There is a great misconception that once hay is “dry” and baled it is devoid of life. But hay is never completely dry; it’s full of microscopic life. If the hay is not dry enough, those life forms can cause major problems – heat damage and hay fires.

Research suggests that hay moisture content should be below 20% for small rectangular bales, 18% for round bales, and 16% for large rectangular bales to prevent heat damage.

These are still good rules of thumb, but also consider the effect that advances in bale package sizes and high-density baling systems have added to the heating problem. It’s more important than ever to bale at a safe moisture content.

But how can these microorganisms – mainly fungi species like Aspergillus and Fusarium, bacteria, and others that are ever-present in hay – cause growers so many problems?

They feed on available carbohydrates on the surface of forage plants and inside stems and leaves. This results in dry matter loss, reduces hay quality and also generates heat. Hay bale, stack and barn temperatures can get very hot. In extreme cases, bales can spontaneously combust. Even if the temperature does not reach these extremes, these microorganisms can form spores that give hay a moldy smell and convert ordinary plant compounds into potent mycotoxins.

Nearly all hay goes through a “sweat” the first few days after baling, when temperature rises. A University of Kentucky study of two alfalfa hay cuttings tracked bale temperature over time. The summer cutting, which was put up at 16% moisture, stayed relatively cool, even during higher average ambient air temperatures. However, the fall cutting was baled at 20% moisture, a little wet for round bales, and it spiked at over 140°F within just three days.

The heat generated when hay sweats is a side effect of the microorganisms consuming the most digestible portions of the forage, such as carbohydrates like sugar and starch. Consequently, a substantial portion of the hay could be consumed and lost during this process.

For every 10°F increase in maximum temperature, nearly 2% of the dry matter (DM) of an alfalfa-orchardgrass hay would be lost during storage, according to research by Wayne Coblentz, USDA-U.S. Dairy Forage Research Center agronomist. He studied the impact that hay moisture and heating have on DM loss, hay quality and heat risk.

It is worth noting, though, that some forage species, including bermudagrass, have fewer readily available carbohydrates than others and may lose DM at a slower rate. Coblentz showed that bermudagrass hay would lose about 1.3% of DM for every 10°F increase in maximum temperature.

Since these losses are coming from the most digestible forms of energy in the forage, hay heating comes at the expense of digestibility and the concentration of energy in the forage.

Coblentz showed that the total digestible nutrients (TDN) of an alfalfa-orchardgrass hay lot can decrease by more than 2 percentage points and a bermudagrass hay by more than 1 percentage point for every 10°F increase in maximum temperature. In other words, a good alfalfa-orchardgrass hay crop that’s a little too wet when baled may have gone into the barn at 62% TDN, but it likely will come out with less than 56% if heated to 140°F or more.

I’ve focused on limiting the maximum temperature, but also be concerned about the amount of time that the hay spends with an elevated temperature. Hay growers should minimize the time their hay spends at even moderate temperatures.

As alluded to earlier, the effects of bale type, size and density, as well as other factors, contribute to the extent of hay heating. The amount of available carbohydrates in the forage crop, air circulation around and in the haystack, relative humidity in the storage area and the ambient temperature and humidity outside can each affect hay heating.

Each producer’s situation will be somewhat different because of equipment, storage technique and climatic differences. So, within the ranges provided in the accompanying graphic, hay growers should allow for the effect of these factors when deciding which target bale moistures are right for their farms.

Read more about moisture content:

Check Every Bale, Maintain Hay Quality

Natural Moisture Loss Protects Stover Bale Quality