Scope and disclosure: XES manufactures bale net wrap. We are not a fire service, forage scientists, veterinarians, or nutritionists. This guide organizes published Extension guidance on hay heating and fire risk. Your local fire department controls decisions during an active heating or fire incident, and a veterinarian or nutritionist controls feeding decisions for hay that got hot.
Quick answer: Yes — a hay bale or stack can heat more than once. The first rise is usually ordinary plant respiration ("sweating"), which fades as hay dries below roughly 15% moisture. But hay above about 20% moisture (18% in large square or round bales, per NDSU Extension) can sustain microbial heat production well past that first sweat. Reviewed Extension sources do not define a distinct "second cycle" mechanism, a fixed number of heat/cool cycles, or a set timeline for any further rise — a falling or cool reading does not end the roughly six-week monitoring window Extension sources flag as highest risk, and any renewed rise is handled the same way as the first: by tracking the temperature trend and acting on thresholds, not a cycle count or the calendar. No action is needed at or below 125°F (keep monitoring at-risk hay); check twice daily at 150°F; every few hours at 160°F; and above 175°F or smoke seen or smelled, call your local fire department or emergency services immediately and do not move the hay. Net wrap binds a bale's shape; it does not dry hay, stop microbial heating, or prevent fire.
1. Why hay heats: respiration, then microbial activity
Cut forage doesn't stop being biologically active the moment it's baled. SDSU Extension describes the first stage as ordinary plant cell respiration — the plant burning its own stored sugars for energy, releasing heat as a byproduct. That's the normal "sweating" or early heating every bale goes through, and SDSU notes it generally continues until the hay is below about 15% moisture.
Above roughly 20% moisture, SDSU Extension says conditions favor mesophilic bacteria, which become active and are "encouraged by the excessive moisture present," producing additional heat as they consume plant sugars. The higher the moisture, the longer that activity runs and the higher the eventual core temperature. Nebraska Extension frames the same process from the storage side: because baled dry hay isn't sealed away from oxygen the way silage or baleage is, heat, moisture, and oxygen let aerobic bacteria grow, consuming plant protein and sugars and producing carbon dioxide, water, and more heat.
This is not "fermentation" in the silage sense. Fermentation describes an oxygen-excluded process; what heats stored dry hay is aerobic microbial activity in the presence of excess moisture. If that heat keeps accumulating, Nebraska Extension notes it can trigger the Maillard (browning) reaction once the hay reaches roughly 170°F — covered below.
2. Can a bale heat more than once?
Yes, directly answering the question in this guide's title, but we're not going to speculate about why. None of the primary Extension sources we reviewed define a distinct mechanism for a "second" heating event, a fixed number of heat-and-cool cycles, or a set schedule for when a further rise might happen. What they describe is continued or renewed moisture and microbial activity while conditions remain favorable for it — not a separate, named second-stage process. That is as far as we'll go on causation.
What Extension sources do give is a monitoring window: both NDSU Extension and SDSU Extension put the highest-risk period at about six weeks after baling. Don't use a cycle count, and don't use a single falling or cool reading, to decide hay is clear — neither ends that six-week window. Instead, keep checking on the schedule in the monitoring section below, and treat any renewed rise the same way you'd treat the first one: by the thresholds in the action ladder above, not by counting how many times it has happened.
3. Baling moisture targets, by package
Package type changes the number — there is no single, universal moisture ceiling. Purdue Extension gives target moisture to begin baling without an effective preservative by package type:
| Package type | Target moisture to begin baling (no preservative) |
|---|---|
| Small rectangular bales | 20% or less |
| Large round bales | 18% or less |
| Large rectangular bales | 17% or less |
NDSU Extension separately frames the moisture level that makes stored hay a fire hazard: 20% or higher in stacked small bales, and more than 18% in stacked large square or round bales. These are two different questions — Purdue's numbers are baling targets before storage, NDSU's are in-storage danger levels — and we're presenting them separately rather than averaging them into one figure. Neither source gives a single "15%" number that applies to every package type; treat that kind of blanket figure as an oversimplification.
A preservative product (SDSU cites propionic-acid-type products as one example) can inhibit or reduce bacterial growth and allow a modestly higher moisture at baling, but the allowable moisture depends on that specific product's label and application rate. Follow the product label and your local Extension office's current guidance rather than a general rule of thumb, and don't assume a preservative removes the need to monitor temperature.
4. The temperature action ladder
The table below follows Purdue Extension's reproduction of the NRAES-18 critical temperatures and action steps, with any movement of hay conditioned on safe access and local fire-service direction.
| Internal hay temperature | Purdue/NRAES action |
|---|---|
| 125°F or lower | No action based on this reading alone. Keep monitoring hay that is at risk (baled wet or without moisture verification). |
| 150°F | Entering the danger zone. Check twice daily. Separate stacked hay only if it's safely possible and consistent with local fire-service direction. |
| 160°F | Reaching the danger zone. Check every couple of hours. Same movement boundary as above. |
| 175°F | Hot spots or fire pockets are likely. Stop air movement if possible and alert the fire service. |
| 190°F | Fire is likely. Remove hot hay only with fire-service assistance; contact with fresh air can produce flames. |
| 200°F or higher | Fire is imminent. Remove hot hay only with fire-service assistance. |
NDSU Extension sets a stronger stop boundary that we are not averaging against Purdue's ladder: above 175°F, or if smoke is seen or smelled, call the fire department or emergency services immediately and do not move any of the hay yourself — exposing an overheated or smoldering core to oxygen can cause a fire to rage out of control. Do not walk on, climb, or dig into a stack that may contain a fire pocket.
5. How long and how often to monitor
SDSU Extension recommends checking stored hay twice daily for about six weeks as it continues to dry down, and NDSU Extension notes hay fires usually occur within that same six-week window — not within the first few days. If your hay was baled at or near the moisture ceilings above, or without a moisture check, treat it as at-risk for the full six weeks rather than stopping once the first "sweat" passes.
6. How to check a hot stack safely
Stop and withdraw first if any emergency indicator is present: smoke is seen or smelled, there's a burning odor, the temperature is at or above the 175°F emergency boundary described above, or reaching the core would require climbing, walking on, or digging into the stack. In any of those cases, do not probe the stack yourself — back away to stable ground and call your local fire department or emergency services.
Short of those emergency indicators, and only from stable ground or a purpose-built, rated access point — never by climbing, walking on, or digging into the stack — use a commercial long hay or compost thermometer, following the manufacturer's instructions and any direction from the fire service or your local Extension office. Check several locations, keeping a partner or bystanders clear of the stack while you work. Never touch-test bare metal as a substitute for a thermometer reading — a rod that feels merely warm can still sit next to hay well past a safe threshold. If the core can't be reached safely by these means, the fire service decides how to proceed; no climbing or probing method should be treated as inherently safe.
7. Airflow: normal drying vs. a dangerously hot stack
These are two different situations, and the right airflow answer is opposite in each. For ordinary dry-hay storage, SDSU Extension recommends site drainage, elevation off wet ground, and enough space between bales for air movement to support continued drying and reduce the risk of heating in the first place.
Once a stack has reached the fire-pocket zone described above (Purdue's 175°F step; NDSU's 175°F stop boundary), the guidance flips: stop air movement around the hay if possible, because introducing fresh oxygen to a smoldering pocket can cause it to flare. Don't apply normal-storage ventilation advice to an already-hot stack, and follow local fire-service direction on when it's safe to disturb it at all.
8. Caramelized hay: heat damage is not automatically safe
Brown, tobacco- or sweet-smelling hay has gone through the Maillard reaction — the same browning chemistry as searing meat or baking bread — which Nebraska Extension says kicks off around 170°F when heat, moisture, and protein and sugars react. The reaction binds some protein into a form that a standard crude-protein test won't flag as unavailable, which means the reported number can overstate what an animal can actually use. That's heat damage to feed quality, not a cosmetic detail, and we are not calling caramelized hay "usually safe" or "usually palatable."
Nebraska Extension recommends adding a Heat Damaged Protein (HDP) or Acid Detergent Insoluble Crude Protein (ADICP) test when you see signs of caramelization, so a ration can be adjusted for the protein that's actually tied up. NDSU Extension similarly recommends testing hay for bound protein before feeding it if you suspect heat damage. The same wet conditions that produce caramelization can also support excess mold, which is a separate risk that a protein test won't catch and that should be assessed on its own. Don't feed caramelized hay free-choice on the assumption it's fine — have a veterinarian or qualified nutritionist weigh the lab report against the specific animal class and ration first.
9. Representative testing and who should interpret it
Define a logical lot first — generally bales made from the same field and cutting on the same day — and ask your receiving laboratory for its current sampling protocol before you take a sample. Penn State Extension recommends a coring device rather than a hand-grab sample and sampling multiple bales per lot, not one. Prefer a laboratory that is NFTA-certified or that documents equivalent analytical methods, proficiency testing, and quality control through the National Forage Testing Association.
A negative or reassuring result is evidence about the submitted sample, not a lot-wide safety clearance — mold, heat damage, or a hot pocket can be uneven through a stack even when a composite sample looks fine. Have a veterinarian or qualified nutritionist interpret any report for the actual animal class, the complete ration, and the specific situation; a protein or mold test alone doesn't make a feeding decision for you. If mold is visible or suspected, let your lab, veterinarian, or nutritionist decide whether an additional mold-species or mycotoxin panel is warranted for that lot — that's a professional call, not a default add-on.
10. Limitations of this guide
This guide summarizes published Extension guidance; it does not replace it, and it does not replace professional judgment. A few specific limits: none of the primary sources we reviewed describe a fixed number of heat-and-cool cycles or a set schedule for a second temperature rise, so we describe the mechanism (moisture and microbial activity) rather than asserting a timeline. Baling-moisture ceilings are Extension figures for typical conditions; your local Extension office, your preservative product's label, and your own moisture testing take precedence over any number in this article. The temperature action ladder is reproduced from Purdue's citation of NRAES-18 and cross-checked against NDSU's independent stop boundary — both are widely used, but neither is a substitute for your local fire department's direction during an actual incident. And nothing here is feeding advice: a veterinarian or nutritionist, not this article, should make the call on a specific lot for a specific animal class.
11. Where net wrap fits — and where it doesn't
Net wrap binds a finished bale's shape. It does not dry hay, does not stop microbial heating, and does not prevent fire. Heating is driven by the moisture the hay was baled at and what happens to it afterward; wrap doesn't change that chemistry. Managing heating risk means baling at a safe moisture for the package type (see the targets above), correct storage, ongoing monitoring, and following the source and label guidance above — not the wrap itself. Weather-protection choices such as site, base, and cover are a separate topic this guide does not address.
Frequently asked questions
Can a hay bale heat twice?
Yes. The first rise is usually ordinary plant-respiration "sweating," which fades as hay dries below roughly 15% moisture. But hay above about 20% moisture (18% in large square or round bales) can sustain microbial heat production well past that first sweat. Reviewed Extension sources do not define a distinct "second cycle" mechanism, a fixed number of heat/cool cycles, or a set timeline for a further rise — a falling or cool reading does not end the roughly six-week monitoring window Extension sources flag as highest risk. Any renewed rise is handled the same way as the first: by tracking the temperature trend against the action-ladder thresholds, not by counting cycles or the calendar.
What temperature is a hay fire emergency?
Purdue/NRAES guidance treats 175°F as the point where hot spots or fire pockets are likely: stop air movement if possible and alert the fire service. NDSU sets a stronger stop boundary: above 175°F or smoke seen or smelled, call the fire department or emergency services immediately and do not move the hay. At 190°F and 200°F or higher, fire is likely or imminent, and hot hay should be removed only with fire-service assistance.
How do I safely measure a bale's temperature?
If smoke is seen or smelled, there's a burning odor, the temperature is at or above the 175°F emergency boundary, or reaching the core would require climbing, walking on, or digging into the stack, withdraw to stable ground and call the fire service — do not probe it yourself. Short of those indicators, and only from stable ground or a purpose-built, rated access point, use a commercial long hay or compost thermometer according to the manufacturer's instructions, fire-service guidance, or your local Extension office, checking several locations with a partner or bystanders clear of the stack. Never touch-test bare metal, and never treat any climbing or probing method as inherently safe — if the core can't be reached safely this way, let the fire service decide.
Should hot bales be moved?
Only if it can be done safely and it's consistent with local fire-service direction. Purdue's guidance conditions moving or separating bales at 150°F and 160°F on safe access; NDSU's stronger rule is that above 175°F or smoke seen or smelled, you should not move any of the hay yourself because exposing an overheated or smoldering core to oxygen can cause a fire to rage out of control. Call your local fire department or emergency services immediately at that point.
Is caramelized hay safe to feed?
Not automatically. Brown, sweet- or tobacco-smelling hay has gone through the Maillard reaction, which begins around 170°F and binds some protein into a form a standard crude-protein test won't flag as unavailable, so the reported number can overstate what an animal can actually use — it's heat-damaged, not simply cosmetic. Extension sources recommend a Heat Damaged Protein (HDP) or ADICP test before relying on the standard crude-protein number. The same wet conditions that cause caramelization can also support excess mold, which should be assessed separately. Never feed it free-choice on the assumption it's fine; a veterinarian or nutritionist should weigh a lab report against the specific animal class and ration before you feed it, and a single test result does not clear an entire lot.
Does net wrap prevent hay from heating?
No. Net wrap binds a finished bale's shape, but it does not dry hay, does not stop microbial heat production, and does not prevent fire. Heating is managed by baling at a safe package-specific moisture, correct storage, ongoing monitoring, and the source and label guidance above — not by the wrap. Weather protection (site, base, and cover) is a separate topic this guide does not cover.
Featured photo: Reeder Creek Ranch, CO by inkknife_2000, licensed under CC BY-SA 2.0, via Wikimedia Commons.
