Measuring spontaneous heating

Under research conditions, spontaneous heating usually is not measured simply as internal bale temperature.

The concept of heating degree days (HDD) is often used as a single index that incorporates both the magnitude and duration of heating during the entire storage period.

Heating degree days usually are calculated by subtracting 86°F (30°C) from the daily internal bale temperature; these differences are then summed over all days in storage. An example of how HDD are calculated is summarized below:

Example:

Day Bale Temperature, °F Degrees > 86°F

This concept is often used to limit effects of ambient air temperature and because negative changes in forage nutritive value are most noticeable when internal bale temperatures exceed 86°F. Heating degree days can be viewed as a relative measure of the heat produced within each bale. Heating degree days totaling 150 or less are indicative of relatively minimal spontaneous heating; conversely, totals in excess of 800 HDD are indicative of hay that was baled excessively wet, probably at about 30 percent moisture.

Of all the factors that affect spontaneous heating, moisture content at the time of baling is the most important. Figure 4 summarizes several alfalfa hay experiments conducted in Kansas. The relationship between moisture content and HDD is quite close (r2 = 0.902). A one percentage unit increase in the moisture content of the forage at baling results in 56 HDD. A similar relationship was observed for bermudagrass hay baled in Fayetteville (Figure 5). In that study, about 43 HDD were accumulated for each increase of one percentage unit in the moisture content at baling. Regardless of the forage type, the level of heating that occurs is primarily driven by moisture content at baling, and this relationship is linear (HDD increases at a constant rate with bale moisture).

Stack of Hay Bales Attribution Stanley Howe

These studies were all conducted with conventional small rectangular bales. While it is generally assumed that similar relationships between moisture content and spontaneous heating exist in large round bales, there is limited documented research to support this.

Typically,the recommended moisture content at baling for larger, round hay bales is lower than is necessary for conventional rectangular bales. A good rule of thumb for maintaining acceptable storage in conventional rectangular hay packages is to bale hay at 20 percent moisture or less; however this guideline is often reduced to 16 to 18 percent moisture for larger hay packages.

A recent study conducted with mixtures of orchardgrass and alfalfa at the University of Tennessee measured the internal bale temperature of 1,373-pound round bales made at 24 percent moisture during a 96-day storage period. These results were compared with those of 25-bale stacks of the same material baled as conventional rectangular bales. Maximum internal bale temperatures for both bale types occurred at about the same time (11 to 12 days of storage); however, the peak internal bale temperature for the round bales was about 190°F compared to only 104°F for the conventional rectangular bales.

Internal bale temperatures in round bales can reach levels comparable to those in the University of Tennessee study through the respiratory processes of plant cells and microorganisms. However, higher temperatures are caused by oxidative chemical reactions that may occur as long as 30 days after baling. Clearly, large round bales are more prone to heat spontaneously and have a higher risk of combustion. Spontaneous combustion is thought to occur when internal bale temperatures reach about 340^°F. Normally, this does not occur in the center of the stack because lower concentrations of oxygen may limit temperature increases and make combustion less likely. It is more commonplace to observe spontaneous combustion near the outside of the stack where concentrations of oxygen are higher.

Figure 5. Relationship between heating degree days > 86°F (HDD) accumulated in conventional rectangular bales of bermudagrass hay and the concentration of moisture in the bale at packaging. Heating degree days can be interpreted as a single number that represents both the magnitude and duration of heating within the bale. Source: W. K. Coblentz.