material, they generate heat as a byproduct of their respiration.
If the hay’s internal temperature rises above approximately 130°F (55°C), microbial activity slows down, and chemical oxidation reactions take over. These reactions are exothermic and cause the temperature to rise further without external oxygen. This self-sustaining chemical reaction can push the internal temperature to a critical point, often between 448°F and 527°F (231°C and 275°C), where the hay ignites without an external spark.
This fire risk is highest in large, dense bales or stacks where the heat cannot easily dissipate, and it typically occurs within the first two to six weeks after baling and storage. Warning signs include a distinct musty or sweet, caramel-like odor, or the presence of visible vapor rising from the stack. If the internal temperature reaches 175°F (79°C), the fire is considered imminent, requiring immediate intervention to prevent a barn fire.
Loss of Nutritional and Mulch Value
Getting hay wet significantly degrades its quality, reducing its value as both feed and mulch. Excessive moisture causes leaching, where water-soluble nutrients are washed out of the forage material. These components include highly digestible elements like carbohydrates, proteins, and vitamins.
The loss of these soluble nutrients directly reduces the hay’s total digestible nutrients (TDN) and overall energy content, making it a less effective feed source for livestock. Repeated wetting and drying cycles increase the fragility of the hay, leading to leaf shatter during handling. Since the leaves contain the highest concentration of nutrients, their loss leaves behind a higher proportion of less digestible stems, lowering the overall quality.
For use as mulch, the rapid breakdown of wet hay is a disadvantage because it shortens the material’s lifespan and effectiveness. Mulch is intended to decompose slowly to suppress weeds and retain soil moisture. When hay is wet, accelerated microbial activity causes it to decompose quickly, necessitating more frequent reapplication to maintain the desired benefits.
Prevention and Salvage Techniques
Preventing hay from getting wet begins with ensuring the forage is properly cured before baling, aiming for a moisture content below 15% for safe storage. Using a hay moisture tester is the most accurate way to confirm this level. Proper storage is equally important and requires the following:
Keeping the hay under cover.
Elevating it off the ground using pallets or gravel.
Storing it away from walls to prevent moisture wicking.
Arranging stacks with small gaps between rows to allow air to circulate and dissipate residual heat and moisture.
If hay does get wet, small amounts can be spread out immediately in a thin layer to dry quickly, which may salvage some of the material. Large, wet bales must be monitored closely for internal temperature using a hay probe, particularly during the first two months of storage. If the internal temperature exceeds 150°F (65°C), the hay should be checked every four hours. If it reaches 175°F (79°C), the fire department should be alerted before attempting to dismantle the stack. Hay that is heavily molded or has reached high internal temperatures should be discarded or isolated from other feed and structures.