How To Store Hay Safely for Horses and Cattle

By Dr Duncan Houston

Hay is too expensive to let it rot in the rain, wick moisture from the ground, grow mould in a poorly ventilated shed, or turn into a fire risk because it was baled too wet.

Good hay storage is not just about saving money. It protects forage quality, reduces waste, preserves nutrients, lowers mould exposure, and helps prevent respiratory disease, colic, poor intake, and feed refusal. For cattle, poor storage can mean major dry matter and nutrient losses. For horses, the bar is even higher because mould and dust can create serious respiratory and digestive problems.

The real question is not just, “Where can I put the hay?” It is, “Will this hay still be clean, dry, safe, palatable, and nutritionally useful when I feed it?”

Quick Answer

Store hay in a dry, well-ventilated area, ideally under cover, off the ground, away from moisture, pests, and direct weather exposure. Horse-quality hay should generally be baled around 10 to 15% moisture, because hay above 17% moisture has increased mould risk, and wet hay can also become a fire hazard. Outdoor round bales should be dense, covered if possible, stored on a well-drained surface such as gravel or pallets, kept out of shade, and arranged in north-south rows with space between rows for airflow and drying. (University of Minnesota Extension)

Why Hay Storage Matters

Poor hay storage creates three main problems:

1. You lose dry matter.

2. You lose nutrient quality.

3. You increase animal health risks.

Hay stored outside on the ground can lose a large amount of feed value. Mississippi State University Extension notes that high-quality hay stored outside on the ground may lose 25 to 30% of total dry matter, and feeding losses can reach 40% in some situations. (MSU Extension Services)

University of Minnesota Extension reports that outdoor round bale storage losses can range from 5 to 35%, depending on precipitation, storage site, and bale condition. The outer 4-inch layer of a 6-foot round bale contains about 25% of the total bale volume, which is why surface weathering matters so much. (University of Minnesota Extension)

That means a “little spoiled outside layer” is not always little. It can be a big chunk of the bale you paid for.

What Moisture Level Is Safe for Hay?

Moisture is one of the most important hay storage factors.

For horse-quality hay, University of Minnesota recommends baling between 10 and 15% moisture. Hay baled over 17% moisture is at increased risk of mould, and hay over 25% moisture is at risk of severe heat damage and possible fire hazard. (University of Minnesota Extension)

General forage moisture targets vary by bale type. North Dakota State University Extension states that moisture at baling should be no more than 20% for small square bales, 18% for round bales, and 15% for large square bales, with larger bale packages needing lower moisture. (North Dakota State University Extension)

A practical rule:

For horses, aim stricter. For cattle, hay quality can vary more by class of animal, but mouldy or heat-damaged hay is still a feed-quality and health concern.

Why Wet Hay Is Dangerous

Wet hay is not just less nutritious. It can mould, heat, lose digestibility, and in severe cases combust.

University of Minnesota’s hay fire guidance states that hay baled below 15% moisture has very low fire risk, 16 to 20% moisture may mould and has slight fire risk, 21 to 25% moisture will likely mould and has moderate fire risk, and hay over 25% moisture has severe heat damage and high fire risk. (University of Minnesota Extension)

North Dakota State University also warns that hay becomes a fire hazard when small stacked bales are 20% moisture or higher, or when stacked large square or round bales are above 18% moisture. Hay fires usually occur within six weeks of baling. (NDSU)

This is the part many owners underestimate. Hay can look calm from the outside while microbial activity is heating the centre.

Best Hay Storage Options

Best Option: Covered, Ventilated Storage

A hay shed, pole barn, open-sided shelter, or well-ventilated covered structure is usually the best option. Indoor storage reduces weather exposure, lowers nutrient loss, and protects hay from rain and ground moisture.

University of Minnesota notes that indoor storage usually reduces storage losses by approximately two-thirds. (University of Minnesota Extension)

Good covered storage should have:

• A dry floor or raised base

• Good airflow

• No roof leaks

• No condensation dripping onto hay

• Space between hay and walls

• Rodent and pest control

• Access to feed older hay first

• Fire safety planning

Do not stack hay tightly against side walls or rafters. Illinois Extension recommends allowing room for ventilation, including gaps in large stacks and at least two feet between bales and rafters. (Illinois Extension)

Good Option: Tarped Hay on a Well-Drained Base

Tarps can work if they are managed properly. A tarp over hay sitting directly on wet ground is only half a solution.

University of Minnesota reports that a good plastic covering or tarp can cut outdoor storage losses in half, but the storage site still needs to be well drained. (University of Minnesota Extension)

For tarped hay:

• Use a gravel pad, pallets, timbers, or tyres under bales

• Secure tarps properly

• Keep water from pooling

• Allow airflow where possible

• Check regularly for condensation, mould, and rodents

• Remove damaged or wet outer hay before feeding

A tarp should shed water, not trap moisture and create a hay sauna.

Higher-Risk Option: Outdoor Uncovered Storage

Outdoor storage can be done, but it needs planning.

Outdoor round bales should be:

• Dense and tight

• Net-wrapped where possible

• Stored on a well-drained site

• Kept off bare soil where possible

• Kept out of shade

• Arranged to allow airflow

• Used before quality declines too far

University of Minnesota notes that dense bales sag less and have less surface area in contact with the ground, while net-wrapped bales can reduce grass hay dry matter losses compared with twine when stored outside. (University of Minnesota Extension)

How To Store Round Bales Outside

Round bales are convenient, but they are vulnerable to weathering, bottom spoilage, and feed loss if stored badly.

Best practice includes:

• Store on a well-drained site

• Avoid shade

• Use a slight slope where possible

• Keep bales off the ground with gravel, pallets, timbers, or a rock pad

• Store rows north to south

• Leave 3 to 4 feet between rows

• Butt flat ends together if storing in rows

• Avoid side-to-side contact between rounded bale surfaces

• Do not pyramid-stack uncovered bales

• Cover stacks if they must be stacked

SDSU Extension recommends north-south rows with a 3 to 4 foot gap between rows, avoiding shade, using a slight south-facing slope, and storing on a well-drained surface such as a rock pad. It also recommends covering outdoor stacks if bales must be stacked to save space. (SDSU Extension)

Oklahoma State University gives similar advice, noting that north-south orientation and at least three feet between rows help sunlight and airflow dry bales after rain. It also states that ground contact can account for more than half of total dry matter losses, so bales should be kept off the ground where practical. (extension.okstate.edu)

Should Round Bales Touch?

Flat ends can be butted together in a row, but rounded sides touching side-to-side is a problem because it traps water between bales.

University of Maryland Extension notes that when bales touch side-to-side, water runs into the crevice and creates high moisture levels where the bales meet. It recommends leaving at least three feet between bale rows, keeping bales off the ground, keeping bales out of shade, and stacking only if the bales are covered. (extension.umd.edu)

This is the storage equivalent of putting wet towels in a pile and hoping they dry. They will not. They will just become a biology project.

How To Store Small Square Bales

Small square bales are often preferred for horses because they are easier to handle, inspect, and feed in controlled portions. They usually store well if kept dry, ventilated, and off the ground.

Good small square bale storage includes:

• Store under cover

• Keep off concrete or damp floors using pallets

• Allow airflow through the stack

• Avoid stacking hard against walls

• Keep away from roof leaks

• Stack with stable, safe structure

• Check for heating after baling

• Keep cats, rodents, and wildlife out as much as practical

• Feed older hay first

For horses, small square bales are useful because you can inspect flakes closely before feeding. If one bale is dusty, mouldy, musty, hot, or discoloured inside, do not feed it.

Hay Safety for Horses

Horses are much less forgiving of dusty or mouldy hay than many owners realise.

University of Minnesota’s horse hay guidance is direct: do not feed horses mouldy hay. Excessive mould may cause coughing, heaves, or allergic reactions, and horses with heaves are often especially sensitive to mould spores or dust. (University of Minnesota Extension)

Mouldy hay can also increase risk of:

• Feed refusal

• Poor intake

• Colic

• Respiratory irritation

• Equine asthma flare-ups

• Coughing

• Poor performance

• Dust exposure for owners and staff

University of Minnesota’s hay fire guidance also lists mouldy hay as especially dangerous to horses because it can cause colic, heaves, and respiratory illness. (University of Minnesota Extension)

For horses, “just feed the outside stuff first” is not good enough if the outside stuff is mouldy, wet, blackened, dusty, or musty.

Hay Safety for Cattle

Cattle can often use lower-quality forage than horses, depending on age, production stage, body condition, and nutritional needs. A dry cow in mid-gestation and a lactating cow in winter do not have the same requirements.

But poor storage still matters.

Mould, weathering, nutrient loss, and palatability loss can reduce intake and performance. Illinois Extension notes that outside storage of round bales can cause substantial losses of nutrient value and palatability, increasing shrink during storage and waste during feeding. (Illinois Extension)

For cattle, the question is often not “is this hay perfect?” It is:

• What class of cattle is being fed?

• What is the forage test result?

• Is there mould or heating?

• Is supplementation needed?

• Is intake adequate?

• Is the hay safe enough to feed?

Cattle hay can be lower quality than horse hay, but it still needs to be safe.

Hay Fire Risk: When Storage Becomes Dangerous

Hay fire risk is highest when hay is baled too wet and stacked before the internal temperature has stabilised.

University of Minnesota provides useful internal bale temperature guidance:

Internal Hay Temperature	Risk Level
Under 130°F	Least risk
130 to 140°F	Little risk, keep checking
150°F	Moderate fire risk, check often
175 to 190°F	Fire is imminent, contact the fire department
Over 190°F	Extreme caution, bales may combust when moved

(University of Minnesota Extension)

North Dakota State University advises calling the fire department immediately if hay temperatures are above 175°F or if you smell or see smoke. It also warns not to move overheated or smouldering hay because exposing it to oxygen can cause a fire to rage out of control. (NDSU)

This is not the time to be heroic with a tractor and a nervous grin. Hot hay stacks are dangerous.

Severity and Risk Framework

Risk Level	What It Looks Like	What It Likely Means	What To Do
Low risk	Hay is dry, smells fresh, stored under cover, no mould, no heating, no pests	Good storage	Feed normally and keep monitoring
Medium risk	Hay stored outside, some weathering, outer layer discoloured but inner hay clean	Some storage loss	Remove spoiled outer material and monitor intake
High risk	Musty smell, visible mould, dust clouds, damp patches, rodent contamination, hot bales	Health and feed-quality risk	Do not feed to horses. Test or discard depending on livestock class and risk
Critical	Smoke, caramel or burning smell, internal hay temperature above 175°F, severe mould, animals sick after feeding	Fire risk or serious feed safety issue	Call fire department or vet depending on the situation

The practical point:

A little weathering may be a feed-quality issue. Mouldy horse hay is a health issue. Heating hay can become a fire issue.

What Else Can Go Wrong With Stored Hay?

Mould

Mould develops when hay is too moist or becomes wet during storage. It can reduce palatability, cause nutrient loss, and increase respiratory and digestive risk.

Heat Damage

Heat-damaged hay may smell caramelised or tobacco-like and can look brown. It may be more palatable but less nutritious because heat can bind protein and reduce digestibility. NDSU notes that heat can cause a Maillard reaction between proteins and carbohydrates, reducing dry matter and digestibility. (North Dakota State University Extension)

Dust

Dust may come from mould spores, soil contamination, leaf shatter, poor storage, or old hay. Dusty hay is especially problematic for horses with equine asthma.

Rodent Contamination

Rodents contaminate hay and feed areas with urine, droppings, nesting material, and pathogens. They also damage twine, bags, tarps, and storage structures.

Ground Moisture

Hay stored directly on soil can wick moisture from the ground. Oklahoma State University notes that as much as 12 inches of the bottom of a bale can be lost through moisture absorption. (extension.okstate.edu)

Poor Airflow

Stacking hay too tightly against walls, ceilings, or other bales can trap moisture and slow drying.

Fire

Wet hay can heat, and hot hay can burn. This is the worst-case storage failure.

When Is This an Emergency?

Call the fire department if:

• Hay temperature is above 175°F

• You smell smoke

• You see smoke

• Hay has a burning or strong caramel smell with rising temperature

• A hay stack or barn is heating rapidly

• You suspect smouldering inside the stack

Do not move smouldering hay unless emergency services advise it.

Call your vet if animals have eaten suspect hay and show:

• Colic signs

• Refusal to eat

• Coughing or respiratory distress

• Fever

• Diarrhoea

• Depression

• Neurological signs

• Multiple animals affected

• Sudden drop in milk production or performance

• Severe weight loss

• Signs after a new hay batch is opened

For horses, coughing, nasal discharge, laboured breathing, colic, or sudden feed refusal after eating dusty or mouldy hay should be taken seriously.

What Should You Do Next?

1. Check Hay Before You Store It

Do not assume every delivered bale is safe.

Check:

• Moisture

• Smell

• Colour

• Dust

• Mould

• Heat

• Weeds

• Foreign material

• Bale tightness

• Bale weight

• Lot consistency

If hay is warm, wet, musty, or suspicious at delivery, do not stack it deep in the barn and hope it behaves.

2. Use a Moisture Probe

A hay moisture probe is one of the best tools for preventing storage problems, especially if buying hay straight out of the field.

Take multiple readings. Moisture varies within and between bales.

3. Store Hay Off the Ground

Use:

• Pallets

• Crushed rock

• Gravel pad

• Timbers

• Rails

• Well-drained concrete

• Rubber or drainage systems where appropriate

The goal is to stop moisture wicking upward into the bale.

4. Keep Air Moving

Ventilation matters in sheds and stacks.

Do not stack hay tight against walls or rafters. Leave gaps where needed and avoid trapping damp air.

5. Store Round Bales Correctly Outside

If outside storage is necessary:

• Use dense bales

• Use net wrap where possible

• Keep bales on a drained base

• Avoid shade

• Run rows north to south

• Leave at least 3 feet between rows

• Do not pyramid-stack unless covered

• Use tarps properly

• Feed outside-stored hay sooner than protected hay

6. Separate Horse Hay From Cattle Hay

If you feed both species, grade your hay.

Keep the cleanest, lowest-dust, least-mould-risk hay for horses. Lower-quality but still safe hay may suit some cattle classes when balanced correctly.

7. Inspect Bales Before Feeding

Open bales and check the inside. Mould can hide in the centre.

Do not feed hay that is:

• Musty

• Mouldy

• Hot

• Wet

• Blackened

• Slimy

• Dusty

• Rodent-contaminated

• Full of weeds or foreign material

8. Test Hay for Nutrition

Hay testing helps you match forage to animal needs. This matters especially for:

• Broodmares

• Growing horses

• Performance horses

• Seniors

• Laminitis-prone horses

• Lactating cows

• Growing cattle

• Thin animals

• Drought hay

• Unknown hay sources

Storage loss is not only about quantity. It also changes the nutritional value of what remains.

9. Feed Older Hay First

Use a first-in, first-out system. Label batches and keep storage organised so older hay is fed before newer hay, unless older hay is poorer quality and should be reserved for animals with lower needs.

10. Keep Records

Record:

• Supplier

• Field or cutting

• Delivery date

• Bale type

• Moisture

• Hay test

• Storage location

• Any mould or heating issues

• Which animals were fed each batch

This is extremely useful if animals become sick after a new hay batch.

Common Mistakes Owners Make

Storing Hay Directly on Soil

Ground moisture is one of the biggest storage-loss drivers. Keep hay off the ground where possible.

Stacking Outdoor Round Bales in a Pyramid Without Cover

This traps water between bales and limits drying. SDSU and Maryland Extension both warn against uncovered outdoor stacking because water shed from upper bales collects in lower bales. (SDSU Extension)

Thinking a Tarp Fixes Everything

A tarp helps only if water sheds properly and the bottom stays dry. Otherwise, moisture still wins.

Feeding Mouldy Hay to Horses

Do not do this. Horses are sensitive to mould and dust, and mouldy hay can trigger colic and respiratory disease. (University of Minnesota Extension)

Ignoring Hay Temperature

Hot hay can become a fire hazard. If hay smells caramelised, musty, smoky, or hot, check it properly.

Buying Hay Without Checking the Inside

Always inspect inside at least one bale before buying a lot. The outside can lie.

Mixing Hay Quality Without a Plan

Do not accidentally feed the dustiest, mouldiest, or poorest hay to the most sensitive animals.

Prevention: A Better Hay Storage System

A strong hay storage system includes:

• Buying from reliable suppliers

• Checking moisture before storage

• Keeping hay under cover where possible

• Keeping hay off the ground

• Using well-drained storage sites

• Creating airflow around stacks

• Avoiding shade for outdoor bales

• Using dense, well-formed bales

• Using net wrap or proper covers for outdoor round bales

• Running outdoor rows north to south

• Leaving 3 to 4 feet between outdoor rows

• Avoiding uncovered pyramid stacks

• Monitoring new hay for heat

• Testing hay quality

• Inspecting every bale before feeding

• Keeping the best hay for horses

• Keeping fire access clear

• Recording hay batches

Good hay storage is not fancy. It is dry, ventilated, organised, boring, and very effective.

FAQ

What moisture should horse hay be stored at?

Horse-quality hay should generally be baled between 10 and 15% moisture. Hay above 17% moisture is at increased mould risk, and hay above 25% moisture is at risk of severe heat damage and potential fire hazard. (University of Minnesota Extension)

Can cattle eat hay that horses should not?

Sometimes cattle can use lower-quality hay than horses, depending on the class of cattle and nutritional balancing, but mouldy, severely spoiled, contaminated, or heat-damaged hay can still be a problem. Horses are particularly sensitive to mould and dust, so the safest hay should usually be reserved for them.

Is it okay to store round bales outside?

Yes, if they are stored properly. Use dense bales, store on a well-drained base, keep them off soil where possible, avoid shade, orient rows north to south, leave space between rows, and cover bales when practical. Outdoor round bale losses can be substantial if bales are left unprotected on the ground. (University of Minnesota Extension)

Can wet hay catch fire?

Yes. Wet hay can heat due to microbial activity, and in severe cases it can combust. Hay temperatures above 175°F are considered a fire emergency, and emergency services should be contacted. (University of Minnesota Extension)

Should I feed mouldy hay if I shake the dust out?

No, especially not to horses. Shaking hay does not remove the mould risk, and it can expose both horses and people to spores and dust. Mouldy hay should not be fed to horses. (University of Minnesota Extension)

Final Thoughts

Hay storage is one of the most practical ways to save money and protect animal health.

The big risks are simple: moisture, poor airflow, ground contact, shade, uncovered outdoor stacking, mould, dust, rodents, and heat. The best fixes are also simple: dry hay, covered storage, raised bases, drainage, airflow, moisture monitoring, proper round bale placement, and regular inspection before feeding.

For cattle, poor storage wastes nutrients and increases feed costs. For horses, poor storage can also become a serious respiratory or colic risk.

Hay is not cheap. Store it like it matters.

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If you are unsure whether hay is safe to feed, your horse is coughing after a new bale, your cattle are refusing hay, or you are worried about mould, heating, or storage losses, ASK A VET™ can help you work through the risk and decide when veterinary care or hay testing is needed.