Moisture content is the share of a bale's weight that is water, measured wet basis. It is the single number that decides whether the bale you just made stores or heats, and the safe band depends on bale size.
Hay moisture content: the baling numbers that matter
window HAY WINDOWchecked 2026-09-16rots yes — these figures moveresponsible Windrow Field Manual (a named masthead, not a person)
[['Small square, 15–25 kg', '15–20%', 'Above about 22% in a tight stack'], ['Large round, soft core', '15–18%', 'Above about 20%'], ['Large round, hard core', '14–16%', 'Above about 18%'], ['Large square, high density', '12–16%', 'Above about 16%']]
| p | r | o | b | e | c | a | r | d | : | b | a | l | i | n | g | m | o | i | s | t | u | r | e | b | y | b | a | l | e | s | i | z | e | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| B | a | l | e | t | y | p | e | ||||||||||||||||||||||||||||||||
| T | y | p | i | c | a | l | s | a | f | e | b | a | n | d | |||||||||||||||||||||||||
| A | b | o | v | e | t | h | i | s | i | t | h | e | a | t | s |
Wet basis, and why the number confuses people
Moisture content is quoted wet basis: the mass of water divided by the total mass of the sample. A bale at 20% wet basis is one fifth water. The confusion arises because drying is often described in dry basis in research papers, where the same bale reads 25%. Both are correct and they are not the same number, so a figure without its basis is unusable.
Why bale size moves the safe band
Heat in a bale comes from microbial respiration, and respiration needs water and oxygen and produces heat faster than the bale can shed it. A large, dense bale sheds heat more slowly than a small one, so the same moisture is safe in one and dangerous in the other. That is the whole reason the safe band is a band and not a number.
| Bale type | Mass | Heat it can shed | Consequence for the moisture band |
|---|---|---|---|
| Small square | 15–25 kg | Quickly, from a large surface per kg | Tolerates the highest moisture |
| Large round, soft core | 300–500 kg | Moderately | Middle of the band |
| Large round, hard core | 500–900 kg | Slowly | Low band, and the core is the risk |
| Large square | 400–900 kg | Slowly, and the stack compounds it | Lowest band in practice |
The bands above are the ranges the extension literature converges on. They are starting points. A bale made at the top of a band and stacked tight is a different object from the same bale left in a single layer.
Boundary: moisture content is not dry matter and not water activity
Dry matter is what is left when the water is gone, so dry matter and moisture content sum to 100% on a wet basis. Water activity is a different measurement again, and it is what the moulds actually respond to. Near kin, different instruments, different numbers.
The common misreading: a probe reading taken from the outside of a bale is read as the bale's moisture. The outside of a large round bale can read four points away from the core, and it is the core that heats.
The sentence the manual will defend
If a probe pushed into the core of a bale reads 18% and a capacitance meter pressed against the side reads 22%, believe the probe and stop baling. The meter is measuring a shell; the probe is measuring the part that will burn.
Where the numbers come from
- S1Round Bale Hay Storage (BAE-1716), Oklahoma State University Extensionsupports the storage-moisture bands for round bales by bale type
- S2Hay Moisture and Baling (PSS-2588), Oklahoma State University Extensionsupports the moisture range in which hay can be baled without heating
- S3Preventing Fires in Baled Hay and Straw, Extension Ag Safety and Health Programsupports the relationship between bale moisture and spontaneous heating
One link, one document. No home pages, no search results, no bare domains.
Put this page down Put this page down when you know which band applies to the bales you are making. The band is the answer; the exact percentage is the detail.