Hay, which is cured forage, forms the basis of many livestock diets, serving as a primary source of fiber and nutrition. Because hay is organic plant material, it remains susceptible to biological and chemical processes that lead to spoilage after baling. When properly cured and stored, hay can maintain its integrity for years, but compromised hay can quickly turn from a valuable feedstuff into a serious hazard.
The Biological and Chemical Causes of Hay Degradation
The largest factor initiating hay spoilage is excessive moisture content at the time of baling. Hay should ideally be baled at or below 15% moisture; anything above 20% dramatically increases the risk of degradation. Excess moisture activates dormant microorganisms, primarily aerobic bacteria and fungi, which consume the hay’s soluble carbohydrates. This microbial respiration is an exothermic process that releases heat as a byproduct.
As the organisms respire, the internal temperature of the bale begins to rise, sometimes reaching 120°F to 140°F within the first few weeks of storage. This heat encourages the growth of heat-resistant fungi and triggers chemical reactions that damage the hay’s nutritional value. Above 140°F, the Maillard reaction occurs, chemically binding proteins and sugars together, which makes the protein indigestible.
If the internal temperature continues to climb, often due to moisture content above 25%, spontaneous combustion becomes a risk. Temperatures exceeding 160°F are dangerous. Once 175°F is reached, a fire is imminent because chemical oxidation reactions rapidly increase the temperature. This heat buildup drastically reduces available energy and vitamins, even before visible mold appears.
How to Visually and Tactilely Identify Spoiled Hay
A thorough inspection provides immediate clues to hay quality. The most obvious visual sign of spoilage is the presence of mold, which may appear as white, green, or black patches on the hay flakes. Hay baled too wet may also exhibit dark discoloration internally due to heat damage, contrasting with the desirable bright green color of freshly cured forage.
Another visual indicator is a dusty appearance, resulting from dried mold spores and fragmented hay particles. Excessive dust, noted when pulling apart a flake of hay, indicates a potential respiratory hazard. Tactilely, hay should feel soft and pliable. A coarse, brittle, or caked texture suggests maturity issues or moisture damage that has led to a hardened mass.
The olfactory sense offers a powerful check for underlying issues, as quality hay should possess a pleasant, sweet, or grassy aroma. Spoiled hay often smells musty, mildewed, or sour, indicating active mold growth and fermentation. A sweet, caramelized, or tobacco-like smell suggests the hay underwent excessive heat damage from the Maillard reaction, even if mold is not visible.
Storage Practices for Extending Hay Shelf Life
Proper storage preserves hay quality by limiting exposure to external moisture and humidity. The ideal location is indoors, such as in a barn or shed, to provide shelter from rain and snow. Stacking hay directly on the ground, especially concrete, allows moisture to wick up into the bales. Therefore, bales should be elevated using wooden pallets or old tires. This elevation creates an air gap that prevents the bottom layer from absorbing ground moisture, reducing spoilage losses.
Air circulation around the stack is necessary to allow residual moisture to escape and prevent temperature pockets from forming. Stacks should not be pushed directly against exterior walls, as this restricts airflow and can lead to condensation and mold growth. When storing bales outside, stack them on a well-drained surface and cover them with a secured tarp to shed water.
Round bales naturally shed water better than square bales, making them more suitable for outdoor storage. However, outdoor storage, even with tarps, results in more dry matter loss than indoor storage. Utilizing plastic wrap to create baleage allows for baling at a higher moisture level by creating an anaerobic environment, but this requires an airtight seal to prevent spoilage.
Risks to Livestock Health from Consuming Damaged Hay
Feeding damaged hay introduces several threats to animal welfare that extend beyond poor nutrition. The inhalation of mold spores and fine dust particles in spoiled hay is a primary concern, particularly for horses. This repeated exposure can cause serious respiratory conditions, including recurrent airway obstruction, sometimes known as Heaves.
Certain molds, especially those belonging to the Aspergillus, Penicillium, and Fusarium genera, produce highly toxic chemical compounds called mycotoxins. These toxins can cause mycotoxicosis, leading to systemic effects. Ruminants, like cattle and sheep, can detoxify some mycotoxins, but young or high-producing animals remain susceptible.
In addition to toxicity, spoiled hay offers diminished nutritional value, even if consumed. The microbial activity and heat damage consume digestible energy and protein, and destroy vitamins like Vitamin A and E. A diet relying on spoiled hay may result in nutrient deficiencies, leading to poor body condition and reduced productivity.