Moldy hay is a major economic and health liability in agriculture. This condition is caused by the proliferation of fungi, whose spores are naturally present but germinate rapidly when environmental conditions are right. Fungal growth consumes the hay’s valuable nutrients, leading to substantial losses in feed value, including reduced total digestible nutrients (TDN) and vitamins. Some molds produce mycotoxins, which are poisonous compounds that can cause severe health issues in livestock, such as liver damage, neurological disorders, respiratory problems, and colic, especially in horses. Preventing mold requires a precise, multi-step approach focused intensely on managing moisture from cutting through long-term storage.
The Critical Role of Hay Moisture
The single most influential factor in mold development is the moisture content of the hay at the time of baling. Mold spores require water activity to germinate and consume the forage’s carbohydrates, which generates heat and causes spoilage. To prevent this, hay must be dried to a threshold where fungal growth is inhibited, typically below 20% moisture.
The maximum safe moisture level varies inversely with the density and size of the bale package. Small square bales can tolerate moisture near 18% to 20%. In contrast, large round bales should be baled at or below 18% moisture, and highly compressed large square bales often require a lower threshold, ideally 12% to 16%.
Relying on touch or sight to gauge moisture is unreliable, making a specialized hay moisture meter necessary. This device uses probes to measure the electrical resistance within the hay, providing an objective percentage reading of water content. Frequent testing across different parts of the field and within the windrow is essential to confirm the highest moisture reading falls within the safe range for the specific bale type being produced.
Optimizing Field Curing and Baling Timing
The curing process is a race against time and weather, making management following cutting paramount. Proper cutting height leaves stubble that allows for airflow beneath the hay, promoting uniform drying. After cutting, the crop should be spread into the widest possible swath, often covering at least 70% of the cut area, to maximize exposure to sunlight and wind.
Mechanical conditioners that crimp or crush the stems accelerate the release of moisture, shortening the overall drying time. Using a tedder or rake to flip the hay moves wetter material from the bottom of the windrow to the top, promoting even curing. This must be done carefully to prevent leaf shatter once the hay is partially dry.
Baling should be timed to avoid high humidity and morning dew, which significantly raise the hay’s surface moisture. Producers often wait until mid-day or afternoon when the sun has driven off surface moisture and maximized dryness. Consistent monitoring of weather forecasts is necessary to ensure the curing window is free of rain, as rewetting hay substantially increases the risk of mold and mycotoxin development.
Creating a Safe Storage Environment
Once baled, the hay’s environment must be controlled to prevent moisture reabsorption and dissipate residual heat from microbial activity. The storage location must be protected from ground moisture by storing bales on elevated surfaces like pallets, old tires, or gravel. This prevents the bottom layer from wicking water from the soil or floor, a common source of spoilage.
Adequate roof coverage and protection from rain and snow are fundamental, but internal airflow is important. Proper ventilation allows the bales to “sweat” or release internal moisture safely by carrying away humid air. For indoor stacks, leaving a gap of six to twelve inches between the stacked bales and exterior walls is necessary to prevent condensation from soaking the hay.
The stacking pattern must facilitate air movement and structural stability. Small square bales are often stacked in a crisscross or brick-like pattern to create channels for ventilation throughout the stack. For large round bales stored outdoors, orienting them end-to-end in narrow rows separated by at least three feet encourages air circulation and drainage.
Applying Chemical Preservative Treatments
When weather delays or tight schedules necessitate baling hay at higher than ideal moisture levels, chemical preservatives offer a practical solution. The most common treatment involves organic acids, specifically propionic acid or blends containing it. These acids create an acidic environment that directly inhibits the growth of mold and other aerobic microorganisms.
Propionic acid allows hay to be safely baled at moisture levels up to 25%, extending the harvest window. The acid is typically applied as a buffered liquid solution to reduce its corrosive effects on baling equipment. It is sprayed directly onto the hay as it enters the baler.
The application rate must be carefully calibrated based on the hay’s precise moisture content; wetter hay requires a higher dosage for effective mold suppression. While preservatives are a valuable tool for salvaging a crop, their effectiveness sharply declines if the hay moisture content exceeds 30%, which risks spontaneous combustion due to excessive heat generation.