Hay, which is dried grass, legumes, or other forage plants, serves as a primary feed source for livestock, especially horses and cattle. The quality of this feed directly impacts animal health and performance, making the identification of spoilage necessary for animal caretakers. Mold growth occurs when hay is baled or stored with excessive moisture, typically above 15-20% moisture content. While nearly all hay contains some mold spores, excessive fungal growth rapidly diminishes the feed’s nutritional value and introduces dangerous contaminants. Recognizing the signs of moldy hay is a safety measure to prevent health risks for the animals consuming it.
Visual Signs of Contaminated Hay
Visible mold appears in a variety of colors and textures, often signaling different types of fungal contamination. A clear indication of mold is the presence of fuzzy, powdery, or slimy patches on the hay strands or within the bale. These patches range in color from white or gray to black, green, or blue, depending on the species of fungus present.
The hay’s overall color should be a vibrant green, indicating proper curing and storage. Moldy hay often shows discoloration, appearing faded, brownish, or yellowed in areas where moisture allowed the fungus to thrive. When pulling apart a flake, look for strands that are clumped, matted, or caked together, as this texture indicates moisture damage and microbial activity.
When moldy hay is disturbed, a visible cloud of fine white or gray dust will billow out. This dust is composed largely of microscopic mold spores and fungal fragments, which become easily airborne. Excessive dustiness, even without obvious mold patches, is a strong indicator of fungal growth and poor quality. The presence of mycelium, the stringy growth structure of fungi, can also be observed as white mold within the bale.
Non-Visual Indicators of Spoilage
Spoilage is not always visible and can be detected through other senses or by measuring the bale’s physical properties. The most immediate non-visual cue is the odor; good hay should smell fresh or like sweet grass. Hay contaminated with mold often emits a sour, musty, earthy, or ammonia-like smell.
An elevated temperature within the bale is a dangerous sign of spoilage, caused by the metabolic activity of bacteria and fungi consuming the plant’s sugars. Hay baled with too much moisture can quickly reach temperatures between 120°F and 150°F due to microbial activity, significantly reducing the hay’s nutritional value. If the internal temperature rises above 170°F, there is a risk of spontaneous combustion and fire.
The feel of the hay can also indicate spoilage; hay that is damp, clammy, or slimy to the touch suggests excessive moisture and microbial growth. This moisture contributes to a loss of total digestible nutrients, even before the bale reaches high temperatures. Hay damaged by heat may develop a caramelized or sweet tobacco smell, indicating a chemical reaction that makes the protein less digestible for the animal.
Health Risks Associated with Moldy Hay
The danger in moldy hay comes from mycotoxins, which are toxic compounds produced by certain molds like Aspergillus, Fusarium, and Penicillium. Ingesting these toxins can lead to liver damage, neurological disorders, and reproductive failure in livestock. Because horses are hindgut fermenters, they are susceptible to mold and mycotoxin-related illnesses compared to ruminants like cattle, which can break down some toxins in their rumen.
The inhalation of mold spores is a major cause of respiratory disease in horses, most notably Recurrent Airway Obstruction (RAO), or “heaves,” which is similar to human asthma. Symptoms of this allergic reaction include chronic coughing, nasal discharge, and labored breathing. Even without mycotoxins, the volume of inhaled spores can cause inflammation and suppress the immune system, leading to pneumonia-like respiratory issues.
Digestive problems are also a concern, as moldy hay can cause feed refusal, diarrhea, and severe abdominal pain, such as colic in horses. Humans handling moldy hay are also at risk from inhaling the spores, which can lead to respiratory irritation and a condition known as “farmer’s lung.” Young, pregnant, or compromised animals are at the highest risk for health effects from consuming contaminated feed.
Methods for Preventing Mold Growth
The most effective method for preventing mold growth begins during baling by ensuring the hay is properly cured to a low moisture content. Hay should be baled when its moisture level is between 12% and 18% to inhibit the germination of mold spores. If hay must be baled at a higher moisture level, a preservative, such as organic acids like propionic acid, can be applied to limit microbial growth and heat production.
Proper storage is the next step in mold prevention, involving protecting the bales from rain, condensation, and ground moisture. Bales should be stored off the ground using pallets, tires, or a layer of gravel to create space for air circulation and prevent moisture wicking. This practice is important because bales resting directly on the soil can absorb enough moisture to cause spoilage.
Adequate ventilation is necessary to allow warm, moist air to escape from the stacks and prevent condensation buildup. Stacking bales with a small gap (6 to 12 inches) between the stack and walls, as well as between rows, encourages air movement. If hay is stored outside, covering it with a UV-resistant tarp is necessary. The cover should be applied loosely, creating a roof-like effect to allow the hay to breathe and prevent air from being trapped.