I always remember being at basketball games growing up and hearing that a close game by two very good teams would be called a “barn burner.” With hay season just around the corner, farmers have the very real risk of dealing with a “barn burner” of a different, more dangerous sort. Improperly cured or stored hay could lead to a biological chain reaction that could result in spontaneous combustion and loss of hay, barn structures, or even life. Knowing the processes involved and when hay is “dry enough” is critical to avoid these issues as well as other potential problems such as mold and degradation in hay quality.
To understand how moisture can actually lead to a burning hay bale, let’s understand the biology and chemistry that is driving it. Even after grass is first cut for hay, it is not “dead”. The plant cells within the grass are still undergoing respiration, (the burning of plant sugars to produce energy) releasing a small amount of heat. The level of respiration, and amount of heat generated, is dependent on the moisture level of the grass. Usually, the heat generated due to respiration is inconsequential as related to hay bale temperature and quality (if hay is allowed to dry to sufficient moisture levels).
Meanwhile, there are microbes ever-present in the environment and on the hay that is being cut. Two fungal species in particular, Aspergillus and Fusarium, feed on carbohydrates both on and inside the grass stems and leaves. This activity can result in losses of forage (dry matter) and also results in the generation of heat through biological activity. These fungi also produce spores, which can become an issue in hay that is put up too wet.
If hay moisture is high enough, the combination of plant respiration and ideal 
growing conditions for a large population of fungi which are also respiring (producing energy) can quickly cause hay bales to reach temperatures of 130° to 140°F. In some instances, this is the highest temperature hay bales will experience and the bales will eventually cool to ambient temperatures. This is what is sometimes referred to as a hay bale “heat” or “sweat” and usually occurs within the first few days of baling. However, this “sweat” is a side effect of microbes feeding on the most digestible parts of the hay, which results in lower quality. For example, studies have shown that the total digestible nutrients (TDN) of bermudagrass hay decreases by 1% for every 10°F increase in temperature over 100°F. This means that a good bermudagrass hay crop that is put in the barn with a TDN of 58% will likely come out of the barn with a TDN less than 54% if it heated up to 140°F or more.
As we mentioned, hay can commonly reach temperatures of 130° to 140°F during the first few days of storage. This will often kill off many of the fungi and bacteria found in the bales, but hay becomes a potential fire hazard when those internal temperatures do not cool off after that first “sweat”. This can occur when the conditions produced by the first set of microbes allow for the growth and multiplication of thermophilic (heat loving) microbes. The respiration of these fungi and bacteria can quickly raise the interior temperature of hay bales to 170°F or more. Temperatures this high bake the hay and convert it into a spongy carbon filled material with microscopic pores similar to charcoal. This material can react with oxygen and can self-ignite. If hay reaches an internal temperature of 175°F, fire is imminent and emergency steps should be taken.
How do we avoid these issues? Monitoring hay bale temperature is important, but checking hay moisture prior to and during baling is critical. Most research confirms that round bales should be baled at moisture levels of 15% or less while square bales should be kept less than 17%. These numbers are a good “rule of thumb”; but other factors can complicate these rules. The density of hay bales plays a major role in temperature. Advances in baling technology have allowed for much denser rolls, but higher density bales enables heat to build up much quicker than similar hay in a looser bale. Other factors, such as air circulation, relative humidity, and ambient temperature in the field and in the hay barn should be considered and play into how closely you monitor moisture levels at baling.
One should check hay bale temperatures twice a day for around six weeks after baling if the moisture levels were higher than recommended. If temperatures 
reach above 150°F, they are likely to go higher, running the risk of fire. If so, calling the fire department immediately is recommended. This should especially be done prior to relocating or handling any bales. The hay should be moved, but only once the fire department is there wetting down the hay as it is being removed. Moving hay introduces oxygen and this greatly increases the risk of fire.
