Hay, which is dried forage, serves as a fundamental feed source for many types of livestock. Haymaking reduces the moisture content of grasses and legumes to halt biological activity within the plant material. Maintaining low moisture preserves the forage’s quality, nutritional value, and safety. When hay is exposed to water, this balance is compromised, initiating biological and chemical reactions that degrade the hay and create health risks for livestock and physical dangers in storage.
The Biological Process of Spoilage
The objective of drying forage is to lower the moisture content below the threshold that supports microbial growth, typically targeting 15 to 20 percent. When water is reintroduced, the hay mass becomes a fertile environment for the reactivation of dormant microorganisms, including bacteria, fungi, and yeasts. These microbes break down digestible nutrients, particularly soluble carbohydrates, through aerobic respiration. This microbial consumption results in the rapid loss of dry matter and a reduction in the hay’s nutritional value, including lower levels of total digestible nutrients and fat-soluble vitamins. As the microorganisms metabolize carbohydrates, they release heat, causing the internal temperature of the bale to rise, often reaching 125°F to 130°F within the first week.
Physical Dangers of Storing Wet Hay
The heat generated by microbial activity within wet hay bales presents the danger of spontaneous combustion. Hay serves as an insulator, meaning heat generated at the core of a tightly packed bale cannot easily escape. If the internal moisture content is high (above 22 percent), microbial respiration drives the temperature into a dangerous range. Once the internal temperature exceeds 130°F, a self-sustaining exothermic chemical reaction begins, accelerating the temperature rise and creating flammable gases that ignite when exposed to oxygen. Temperatures of 150°F represent the beginning of the danger zone, while 175°F signals that a fire is imminent.
Health Risks for Livestock
The primary health risks for livestock consuming wet hay stem from the proliferation of molds and the toxins they produce. Molds such as Aspergillus, Penicillium, and Fusarium thrive in high-moisture conditions and can produce mycotoxins that cannot be visually differentiated from non-toxic molds. Ingesting mycotoxin-contaminated hay can lead to severe systemic effects, including:
- Liver and kidney damage.
- Neurological disorders.
- Reproductive problems like reduced fertility or abortion.
- Suppression of the animal’s immune system.
Horses are particularly susceptible to mold spores, as they are non-ruminants and lack the digestive protection of cattle and sheep. Inhaling dust and spores from moldy hay can trigger Recurrent Airway Obstruction (RAO), commonly known as “heaves,” which is a chronic, asthma-like respiratory disease. Symptoms of RAO include coughing, nasal discharge, and labored breathing. Consuming fermented or heavily spoiled hay also poses a risk of digestive upset, most notably colic in horses. The spoilage process reduces the hay’s palatability, often leading to feed refusal, poor performance, weight loss, or nutritional deficiencies.
Mitigating Damage and Preventing Future Wetting
Producers must act quickly to manage hay exposed to excessive moisture. For hay suspected of heating, the internal temperature should be monitored daily using a specialized hay probe thermometer. If the temperature rises to 160°F, it should be checked every four hours; hay reaching 175°F warrants a call to the fire department. Preventing future wetting requires baling hay only when the moisture content is within safe limits—typically 18 to 20 percent for small square bales and 16 to 18 percent for large bales. Once baled, hay should be stored in a well-ventilated area, stacked to promote air circulation, and placed on pallets or railroad ties to prevent absorbing ground moisture.