Two bulletins disagree by about 4 °C on the dew-point threshold for safe overnight drying. The disagreement is real, it comes from climate, and neither number travels.
Two bulletins, two dew-point thresholds
window HAY WINDOWwritten 2026-09-08rots yes — these figures moveresponsible Windrow Field Manual (a named masthead, not a person)
[['Bulletin A', 'Dew point 4 °C or more below the forecast low', 'A continental climate with dry nights'], ['Bulletin B', 'Dew point within 8 °C of the forecast low is still workable', 'A humid climate with heavy dew'], ["This manual's reading", 'Use the spread, and watch the first hour after dawn', 'Neither number is portable']]
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What each bulletin actually measured
Bulletin A was written for a region where summer nights are dry and the dew point falls well below the low temperature. In that setting a 4 °C spread is generous, and the advice to cut is safe. Bulletin B was written where humid air arrives overnight and the dew point sits within a few degrees of the low for hours. In that setting a 4 °C spread would rule out most of the season, so the bulletin uses a wider one and warns about the morning.
| Condition | Bulletin A's advice | Bulletin B's advice | What actually happens |
|---|---|---|---|
| Spread above 8 °C | Cut | Cut | Both agree; the row stays dry |
| Spread 4–8 °C | Cut | Cut with caution | Light dew on the top inch |
| Spread below 4 °C | Do not cut | Cut and bale in the evening | Heavy dew; the morning is lost |
Why the manual will not pick a winner
The manual holds that both are correct and that the useful output is not a threshold but a habit: compare the dew point with the forecast low, and expect the top of the row to rewet whenever the two come close. A threshold copied from a bulletin written in another climate is a number with no measurement behind it in your field.
There is a cost to this position. Readers want a number. The manual is declining to give one, and the reason is that the number would be less accurate than the reader's own first-week observation.
What to do instead
- Record the dew point and the forecast low for one season, and note on which mornings the row was wet at dawn.
- Record the moisture reading at seven in the morning on those days. That is your local threshold, measured.
- Compare your threshold with both bulletins and see which climate you are in.
Where the numbers come from
- S1Introduction (Chapter I), Hay and Straw Conservation, FAO Agricultural Services Bulletin 154supports the field conditions under which the two bulletins were written
- S2Large Round Bale Hay Storage (BSE-332), Virginia Cooperative Extensionsupports the storage moisture the overnight rewetting threatens
- S3Hay Storage (FSA-1057), University of Arkansas System Division of Agriculturesupports the storage-loss framing used to judge whether cutting was right
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Put this page down Put this page down when you have decided to record your own threshold rather than borrow one.