Quick answer: Yes — it's normal for hay to "sweat" in the first days after baling, and for a probe meter to read several points higher than it did at baling. Bales baled at 15% can read low-to-mid 20s within a week, then settle back. The cause is stem-core moisture migrating out (a probe only reads the surface) plus a brief flush of microbial respiration. If the hay was truly dry when you baled it and you give bales air circulation, a normal sweat is harmless. Start worrying only if bales heat — climbing past about 120–130°F is your real warning sign, not the moisture number alone.
You baled beautiful hay at 15%. Three days later the meter says 21% and the bales feel faintly damp. Nothing is dusty, nothing is moldy — so where did the moisture come from, and should you panic? This question comes up every June, and the producers who've lived it have a clear, reassuring answer. Here's what "going through a sweat" actually is, why your meter climbs, and the one signal that separates a normal sweat from a real problem.
Is it normal for hay moisture to rise after baling?
Yes. A moisture-reading bump in the first few days is routine, and old-timers have a name for it:
"Yes, it's normal for baled hay moisture % to rise in the first few days after baling. I've had round bales that tested 15% moisture test in the low-mid 20% range in the next few days. Old timers called it 'going through a sweat'."
— Tx Jim · HayTalk thread 98516
One Massachusetts producer laid out a textbook case: a Delmhorst meter agreed with the baler probes at 8–10% during baling, yet 48 hours later the same bales read 15–22%, with a faint dampness to the touch — and zero heat or mold. That gap between "dry at baling" and "damp two days later" is the sweat, and understanding the mechanism is what lets you tell harmless from harmful.
Why your probe reads higher than the hay really is
A handheld probe doesn't measure the whole plant — it senses the moisture at the surface of the stems and leaves it touches. But a plant isn't uniform. Water locked deep in the stem cells releases slowly, and keeps migrating outward for days after baling. The best plain-English explanation on the forums is worth quoting in full:
"Think of a fresh loaf of bread out of the oven... Fresh out of the oven, feel it — it has a nice dry crust. Cut into it and it's super moist and soft. Put it in a bag (roughly equivalent to baling) and the moisture from the middle of the bread comes out, softens the crust, and condenses on the bag. If you guessed the dryness touching the crust when it was first taken out of the oven, you are assessing only on the outside. That's like a hay probe."
— slowzuki · HayTalk thread 93158
So a rising number doesn't necessarily mean the hay got wetter — it means the moisture that was always inside the stems is now reaching the surface where the probe (and any mold spores) can find it. Another producer put the same idea in numbers:
"Probes measure moisture on what the probe is touching, but if the inside of the stem is wetter, the probe may say 15% but the inside maybe 25% and in reality it's 20% hay — that's one reason it reads wetter after it is baled a day or 2. It can also happen in reverse: if you have bone dry hay and bale at night the monitor may say 30% but a day later may say 18% because the stems are dry inside."
— swmnhay · HayTalk thread 98516
For more on getting an accurate reading in the first place, see our hay moisture tester guide and the targets in baling moisture for net-wrapped bales.
The twist test: read the stem, not the surface
Because the probe misses stem-core moisture, experienced hands cross-check it by hand before baling. The most useful field method on the threads is the twist test, described precisely:
"The twist test doesn't lie like a meter. If you grab a little bundle of stems about 4" apart between your hands, offset it into a set of bike-pedals shape and 'pedal,' anything much over 3 twists before it breaks, you will see quite a bit of stem moisture release after baling and need acid or single-layer stacking to let it dry in the bale."
— slowzuki · HayTalk thread 98516
The companion rule is to separate dew from stem moisture. Surface dew burns off as you bale; stem moisture doesn't — and it's stem moisture that drives the sweat:
"Big difference between moisture from dew and stem moisture. If it's cured, dew moisture will burn off quickly as you are baling. Stem moisture won't change during baling — [it] will give you a false sense of security that it's right to bale. I ALWAYS make sure my hay is bone dry in the row the late afternoon before I make it."
— Hay diddle diddle · HayTalk thread 98516
A useful predictor: more stem means more sweat, so first cutting sweats the most. As ttazzman observes, "the more stem in your hay the more sweat you will have, so first cut will naturally have more — we have seen first cut bales at 15% or less rise to 20% in the barn with no issues."
Sweat vs. dangerous heating: know the difference
Here's the key distinction. A sweat is moisture moving and a brief, mild microbial flush. Dangerous heating is sustained microbial growth burning the plant's sugars as fuel — and it's the microbes that drive the temperature up, not the other way around:
"The mould / bacteria growing is what causes the heating, not the other way around."
— slowzuki · HayTalk thread 98516
Forage agronomists describe the same process: heat is generated when microbial growth uses the hay's carbohydrates for energy, converting carbohydrates and oxygen into carbon dioxide, water, and heat — which is why a heating bale loses dry matter and gains temperature at the same time (University of Idaho forage specialist Glenn Shewmaker, via Hay & Forage Grower). That means temperature, not the moisture reading, is your real alarm. Watch the bale's internal temperature, and use roughly these thresholds:
| Internal bale temp | What it means | What to do |
|---|---|---|
| Up to ~120°F | Normal sweat — mild respiration | Nothing; let it cure with airflow |
| 130°F | Active heating starting; mold likely forming | Move bales apart for airflow; monitor daily |
| 140°F | Self-heating is running away | Get bales out of the barn now |
| 150°F | Fire risk is real | Call the fire department before disturbing the stack — adding oxygen can ignite it |
| 160–175°F | Spontaneous combustion can occur on its own | Treat as an active fire emergency |
Field experience lines up with that ladder — producers treat a climb past about 120°F as the moment to start paying attention, while a 15%-to-20% moisture wobble with bales sitting at 70°F is a non-event. For the full danger-temperature playbook and how to probe a stack safely, read hay bale heating and fire prevention.
How to manage hay through a normal sweat
If the hay was genuinely dry when you baled it, you mostly just need to let the moisture escape rather than trap it. The forum consensus:
- Give bales air circulation. The single most-repeated tip: "Make sure the bales have air circulation around them so any moisture that wants to leave the bale can get away. If there is no heating I would not be concerned," says Gearclash. Don't pack a fresh, sweating batch tight into the center of a stack.
- Let marginal hay sweat before stacking it away. Producers hold back hay baled at the high end and let it ride: "If we start with marginal moisture hay, 18%-ish, we will let it sweat on the wagon for several days before barning it, or stack it separately" (ttazzman). Single-layer, cut-side up is a common precaution.
- Don't over-probe with a handheld. Repeated probing can show a false temperature rise from friction at the probe tip. To monitor heat, "stab the tester in a bale and leave it there, check it on a daily basis," as Gearclash's brother does — or use a dedicated long-stem compost/hay thermometer.
- Crimping helps the meter tell the truth. A well-set conditioner cracks the stem so internal moisture is more uniform — and your in-baler monitor can actually "see" it. One producer found that after fitting aftermarket rollers he only needed preservative when moisture topped 18%. (More on conditioners in our conditioner guide.)
- Use preservative when you must bale a touch wet. Buffered propionic-acid products are the standard tool to carry slightly tough hay safely through the sweat.
And the oldest rule still holds: "You make hay in the field, not in the barn." If it's dry in the windrow, a normal sweat resolves on its own. As one producer summed it up, "Trust the tester when you bale it. If it's dry when I bale it, I never worry about it after — it will just make me lose sleep over the sweat" (sprout).
The bottom line
- A post-baling moisture rise is normal. Bales baled at 15% reading low-20s a few days later is "going through a sweat," not a crisis.
- The probe reads the surface, not the stem core. Cross-check with the twist test and make sure hay is bone-dry in the row before baling.
- Watch temperature, not the moisture number. A normal sweat stays cool; sustained heat past ~120–130°F is the real warning sign.
- Give bales airflow, hold marginal hay separately, and reach for preservative when you have to bale a touch tough.
Cure it right, store it dry, and wrap it tight — a uniform, well-formed bale in good net wrap sheds water and breathes far better than a loose one. To speed the dry-down that prevents a heavy sweat in the first place, see how to make dry hay faster.
Frequently asked questions
Why does my hay moisture reading go up after baling?
Because a probe reads only the surface of the stems and leaves, while moisture locked in the stem core releases slowly and keeps migrating outward for days. As that internal moisture reaches the surface, the meter climbs even though no water was added. This is "going through a sweat," and it's why hay baled at 15% can read in the low-to-mid 20s a few days later, then settle back down.
How long does hay sweat after baling?
Usually about one to two weeks, depending on stem content, baling moisture, weather, and how the bales are stored. First cutting, with more stem, sweats the most. Bales with good air circulation finish the sweat faster; tightly packed or marginal-moisture bales take longer and need closer watching. The moisture reading typically peaks within the first several days, then gradually settles as the bale equalizes.
At what temperature should I worry about hay bales heating?
Temperature, not the moisture reading, is the real warning sign. Up to about 120°F is a normal sweat. At 130°F, move bales apart for airflow; at 140°F, get them out of the barn; at 150°F the fire risk is real, so call the fire department before disturbing the stack; and spontaneous combustion can occur around 160–175°F. Watch temperature rather than the moisture number alone.
What is the twist test for hay moisture?
Grab a small bundle of stems about four inches apart between your hands and twist it back and forth like pedaling a bicycle. If it takes much more than three twists before the stems break, there is significant stem-core moisture that a probe won't show, and the hay will release moisture after baling — meaning you should use preservative or stack single-layer so it can finish drying in the bale.
Will hay that sweats turn moldy?
Not necessarily. A normal sweat in hay that was genuinely dry at baling, with airflow around the bales and no sustained heating, typically cures out fine. Mold becomes a risk when surface moisture stays high and bales heat — microbial growth is what drives the temperature up. Watch for heat rather than the moisture number alone, give bales air circulation, and separate any marginal-moisture batches.
The XES Netting team manufactures bale net wrap for round balers and writes these guides so forage operators can find clear, source-cited answers. Every farmer quote in this post is verbatim with a link to the original HayTalk thread — go read the discussions in full.
Featured photo: Cylindrical Hay Bales by Stephen Burton, licensed under CC BY-SA 2.0, via Wikimedia Commons.
