Heated Bales: Management and Handling - Frequently Asked Questions

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 I baled some of my hay when it wasn't quite dry, what will happen?
Chances are if you baled your hay too wet (over 15% moisture) there is a risk of prolonged heating. Moisture levels above 20 percent allow the respiration process to continue and mould to develop, producing heat. When hay heats beyond 100 F (38 C), browning or carmalization occurs. Wet hay bales also produce internal heat that may result in spontaneous combustion.

What are the symptoms of heating?
Check your hay or greenfeed regularly for a slight caramel or tobacco odour, a strong musty smell, a strong burning odour, visible steam, or if the feed feels hot to the touch. Another indication that your bales have heated is that the bales will lose their initial shape and flatten out.

How does heating occur?
Plant material and microorganisms on the plants continue to respire after cutting. Once the forage is baled up, it will go through a heating or sweating out process. Under normal conditions, the bale will reach temperatures no higher than 130 F (54 C). The factors that govern the extent of heating include: forage moisture content at baling, bale type (large vs small, round vs. square), bale density (soft core or hard core), environmental factors (relative humidity, ambient temperature and air movement), storage site, and use of preservatives.


Characteristics of heating

Up to 120 F (49 C)

Not considered to cause serious forage quality loss.Mold or mustiness may develop at this temperature range. No problem.

121 to 140 F (50 to 60 C)

Heating can cause some of the protein and fibre to become less digestible. Hay will caramelize, smell like tobacco and have a brown colour. Loss in digestibility is greater at these temperatures than at lower temperatures. The level of heat damaged proteins increase. If excess heat can be released from the stack or bale, temperatures do not generally rise above 130 to 140 F (54 to 60 C). Temperature may go up and down, recheck in a few hours. Caution zone.

141 to 160 F (61 to 71 C)

Heating dominated by the respiration of fungi. At 150 F (65.5 C), check the temperature everyday! At temperatures above 160 F chemical reactions dominate the heating process these can escalate very rapidly. If temperatures continue to rise and heat cannot be released from the storage site a dangerous condition can occur. At 160 F check the temperature every four hours.

175 F (80 C)

Check temperature every few hours. Notify the fire department that you have a potential problem and ask them for their recommendations. Danger of fire.

195 F or hotter(90.5 C)

Spontaneous combustion is possible. Do not attempt to move hay without fire department assistance. Moisture loss escalates until the forage is dry enough to burn, material will ignite at 450 to 525 F (232 to 274 C).
Adapted from "Hot hay! How hot is too hot?" by Dr. Steve Barnhart Iowa State University Department of Agronomy, 1998. "Guarding Against Hay Fires" by Dr. Charles B. Ogburn, Alabama Cooperative Extension System, Auburn University, Department of Agricultural Engineering. 1995.

How long will the heating occur?
The internal bale temperature should peak at about 3 to 7 days after baling. After that, the temperature should decline within 15 to 60 days, depending on the moisture content of the forage, external humidity, bale density, and the amount of water absorbed by rain or snow.

A cycle of heating and cooling may occur several times during the weeks after baling as the microbial population increases and decreases. However, the maximum temperature decreases during each subsequent cycle. The internal bale temperature will eventually stabilize near the ambient temperature. Baled feed becomes a potential fire hazard when the internal bale temperature does not cool after the first heating cycle.

How do I monitor the bales?
It is recommended that bales be checked four or five days after baling, to monitor their initial temperature level. To monitor your baled feed, buy a thermometer that will read up to 200 F (94 C) with an accuracy of plus or minus 5 F(15 C). Thermometers used in laboratories, for candy making or those produced to monitor compost are all sufficient. Spirit filled thermometers or electronic thermometers should be used rather than mercury filled ones so that if it breaks it will not contaminate the feed.

Check the most tightly packed part of the bale, this is where the heat tends to build, generally 6 to 12" from the centre, therefore, a 18 to 36" thermometer is adequate.

Stack evaluation can be achieved by reaching 5 to 10' down from the top or in from the side. A hollow probe can be placed through the stack and then place the thermometer inside to take the temperature (leave the thermometer in the probe for 10 to 15 minutes).

How long should I leave the bales in the field or unstacked?
Research has shown that the maximum heating of hay usually occurs within one week of baling. Ideally, hay should remain unstacked for the first three weeks after baling. Dry hay does not heat excessively because it lacks the necessary moisture to support any significant microbial growth.

How do I determine the extent of heat damage in the forage?
The extent of heat damage can be estimated in a feed lab by measuring the amount of crude protein associated with the acid detergent fibre (ADF) fraction. When you submit your forage for a feed test, ask the laboratory to measure the amount of unavailable protein. Depending on the lab's analytical procedures, the test may be reported as acid detergent insoluble nitrogen (ADIN %), acid detergent fibre protein (ADF-P %) or acid detergent fibre nitrogen (ADF-N %). Values reported as N % can be converted to protein % by multiplying by 6.25.

Other considerations:
If you have greenfeed that is high in nitrates, and if the bales suffered from heat damage, you should test the nitrite level in the feed. As the forage heats, nitrate converts to nitrite, and nitrite is ten times more toxic than nitrate.

For more information please refer to the following links:
Spontaneous Combustion and Hay Fires
Hay Fire Prevention and Control
Preventing Fires in Baled Hay and Straw
Nitrate Poisoning and Feeding Nitrate Feeds to Livestock

Or contact the Alberta Ag-Info Centre at 310-FARM (3276)

Prepared by Ag-Info Centre, Alberta Agriculture & Forestry 310-3276
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For more information about the content of this document, contact Barry Yaremcio.
This document is maintained by Brenda McLellan.
This information published to the web on June 1, 2005.
Last Reviewed/Revised on January 30, 2019.