Does Hay Go Bad? How Long Hay Lasts and Feeding Year-Old Hay

By the XES Netting team — bale net-wrap manufacturer · Updated August 16, 2026 · ~16 min read

When people ask, “Does hay go bad?” they usually need a decision, not an expiration date: Can this particular lot be bought, sold, or included in a ration for these particular animals? The answer depends on what happened before, during, and after baling; what hazards may be unevenly distributed through the lot; and whether a representative analysis matches the intended animal.

This article owns that lot-condition + intended-animal decision. It summarizes, but does not replace, specialized fire-response, moisture-testing, mold, forage-analysis, storage-design, toxic-plant, pesticide-label, or feeding guidance. University of Nebraska-Lincoln Extension makes the core distinction plainly: time alone does not drive hay-quality decline; storage conditions determine how much moisture, oxygen, weathering, and handling the hay experienced.

The old-hay decision in one table

Question	What supports a decision	What does not clear the lot
Is there an active fire risk?	Baling date, measured moisture, temperature trend, maximum recorded heat, odor or smoke, and fire-service direction	Assuming the danger passed after a few days
Did storage preserve the dry matter?	Roof, drainage, base, cover, row layout, rainfall or flooding, ground contact, leaks, bale integrity, and handling history	Age, exterior color, binding type, or a dry reading from one spot
Is the lot free of known hazards?	Field and chemical records, inspection across the lot, targeted laboratory tests, and professional interpretation	Smell, a slap test, animal interest, or one clean-looking bale
Does it fit the intended animal?	Representative analysis on a stated basis, actual intake, the rest of the ration, species, age, body weight, production stage, health, and performance goal	Calling hay “good,” “cow hay,” or “horse hay” without a defined lot and use

Core rule: A lot is not accepted because it is young, and it is not rejected because it is old. It is accepted only for a defined use after the history, current condition, representative test, and animal requirements agree.

Four phases that should not share one loss rate

Statements such as “hay loses X% per year” combine mechanisms that occur at different times and under different conditions. Separate these four phases before interpreting a loss figure or test result:

1. Post-baling respiration, heating, and fire risk. Plant respiration and microbial activity can raise temperature, consume dry matter, grow mold, and bind protein. This is an early storage hazard, not ordinary calendar aging. North Dakota State University Extension says hay fires usually occur within six weeks of baling, while Purdue Extension notes spontaneous combustion may take three to four weeks.
2. Protected-storage aging. Under a sound roof with stable, dry conditions, biological activity can be limited. Even then, handling losses, pests, leaks, oxidation, and vitamin changes can occur. A nutrient concentration expressed as a percentage may also hide a loss of total dry matter.
3. External weathering and ground loss. Rainfall, runoff, wicking from soil, humidity, bale density, shape, binding, base, cover, spacing, and storage duration affect the rind and bottom differently from the core. These losses are climate- and system-specific.
4. Opened-bale and feed-out contamination. Once hay is moved, opened, processed, or exposed at a feeder, new moisture, soil, manure, wildlife, carcass material, and physical waste can change what animals actually encounter and consume.

A storage study, laboratory result, or anecdote from one phase should not be transferred to another without its scope. That is why this guide does not publish a universal annual loss rate or a universal shelf-life number.

Check heating and fire risk first

Do not begin a feed-quality discussion while a stack may be heating. Purdue Extension’s hay-temperature ladder and NDSU’s fire-response boundary support the following actions. Measurements should be taken with appropriate equipment and without walking on an unstable hay mass or entering a suspected fire pocket.

Hay temperature	Action boundary
125°F or lower	Purdue lists no action, but keep the lot’s monitoring plan when moisture or prior heating makes it at risk.
150°F	Entering the danger zone; check twice daily and obtain local fire or Extension guidance before changing a stack.
160°F	Reaching the danger zone; check every couple of hours and escalate the response plan.
175°F	Hot spots or fire pockets are likely. Alert the fire service. NDSU says to call immediately above 175°F and not move hay.
190°F	Fire is likely. Removal is only with fire-service assistance; fresh air can cause hot hay to burst into flame.
200°F or higher	Fire is imminent. Fire-service assistance is required.

Monitor an at-risk lot for about six weeks after baling, not merely for the first few days. If the lot heated, record the highest temperature, where it occurred, how long monitoring continued, and what was done. Once the fire authority clears the hazard, test heat-damaged protein before making a ration decision.

What age can and cannot tell you about nutrients

Beta-carotene declines, but there is no universal percentage

Plants contain carotenoids such as beta-carotene, which animals can convert to vitamin A; hay does not contain one fixed amount of ready-made vitamin A. Crop species, maturity, leaf retention, curing weather, storage time, heat, light, oxygen, and humidity all affect the precursor supply. NDSU Extension and Kansas State’s veterinary diagnostic guidance both identify harvested forage and storage exposure as reasons vitamin A activity can decline.

That evidence supports direction, not a blanket claim that every hay lot loses a fixed percentage by a given birthday. It also does not support automatic supplementation. Requirements depend on species, body weight, production class, the rest of the ration, intake, and the form and stability of the supplement. Kansas State cautions that excessive vitamin A can be toxic and advises using a veterinarian for product and dose decisions.

Protein, fiber, and energy can change in different ways

Dry-matter loss is not the same as a change in nutrient concentration. Weathering can remove soluble carbohydrates and leaves; microbes can consume digestible material; handling can shatter protein-rich leaves; and leaching can move nutrients. As a result, crude protein reported as a percentage of dry matter may fall, remain similar, or even appear higher while the bale has fewer total pounds of feed and less useful energy. UNL Extension specifically warns that a stable percentage does not mean the lot improved.

Heating adds another problem: the Maillard reaction can bind protein to fiber. Crude protein may still appear in a basic report even though less is available to the animal. Rutgers Cooperative Extension recommends acid detergent insoluble nitrogen (ADIN) or a heat-damaged protein test when overheating is suspected; laboratories may report related measures as ADICP, NDICP, unavailable protein, or heat-damaged protein. Ask the laboratory and nutritionist which method fits the species and ration model.

Compare analyses on a stated basis. Oregon State University Extension explains that animal requirements are generally expressed on a dry-matter basis, while the as-fed amount includes water. A buyer, seller, or feeder should not compare numbers until both reports use the same basis.

How storage and binding affect the lot

Inside or under a sound roof is preferred because it limits direct precipitation and ground wetting. It does not erase baling moisture, earlier heating, leaks, condensation, pests, flood exposure, or contamination. Inspect the building, stack edges, floor, roofline, and the lot’s history rather than assuming “barn stored” is a complete condition report.

When outdoor storage is unavoidable, use local Extension guidance for the climate and bale system. A well-drained base, separation from runoff, intact binding, layouts that do not trap water, and a correctly secured cover can reduce exposure. Rainfall, humidity, freeze-thaw cycles, bale width × diameter, density, baling moisture, forage, binding, base, cover, row layout, and duration all affect the result.

Why this guide does not publish a generic storage-loss table

A USDA Agricultural Research Service abstract compared alfalfa and grass/alfalfa round bales bound with sisal twine, plastic twine, two net-wrap configurations, or bale bonnets, plus indoor storage. Outdoor bales were placed either on the ground or on a well-drained surface for five or 12 months; outcomes included rind moisture, dry-matter loss, and nutrient change. Net treatments performed better than twine in those trials, indoor storage and bale bonnets performed better than the uncovered bindings, and a well-drained surface reduced losses across treatments.

However, the accessible abstract does not report bale width × diameter, density, or dry-hay baling moisture. Those omissions matter. This article therefore uses the study for directional conclusions and does not transfer its percentages into a universal range for another climate, bale, forage, or storage duration.

Net wrap is binding, not a moisture barrier. Open mesh does not waterproof a bale, make wet hay dry, or create a validated “breathing” process. The USDA study supports a narrower statement: under its specific outdoor conditions, net-bound bales had lower rind moisture and dry-matter loss than twine-bound bales, while the bottom of every treatment still benefited from a well-drained surface.

Why inspection cannot clear mold or contaminants

Visible mold and dust matter, but mold is not only a respiratory issue. Penn State Extension reports that mold spores can contribute to equine recurrent airway obstruction and that mold or associated mycotoxins may reduce intake, impair nutrient use, suppress immunity, reduce fertility, contribute to abortions, and damage organs. The outcome depends on the animal species and class, the mold or toxin, dose, duration, and the rest of the ration.

Soaking or steaming may change airborne dust exposure, but it does not establish that a suspect lot is free of mycotoxins. Penn State specifically notes that soaking dusty hay does not reduce the threat of mycotoxin contamination. This guide does not clear moldy hay for any species, recommend dilution, or redirect suspect hay to cattle, horses, sheep, goats, pregnant animals, young animals, or lactating animals. Those decisions require a veterinarian or nutritionist with the lot history and targeted results.

Also inspect and document hazards that can cluster in only part of a field or bale:

• Foreign material and flood contamination: wire, trash, petroleum products, pesticides, soil, manure, and biological contaminants can make a visually intact bale unsuitable. The U.S. Food and Drug Administration says flood-impacted crops intended for animal food are usually unacceptable because floodwaters can contain sewage, pathogens, pesticides, chemical waste, and other toxic substances; each situation requires case-specific evaluation.
• Carcass material: UConn Extension identifies decomposed carcasses in forage or feed as a route for botulism toxin exposure, especially in horses. One clean core elsewhere in the lot cannot clear a carcass pocket.
• Blister beetles: cantharidin remains toxic in dead beetles, and swarms can be patchy. NDSU Extension warns that horses are particularly sensitive, while cattle and sheep can also be poisoned.
• Toxic plants and nitrate risk: the relevant plant, growth conditions, harvest timing, preservation, dose, and animal species determine risk. Request a targeted test or identification when field history warrants it.
• Pesticide and manure restrictions: obtain the product, active ingredient, application date, rate, field, harvest interval, feeding restriction, and current label. NC State Extension notes that some persistent herbicides can remain in hay and pass into manure; hay sellers should communicate those restrictions in writing.

Do not use smell, color, softness, a slap test, one moisture reading, animal enthusiasm, or “looks fine” as a safety certificate. These observations can trigger more investigation, but they cannot identify every toxin, nutrient gap, heat-damaged protein fraction, carcass, beetle cluster, or chemical restriction.

Build a representative test, not a convenient sample

First define the lot. Mississippi State Extension defines a hay lot as hay from the same field and cutting, harvested under the same environmental conditions with a uniform forage composition. For an old inventory, split that further when storage exposure differs: roof leak versus dry bay, ground-contact row versus raised row, heated stack versus unheated stack, or flooded versus unaffected bales should not be represented by one composite sample.

The receiving laboratory’s current package-specific protocol governs. Before coring, ask the accredited or proficiency-tested laboratory how it wants that bale package sampled and follow its instructions. Preserve in the report the field/cutting/storage lot, bale package, exact core count, insertion location and orientation, insertion depth, corer make or dimensions, sample handling, and whether results are as-fed or dry-matter basis. A convenient hand grab, too few packages, or a core taken in the wrong orientation is not representative merely because it reaches the laboratory.

Authoritative examples illustrate why one instruction should not be universalized:

Authority and scope	Source-specific example
Penn State: its baled-hay procedure	Take at least 20 widely separated cores per lot and insert the sampler to full depth into the end of each sampled bale. The article does not divide this instruction by bale package.
NFTA: its integrated hay-probe program	Take at least 20 cores at 90 degrees to bale butt ends; the probe guidance generally describes 14–24 inches of penetration and equipment that preserves the leaf-to-stem ratio.
Mississippi State: package-specific locations	For round bales, sample 15–20 bales from the curved side. For small squares, sample at least 10 near the center of the ends. For large squares, sample at a 45-degree angle from the side or at 90 degrees from the end.

These are examples, not a menu for mixing counts from one source with the location or depth from another. Confirm the current protocol with the laboratory receiving the sample, use a sharp corer that does not bias the leaf-to-stem ratio, then mix, label, store, and ship the composite exactly as that laboratory directs. An NFTA-certified laboratory or another laboratory that documents equivalent proficiency testing and quality control is preferred.

Test layer	What to discuss with the laboratory	Why it matters
Basic forage panel	Moisture and dry matter, crude protein, NDF, ADF, an animal-appropriate energy estimate, ash, and minerals	Estimates intake, nutrient density, mineral balance, and the dry-matter quantity available for ration formulation
Heating history	ADIN, ADICP, NDICP, unavailable protein, or heat-damaged protein, depending on laboratory method	A basic crude-protein number can overstate usable protein after severe heating
History-triggered hazards	Mold identification or count, specific mycotoxins, nitrate, toxic-plant identification, cantharidin, pesticide residues, or other targeted work	A generic nutrient panel does not answer a specific toxicology question

A negative result has limits when contamination is clustered. A composite can miss one carcass, one beetle swarm, one mold pocket, one roof-leak column, or one chemically different field edge. The veterinarian, nutritionist, toxicologist, Extension specialist, and laboratory should interpret the result alongside the sampling design and lot history.

Decision flow for an old lot

1. Identify the lot and reconstruct its history. Record field, cutting, harvest date, forage, baling conditions, preservative, storage location, moves, weather exposure, and known chemical or contaminant events. If histories differ, split the lot.
2. Check heating and fire risk before moving or opening hay. Use the temperature and smoke boundaries above. Fire-service direction overrides feed evaluation.
3. Inspect the storage system and lot. Check roof and floor, base and cover, ground contact, flood line, pests, foreign material, carcass evidence, toxic plants, beetles, mold, dust, and chemical records. Inspection identifies red flags; it does not certify safety.
4. Take a representative sample and add targeted tests. Ask the receiving qualified laboratory for its current package-specific protocol, then document the exact count, package, insertion location/orientation, depth, corer, handling, and basis actually used. Order additional analyses that the heating, crop, weather, chemical, or animal history warrants.
5. Compare the result with the intended animal and complete ration. Use the same dry-matter basis as the requirement model. Account for realistic intake and sorting. A veterinarian or nutritionist should address pregnancy, lactation, growth, disease, sensitive species, and supplements.
6. Choose and document one outcome. Accept the defined lot for the defined ration; have a qualified professional set conditions for limited use or non-feed use; or reject and dispose of the lot under local fire, environmental, pesticide, and waste guidance. Do not turn an uncertain lot into an animal trial.

What buyers and sellers should disclose

This is a best-practice transaction record, not a universal legal checklist. State law, contracts, pesticide labels, and regulated-feed rules may add requirements. The seller should avoid an animal-suitability guarantee; the buyer’s veterinarian or nutritionist must match the lot to the ration.

• Lot identity: field, cutting, forage species or mixture, harvest date, lot size, and whether any bales were separated because their history differed.
• Baling: measured baling moisture, instrument and method, bale type, preservative product and rate, and any rain before or after baling.
• Heating: maximum measured temperature, measurement locations, monitoring dates and duration, odor or smoke events, and fire-service or Extension involvement.
• Storage: inside or outside, roof or cover, base and drainage, row or stack layout, ground contact, moves, rainfall, flood, leak, condensation, and visible weathering.
• Known hazards: mold, dust, pests, blister beetles, weeds or toxic plants, carcass or foreign material, manure exposure, floodwater, and other contamination.
• Chemicals: pesticide product and active ingredient, application date and rate, preharvest or feeding restrictions, and any manure or compost advisory that must follow the hay.
• Analysis: receiving laboratory and its package-specific protocol; field/cutting/storage lot represented; exact core count; bale package; insertion location, orientation, and depth; corer; sample handling; report date; analytes and methods when relevant; and whether values are as-fed or dry-matter basis.
• Bale and sale terms: binding, bale width × diameter or package dimensions, actual measured weight with moisture at weighing, quantity, unit of sale, price, loading, delivery, rejection terms, and who bears weather or handling risk in transit.

Good disclosure does not prove suitability. It gives the buyer, laboratory, veterinarian, and nutritionist enough information to ask the right questions instead of relying on the word “old.”

Frequently asked questions

The bottom line

Hay does not become safe or unsafe when a calendar page turns. Check the defined lot’s history, clear heating and fire risk first, inspect for uneven hazards, obtain a representative and appropriately targeted analysis, and compare the result with the complete ration for the intended animal. The defensible conclusion is not “old hay is fine” or “old hay is bad.” It is one documented decision for one lot and one use.

Sources

1. Purdue University Extension, “Is Your Hay Too Hot?” — temperature ladder, heating, mold, protein binding, and extended monitoring.
2. North Dakota State University Extension, “Preventing Hay Fires” — six-week risk window, smoke or odor warning, 175°F fire-service and no-move boundary.
3. University of Nebraska-Lincoln Extension, “Why Storage, Not Age, Determines Hay Quality” — storage mechanisms, dry-matter loss, nutrient interpretation, and heat-damaged protein.
4. NDSU Extension, “Evaluate Vitamin A Supplementation for Beef Cattle” — beta-carotene, storage exposure, production-class requirements, forage analysis, and professional consultation.
5. Kansas State University Veterinary Diagnostic Laboratory, “Vitamin A: A Major Player in Stillborn and Weak Calf Syndromes” — harvest and storage losses, supplement instability, toxicity warning, and veterinary dosing.
6. Penn State Extension, “Mold and Mycotoxins in Horse Hay” — respiratory disease, intake effects, mycotoxin outcomes, and limits of soaking.
7. Penn State Extension, “Forage Quality Testing: Why, How, and Where” — field/cutting lots, full-depth end cores, at least 20 cores per lot, laboratory quality control, and buying or selling with analysis.
8. Mississippi State University Extension, “Hay Testing and Understanding Forage Quality” — lot definition and package-specific counts, locations, and orientations for round, small-square, and large-square bales.
9. National Forage Testing Association, Laboratory Certification and Hay Probes — proficiency, laboratory reproducibility, at least 20 cores, butt-end orientation, 14–24-inch depth, and probe design.
10. Rutgers Cooperative Extension, “Testing for Heat Damaged Protein in Hay/Forage Put Up Too Wet” — Maillard reaction, ADIN, and heat-damaged protein testing.
11. Oregon State University Extension, “Understanding Your Forage Test Results” — as-fed versus dry-matter basis and ration comparison.
12. USDA Agricultural Research Service, large-round-bale storage study abstract — scoped binding, base, duration, rind-moisture, dry-matter, and nutrient outcomes.
13. NDSU Extension, “Watch for Poisonous Blister Beetles When Cutting Hay” — clustered contamination, persistent cantharidin, species risk, and label restrictions.
14. NC State Extension, “Herbicide Carryover in Hay, Manure, Compost, and Grass Clippings” — label records, seller disclosure, persistent residues, and manure advisories.
15. UConn Extension, “Equine Botulism” — decomposed carcasses in feed, round-bale inspection, and veterinary prevention.
16. U.S. Food and Drug Administration, “Safety of Food and Animal Food Crops Affected by Hurricanes, Flooding, and Power Outages” — animal-feed contamination, testing, and case-specific regulatory review.

Featured photo deployment source: Marion Post Wolcott Blairs Virginia, showing a farm worker loading hay into a barn on the A. B. Douglas tobacco farm in Blairs, Virginia, September 1939. Photograph by Marion Post Wolcott for the Farm Security Administration; public domain via the Library of Congress and Wikimedia Commons.