SPONTANEOUS HEATING
Wayne K. Coblentz1
ABSTRACT
Increased use of large-rectangular or large-round balers has been observed throughout the US. In part, this
move away from small- rectangular (100-lb) hay bales has occurred because of the high cost and limited
availability of labor required to handle these bales. Although the efficiency of harvest is improved with
large hay packages, there also is greater susceptibility within these large bales for spontaneous heating,
and in extreme cases, spontaneous combustion. Furthermore, heating is directly associated with losses of
DM and undesirable changes in forage nutritive value. Therefore, producer diligence and proactive action
may be required to limit or prevent this undesirable phenomenon.
Keywords: Alfalfa, combustion, hay, spontaneous heating
INTRODUCTION
Spontaneous heating is the most obvious result of plant and microbial respiration. In this process, plant
cells and different microorganisms consume plant sugars in the presence of oxygen to yield carbon
dioxide, water, and heat:
plant sugars + oxygen ≡≡≡≡≡ carbon dioxide + water + heat.
This process causes the internal temperature of any hay bale to increase, thereby facilitating drying by
encouraging evaporation of water, and ultimately lowering the energy content and digestibility of the
forage. Numerous factors contribute to the extent of spontaneous heating; a partial list includes: i)
moisture concentration at baling; ii) bale type; iii) bale density; iv) environmental factors, such as relative
humidity, ambient temperature, and air movement; v) storage site; and vi) use of preservatives. Normally,
the extent of heating that occurs in any hay bale is an excellent indicator of (undesirable) changes in
nutritive value that are often observed after storage.
Patterns of Heating
Figure 1 illustrates the typical patterns of spontaneous heating that occur during storage for smallrectangular (100-lb) bales of alfalfa hay made at 20 and 30% moisture (Coblentz et al., 1996). Beginning
immediately after baling, the internal bale temperature increases as a result of respiration by both plant
cells and microbes associated with the plant in the field (Roberts, 1995). This period of spontaneous
heating often lasts less than 5 days. Following a short period in which internal bale temperatures may
recede (at 4 to 5 days post-baling), a prolonged period of heating begins that can last for several weeks.
This heating occurs primarily as a result of respiration by microorganisms that proliferate during bale
storage. Furthermore, it should be noted that bales packaged at 30% moisture maintained a greater
internal bale temperature than the drier hay for about 25 days. Similar trends can be observed for
spontaneous heating within small-rectangular bales of bermudagrass hay (Figure 2), which is an important
cash crop throughout much of the southeastern US.
___________________________________
1
Wayne K. Coblentz, USDA-ARS; US Dairy Forage Research Center, 2615 Yellowstone Drive,
Marshfield, WI 54449. Published In: Proceedings of the Idaho Hay and Forage Conference. 28 February
to 1 March 2013. Burley, ID. University of Idaho Extension. http://www.extension.uidaho.edu/forage/
24
Bale Moisture
Of all the factors that affect spontaneous heating within a specific bale type and/or size, the moisture
concentration at the time of baling is the most important. This concept is illustrated in Figure 3, where
several alfalfa hay experiments conducted in Kansas during the 1990's are summarized. A heating degree
day (HDD) concept has been used to integrate the magnitude and duration of heating in hay bales; this
index often is used as a response variable in hay preservation studies, and is a better indicator of the total
heating incurred within hay bales than single point-in-time indices, such as maximum internal bale
temperature. For a biological system, the positive linear relationship between HDD and initial bale
moisture is remarkably close (r2 = 0.902), and clearly indicates that heating within small-rectangular bales
is primary a linear function of initial bale moisture. All bales within this summary were packaged as
small-rectangular (100-lb) bales, and the summary includes studies conducted at different times with
bales of different densities, including some that were mismanaged intentionally to induce heating.
Bale Size and Density
Both size and density of bales have a positive effect on spontaneous heating in hay packages. Density
enhances spontaneous heating simply by packing more DM into the bale, but not (generally) by causing
any change in the heat produced per unit of forage DM (Nelson, 1966; Rotz and Muck, 1994). However, a
mean density difference of 1.4 lbs/ft3 for bales packaged at five moisture concentrations ranging from 18
to 33% did not result in detectable differences in heating characteristics within small-rectangular bales of
bermudagrass hay produced in Arkansas (Coblentz et al., 2000). Large-hay packages are much more
likely to heat spontaneously, achieve greater temperatures, and maintain these elevated temperatures
longer than small-rectangular bales. These concepts are illustrated in Figure 4; large-round bales of
alfalfa-orchardgrass hay were made in 3-, 4-, and 5-ft diameter round bales over a wide range of bale
moistures in a recent Wisconsin study (Coblentz and Hoffman, 2009). Several important points can be
gleaned from these data that include: i) the relationship between spontaneous heating and bale moisture is
again positive; ii) greater HDD were measured in larger diameter bales; iii) for 3- and 4-ft diameter bales,
the relationship between HDD and bale moisture was linear (as observed previously for small-rectangular
bales), but linearity was lost and HDD were accumulated at a more rapid rate within larger 5-ft diameter
bales; and iv) the variability of data points around the regression lines was limited (r2 or R2 ≥ 0.880),
indicating initial bale moisture was the primary factor driving heating within any specific bale diameter.
The unique response of 5-ft diameter round bales may be partly related to the reduced surface area per
unit of DM, which restricts dissipation of heat and moisture from the bale. Commonly recommended
moisture thresholds for safe storage vary with bale type; generally, these are 18 to 20% moisture for small
(< 100-lb) bales, but usually are 3 to 5 percentage units drier for larger bales.
Preservatives
Preservatives have been used to reduce spontaneous heating and improve storage characteristics of moist
hays for several decades. Although there are numerous types of preservatives, those used most commonly
are organic-acid products, particularly propionic acid or propionic-acid-based formulations. These
products are usually buffered (pH stabilized) to prevent the corrosion of expensive hay equipment. In the
past, numerous researchers have reported reduced spontaneous heating when these products have been
properly applied to small-rectangular bales. However, recent studies at Marshfield (WI) have produced
mixed results within larger hay packages. Our recent work has demonstrated excellent effectiveness with
large (3 × 3 × 6-ft; 600 to 650-lb) rectangular bales (Figure 5), but poor effectiveness with large (5-ft
diameter) round bales (Figure 6). Reasons for this discrepancy remain unclear, but a couple of clear
points, as well as cautions, can be made that may have relevance for hay producers. First, hays treated
with propionic-acid-based preservatives tend to retain water, probably due to the hygroscopic nature of
the acid. After extended storage periods ranging up to 3 months, it is not uncommon to observe moisture
25
concentrations in treated hays that are several percentage units greater than observed in untreated
controls. Within our studies with large-round bales, this phenomenon may explain why treated hays
retained elevated temperatures (essentially a low-grade fever) relative to untreated control hays for
extended periods of time. This can be visualized from Figure 6; acid treatment generally suppressed HDD
during the first 28 days of storage, but these modest benefits generally were lost over the entire storage
period. Although temperature suppression was excellent for large-square bales (Figure 5), the
characteristic retention of moisture in treated hays was still observed. Lastly, a word of caution is advised;
all of our studies at Marshfield were conducted with individual (unstacked) bales, primarily to facilitate
statistical analysis. Typically, highly valued hays are stacked and often stored under roof to limit rain
damage prior to feeding or cash sale. It should not be assumed that responses reported in these projects
would be duplicated with hays stored in large stacks in which the overall forage mass is much greater than
that of individual bales.
Combustion
Festenstein (1971) suggested that internal bale temperatures in excess of 158oF are likely generated by
oxidative reactions, rather than by microbial and plant respiration, largely because enzymes can be
denatured and their systems rendered inactive at high temperatures. Elevated (> 158oF) temperatures
caused by oxidative chemical reactions may occur more than 30 days after baling. Clearly, large-round or
large-square bales are more prone to heat spontaneously and have a higher risk of spontaneous
combustion that may occur when internal bale temperatures reach about 340oF (Collins, 1995). 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 for spontaneous
combustion to occur near the outside of the stack where more oxygen is available.
SUMMARY
The process of spontaneous heating results from respiration of (primarily) plant sugars into carbon
dioxide, water, and heat. Typically, this phenomenon occurs in consistent patterns over time within smallrectangular (100-lb) bales, and is highly dependent on the initial moisture concentration of the hay within
the bale. Larger bale packages are more prone to heat spontaneously; in part, this occurs because there is
less surface area per unit of DM within the bale, thereby making it more difficult for large hay packages
to dissipate heat and water. Recent work with propionic-acid-based preservatives has generated mixed
results. Substantial reductions in spontaneous heating have been observed within treated 3 × 3 × 6-ft
large-rectangular bales, but results were disappointing for 5-ft diameter large round bales. For these recent
studies, all bales were stacked individually; therefore, the impact of large stacks remains unclear.
Generally, hay producers should be cautious when using large bale packages because the increased
sensitivity to spontaneous heating has greater potential to lead to spontaneous combustion.
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REFERENCES
Coblentz, W. K., and M. G. Bertram. 2012. Effects of a propionic acid-based preservative on storage
characteristics, nutritive value, and energy content for alfalfa hays packaged in large-round bales. J.
Dairy Sci. 95:340-352.
Coblentz, W. K., K. P. Coffey, A. N. Young, and M. G. Bertram. 2013. Storage characteristics, nutritive
value, energy content, and in vivo digestibility of moist large-rectangular bales of alfalfaorchardgrass hay treated with a propionic-acid-based preservative. J. Dairy Sci. (in press).
Coblentz, W. K., J. O. Fritz, K. K. Bolsen, and R. C. Cochran. 1996. Quality changes in alfalfa hay during
storage in bales. J. Dairy Sci. 79:873-885.
Coblentz, W. K., and P. C. Hoffman. 2009. Effects of bale moisture and bale diameter on spontaneous
heating, dry matter recovery, in-vitro true digestibility, and in-situ disappearance kinetics of alfalfaorchardgrass hays. J. Dairy Sci. 92:2853-2874.
Coblentz, W. K., J. E. Turner, D. A. Scarbrough, K. E. Lesmeister, Z. B. Johnson, D. W. Kellogg, K. P.
Coffey, L. J. McBeth, and J. S. Weyers. 2000. Storage characteristics and nutritive value changes in
bermudagrass hay as affected by moisture content and density of rectangular bales. Crop Sci.
40:1375-1383.
Collins, M. 1995. Hay preservation effects on yield and quality. Page 67 In Post-Harvest Physiology and
Preservation of Forages. CSSA Special Publication No. 22. K. J. Moore and M. A. Peterson, ed. Am.
Soc. Agron., Crop Sci. Soc. Am., and Soil Sci. Soc. Am., Madison WI.
Festenstein, G. N. 1971. Carbohydrates in hay on self-heating to ignition. J. Sci. Food Agric. 22:231-234.
Nelson, L. F. 1966. Spontaneous heating and nutrient retention of baled alfalfa hay during storage. Trans.
ASAE. 9:509-512.
Roberts, C. A. 1995. Microbiology of stored forages. Page 21 In Post-Harvest Physiology and
Preservation of Forages. CSSA Special Publication No. 22. K. J. Moore and M. A. Peterson, ed. Am.
Soc. Agron., Crop Sci. Soc. Am., and Soil Sci. Soc. Am., Madison WI.
Rotz, C. A., and R. E. Muck. 1994. Changes in forage quality during harvest and storage. Pages 828-868
in Forage Quality, Evaluation, and Utilization. 13-15 April 1994, Nat. Conf. on Forage Quality,
Evaluation, and Utilization, G. C. Fahey, M. Collins, D. R. Mertens, and L. E. Moser, ed. Univ.
Nebraska, Lincoln. ASA-CSSA-SSSA, Madison,WI.
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140
120
Temperature (oF)
100
30%
20%
80
60
Plant-associated
respiration
40
Microbial respiration
20
0
0
10
20
30
40
Storage Time (Days)
Figure 1. Internal bale temperature vs. storage time for small-rectangular (100-lb) bales of alfalfa hay
packaged at 20 and 30% moisture (adapted from Coblentz et al., 1996).
150
Temperature (oF)
120
90
60
31 3%
26.6%
16 9%
Plant30
Microbial
0
0
10
20
30
40
Storage Time (Days)
Figure 2. Internal bale temperature vs. storage time for small-rectangular bales of bermudagrass hay
packaged at 16.9, 26.6, and 31.3% moisture (adapted from Coblentz et al., 2000).
28
o
Heating Degree Days >86 F
1200
1000
Y = 56 X - 891
2
r = 0.902
800
600
400
200
0
15
20
25
30
35
40
Moisture at Baling (%)
Figure 3. Relationship between heating degree days > 86oF (HDD, a numerical index that integrates the
magnitude and duration of heating) and moisture at baling for small-rectangular bales of alfalfa hay.
4000
Heating Degree Days > 86oF
3500
3000
2500
2000
1500
1000
500
0
5
10
15
20
25
30
35
40
45
50
Bale Moisture, %
Figure 4. Relationship between heating degree days > 86oF and initial bale moisture for large-round bales
of alfalfa-orchardgrass hay made in Marshfield, WI. Regression relationships were defined by: 3-ft bales
(- - o - -), Y = 34.3 x – 339, r2 = 0.880; 4-ft bales (—●—), Y = 48.4 x – 425, r2 = 0.895; and 5-ft bales (—
●—), Y = 1.79 x2 – 147, R2 = 0.957 (adapted from Coblentz and Hoffman, 2009).
29
130
1.0%
0.6%
0%
Maximum Temperature, oF
120
110
100
90
80
High (27.4%)
Medium (23.8%)
Low (19.6%)
Moisture, %
800
1.0%
700
0.6%
0%
Heating Degree Days > 86oF
600
500
400
300
200
100
0
High (27.4%)
Medium (23.8%)
Low (19.6%)
Moisture, %
Figure 5. Maximum internal bale temperature (top) and heating degree days > 86oF (bottom) for largerectangular (3 × 3 × 6-ft) bales of alfalfa-orchardgrass hay made at 27.4, 23.8, or 19.6% moisture and
treated at rates of 0, 0.6, or 1.0% of weight bale weight with a commercial propionic-acid-based hay
preservative at Marshfield WI (adapted from Coblentz et al., 2013).
30
Heating Degree Days > 86oF
2500
○ Untreated ● Treated (0.5% of wet weight)
2000
1500
1000
500
0
10
15
20
25
30
35
40
Initial Bale Moisture, %
7000
Heating Degree Days > 86oF
○ Untreated ● Treated (0.5% of wet weight)
6000
5000
4000
3000
2000
1000
0
10
20
30
40
Initial Bale Moisture, %
Figure 6. Heating degree days > 86oF for large-round bales of alfalfa hay treated with a commercial
propionic-acid-based preservative at 0.5% of wet weight or untreated with the preservative (adapted from
Coblentz and Bertram, 2012). Results are shown for the first 28 days of storage (top) and for the entire
storage period (bottom).
31