Push a probe into the core and read the temperature ladder. Warm is normal for a few days; hot is a decision; and past the top rung the stack comes apart whether or not you want it to.
How to tell whether a bale is heating
window HAY WINDOWchecked 2026-09-16responsible Windrow Field Manual (a named masthead, not a person)
What you need A temperature probe with a stem long enough to reach the core of your bale; a notebook; a plan for where the bales go if they have to be separated.
[['Below 40 °C', 'Normal respiration after baling', 'Watch it daily for a week'], ['40–55 °C', 'Active heating, usually self-limiting', 'Check daily; separate if it keeps climbing'], ['55–65 °C', 'Sustained heating, moisture is too high', 'Break the stack apart and spread the bales'], ['Above 70 °C', 'Approaching the range where fire starts', 'Do not move bales into a building; call for advice']]
| p | r | o | b | e | c | a | r | d | : | t | h | e | t | e | m | p | e | r | a | t | u | r | e | l | a | d | d | e | r | ||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| C | o | r | e | t | e | m | p | e | r | a | t | u | r | e | |||||||||||||||||||
| W | h | a | t | i | t | m | e | a | n | s | |||||||||||||||||||||||
| W | h | a | t | t | o | d | o |
Steps
- Insert the probe to the centre of the bale, not the end. On a large round bale that means the middle of the flat face.
- Leave it long enough for the reading to settle. A probe pulled too early reads the shell.
- Record the temperature with the date and the bale's position in the stack. One reading is a point; a series is a trend.
- Repeat daily for the first week after baling, then weekly. Heating peaks in the first two to six weeks.
- If the temperature is climbing past 55 °C, separate the bales into a single layer in the open. Separation is the only reliable intervention.
Why the core is the only reading that matters
| Measurement position | What it tells you | Verdict |
|---|---|---|
| Outside surface of the bale | The weather, and nothing else | Useful only for detecting rain |
| 10 cm inside the flat face | The shell, which runs cool | Under-reports the risk badly |
| Centre of the bale | Where respiration heat accumulates | The reading the ladder is written for |
| Between two bales in a stack | The hottest place in the stack | Worth checking as well as the core |
The disagreement worth stating
The manual holds that a stack of large bales should be probed between bales as well as inside them, because the interface between two bales is where heat accumulates and it is the one place a bale-by-bale check misses. Most checklists stop at the bale core.
Where the numbers come from
- S1Power Take-Off Safety, Extension Ag Safety and Health Programsupports the handling safety context for working around heating bales
- S2Power Take-Off (PTO) Safety, Penn State Extensionsupports the equipment safety requirement when bales are being moved
- S3Propionic Acid Preservatives for Hay (Focus on Forage), University of Wisconsin-Madisonsupports the preservative alternative that changes the heating risk
One link, one document. No home pages, no search results, no bare domains.
Put this page down Put this page down when you have today's core temperature written down and a plan for 55 °C.