Rain-Damaged Hay: Can Horses and Cattle Eat It Safely?

By Dr Duncan Houston

Rain on cut hay is one of those farm moments that makes everyone stare at the sky like it personally betrayed them.

The frustrating part is that rain-damaged hay is not always ruined. Some rained-on hay can still be useful, especially if the rain was brief, happened soon after cutting, and the hay was dried properly before baling. Other hay becomes dusty, mouldy, nutrient-poor, heat-damaged, or unsafe, especially for horses.

The key is not asking, “Did it get rained on?” The better question is: how much rain, at what stage of drying, how was it handled afterwards, how was it baled, how was it stored, and which animals are you planning to feed it to?

Quick Answer

Rain-damaged hay can sometimes be fed safely, but it must be assessed carefully. Horses should not be fed mouldy, musty, dusty, damp, or visibly spoiled hay because they are highly sensitive to respiratory and digestive problems from poor-quality forage. Cattle can often use lower-quality hay more safely than horses, but rain-damaged hay should still be tested for nutrient value and checked for mould, heating, mycotoxin risk, nitrates where relevant, and suitability for the class of cattle being fed. The best way to judge rained-on hay is forage testing plus a proper physical inspection. University of Minnesota Extension notes that rained-on hay can be suitable forage in some cases, and that testing is the best way to determine quality. (extension.umn.edu)

What Happens When Hay Gets Rained On?

Rain can damage cut hay in several ways. The severity depends on how dry the forage was when the rain hit, how long it stayed wet, how many times it was re-wetted, how much mechanical handling was needed afterwards, and whether it was baled at a safe moisture level.

The main problems are:

• Leaching of soluble carbohydrates, vitamins, minerals, and soluble nitrogen

• Continued plant respiration, which burns energy and dry matter

• Leaf shatter, especially in legumes like alfalfa

• Microbial breakdown from fungi, moulds, and bacteria

• Reduced palatability

• Higher fibre percentage and lower digestible energy

• Increased mould risk if drying is poor

• Heating and fire risk if baled too wet

Oklahoma State Extension describes four major mechanisms of rain damage: leaching, prolonged respiration, leaf shattering, and microbial breakdown. It also notes that prolonged rain after hay has partly dried is often more damaging than a quick rain soon after cutting. (sites.udel.edu)

In practice, a short shower on freshly cut grass hay may be annoying but manageable. Repeated rain on nearly dry alfalfa windrows is a different beast entirely.

Why Timing of Rain Matters

Rain does not damage hay equally at every stage.

Rain Soon After Cutting

If rain falls soon after cutting, before the plant has dried much, nutrient leaching may be less severe. The forage still contains a lot of moisture, so the rain has less effect on washing out concentrated soluble nutrients. University of Minnesota notes that rained-on hay tends to retain more quality when rain occurs soon after cutting, when the forage has had little time to dry. (extension.umn.edu)

Rain on Partly Dried Hay

This is often more damaging. As forage dries, soluble nutrients become more concentrated and easier to leach. If the hay gets wet again, respiration restarts or continues, drying is delayed, and quality drops.

Rain on Nearly Dry Hay

Rain on nearly dry hay can cause major dry matter and nutrient losses, especially if more raking or tedding is needed afterwards. University of Minnesota reports that Wisconsin rainfall studies found the maximum dry matter loss occurred when rain fell on nearly dry hay, and that rained-on hay can have digestibility declines of 6 to 40%. (extension.umn.edu)

That is why “it only rained once” is not the whole story. The stage of drying matters.

How Much Nutrient Loss Can Rain Cause?

Losses vary widely.

Oklahoma State Extension reports that research from the University of Kentucky found cut hay can lose up to 5% of dry matter per inch of rain, and digestibility can be reduced by 10% or more. (sites.udel.edu)

University of Minnesota summarises several rainfall studies showing large variation. For example, 1 inch of rain on alfalfa after one day of drying caused 22% dry matter loss in a Wisconsin study, while 1.6 inches over several days caused 44% dry matter loss. Other studies found lower losses depending on rainfall pattern, intensity, and forage type. (extension.umn.edu)

The real takeaway: rain damage is variable enough that guessing from the field is not good enough. Test the hay.

Can Rain-Damaged Hay Ever Be Useful?

Yes.

Rain-damaged hay is not automatically rubbish. In some cases, the lower soluble carbohydrate level may even be useful for selected horses that need lower sugar forage, such as horses prone to laminitis. University of Minnesota notes that rained-on hay may be suitable for horses prone to laminitis because rainfall can reduce soluble carbohydrate levels. (extension.umn.edu)

But that only helps if the hay is otherwise safe.

Low sugar does not make mouldy hay acceptable. Reduced energy does not make dusty hay safe for an asthmatic horse. A lower carbohydrate result is useful only when the hay is dry, clean, palatable, and nutritionally balanced.

How To Inspect Rain-Damaged Hay

Before testing, inspect the hay properly.

Do not feed hay that has:

• Musty smell

• Mould smell

• Sour or fermented smell

• Visible mould

• White, grey, black, blue, or green fungal growth

• Damp or heavy patches

• Heat in the bale

• Brown, caramelised, or tobacco-like heat damage

• Slimy or rotten sections

• Excessive dust

• Clumping

• Rodent contamination

• Insect contamination

• Refusal by multiple animals

Open several bales from different areas of the stack or batch. The outside of a bale can lie. The middle tells the truth.

Why Forage Testing Matters

Rain-damaged hay should be tested if you plan to use it for horses, pregnant animals, growing animals, lactating cows, thin cattle, or any ration where nutrition matters.

A standard forage test can assess:

• Dry matter

• Crude protein

• Digestible energy or total digestible nutrients

• Neutral detergent fibre

• Acid detergent fibre

• Relative feed value or relative forage quality

• Calcium and phosphorus

• Other minerals

• Water-soluble carbohydrates and nonstructural carbohydrates for horses at laminitis risk

• Nitrates where drought, stress, or certain forage species make this relevant

• Mould and mycotoxin screening where spoilage is suspected

Oklahoma State Extension states that hay quality cannot be known accurately until it is tested, and that forage testing allows a supplementation program to be designed around the actual nutrient value. (sites.udel.edu)

For cattle, a “low-quality but safe” forage may still be usable if you balance the ration. For horses, safety and dust/mould quality are often the limiting factors before nutrient value.

Feeding Rain-Damaged Hay to Horses

Horses are the strictest category.

A horse may be able to eat rained-on hay if it is:

• Dry

• Clean

• Free from visible mould

• Low in dust

• Not musty

• Not heat-damaged

• Palatable

• Tested and balanced for the horse’s needs

Do not feed rain-damaged hay to horses if it is mouldy, dusty, musty, damp, or spoiled. University of Minnesota warns that mouldy hay is especially dangerous to horses and can cause colic, heaves, and respiratory illness. (extension.umn.edu)

Horses with the lowest tolerance for questionable hay include:

• Horses with equine asthma

• Horses with heaves or recurrent airway obstruction

• Seniors

• Foals and growing horses

• Broodmares

• Performance horses

• Horses with a history of colic

• Laminitis-prone horses where sugar levels are unknown

• Horses with poor immune health

If you have to talk yourself into feeding a bale to a horse, that is usually your answer.

Should You Soak or Steam Rain-Damaged Hay for Horses?

Soaking or steaming can reduce dust exposure, but it does not turn unsafe hay into safe hay.

University of Minnesota notes that horses are highly sensitive to mould and dust from hay and bedding, and that soaking hay for no more than 60 minutes before feeding can help horses with respiratory problems, although soaking can leach nutrients. It also states that steaming should not replace the main goal of feeding hay with low mould and dust content. (extension.umn.edu)

A practical rule:

Soaking or steaming may help dusty but otherwise acceptable hay. It should not be used to rescue mouldy, rotten, wet, or dangerous hay.

Feeding Rain-Damaged Hay to Cattle

Cattle are generally more tolerant of lower-quality hay than horses, but that does not mean all spoiled hay is safe.

Rain-damaged hay may be usable for cattle if:

• It is not severely mouldy

• It is not rotten or wet

• It is not heating

• It is tested for nutrient value

• It is matched to the class of cattle

• Supplements are provided where needed

• Pregnant or high-risk animals are protected

• Nitrate or mycotoxin concerns are investigated where relevant

University of Nebraska Extension notes that horses may be the most sensitive common livestock species to mould, while cattle are generally less affected. However, it also warns that certain moulds can cause problems such as mycotic abortions or aspergillosis, and that feeding mouldy hay to sensitive animals, including horses and pregnant cows, should be minimised. (University of Nebraska Newsroom)

For cattle, the class of animal matters enormously.

Lower-Risk Cattle Uses

Lower-quality rain-damaged hay may sometimes suit:

• Dry mature cows in mid-gestation

• Beef cattle with lower nutrient demands

• Animals receiving balanced supplementation

• Situations where hay can be sorted and refused portions removed

Higher-Risk Cattle Uses

Be more careful for:

• Lactating cows

• Growing calves

• Late-gestation cows

• Thin cows

• Sick or stressed cattle

• Pregnant animals if mould or mycotoxin risk is present

• Dairy cattle where intake and milk production are critical

A dry beef cow and a lactating dairy cow are not eating for the same job. The hay should not be judged the same way.

Mould, Mycotoxins and Rain-Damaged Hay

Mould is not just a cosmetic problem.

Mould can reduce palatability, reduce intake, irritate airways, and sometimes indicate mycotoxin risk. Not every mould produces dangerous toxins, but you cannot safely judge that by smell or colour alone.

For horses, the practical rule is simple: do not feed mouldy hay.

For cattle, some mildly mouldy hay may be managed with caution in certain situations, but it should not be forced into animals that would otherwise refuse it. University of Nebraska warns that mixing mouldy hay with other feed may dilute the issue in some cases, but owners should be careful not to trick animals into eating bad hay they would normally reject. (University of Nebraska Newsroom)

If you suspect mycotoxins, laboratory testing is the only reliable way to assess risk.

Rain-Damaged Hay and Fire Risk

Rain damage becomes a different problem if the hay is baled too wet.

Wet hay can heat after baling because microbes continue breaking down plant material. That can lead to mould, heat damage, and in severe cases, spontaneous combustion.

University of Minnesota states that hay baled at less than 15% moisture has very low fire risk, while 16 to 20% moisture can mould and carries slight fire risk, 21 to 25% will likely mould and has moderate fire risk, and over 25% moisture has severe heat damage and high fire risk. It also lists internal bale temperatures of 175 to 190°F as “fire is imminent” and recommends contacting the fire department. (extension.umn.edu)

North Dakota State University gives a similar warning, stating that hay becomes a fire hazard when moisture is 20% or higher in small stacked bales or above 18% in stacked large square or round bales, and that hay fires usually occur within six weeks of baling. (ndsu.edu)

If rain-damaged hay was baled wet, do not stack it deep in a barn and hope it behaves. Hot hay is not “curing nicely.” It may be turning your hay shed into a very expensive bonfire.

Severity and Risk Framework

The most important decision point is this:

Clean but lower-quality rain-damaged hay may be a nutrition problem. Mouldy hay is a health problem. Hot hay is a fire problem.

What Else Can Make Rain-Damaged Hay Unsafe?

Heat Damage

Wet hay can heat in the bale. Heat-damaged hay may smell caramelised or tobacco-like and look brown. It may have reduced protein availability and lower feed value.

Nitrates

Rain itself does not automatically mean nitrate danger, but stressed forage systems can. Drought-stressed crops, annual grasses, sorghums, millet, oats, and certain weeds may need nitrate testing, especially for cattle. Nitrate risk is more about plant stress and species than rain damage alone.

Botulism Risk in Wet Forage or Baleage

If hay is baled as baleage or high-moisture forage, poor fermentation, damaged wrapping, carcass contamination, or soil contamination can create botulism risk, especially in horses. This needs separate handling from dry hay.

Soil and Mud Contamination

Hay left on wet ground can pick up soil, mud, manure, and bacteria. This lowers palatability and may increase health risk.

Leaf Loss in Legume Hay

Alfalfa and other legumes lose a lot of feed value when leaves shatter. The stems may remain, but the best nutrition may be lying on the ground.

Poor Storage After Baling

Even hay that survived rain in the field can be ruined by being stored outside on wet ground, uncovered, tightly stacked while damp, or exposed to more weather.

When Is This an Emergency?

There are two emergency categories: fire risk and animal health risk.

Fire Emergency

Call the fire department or emergency services if:

• Internal hay temperature reaches 175°F or higher

• You see smoke

• You smell smoke

• Hay has a strong burning smell

• Bale temperature is rising rapidly

• Bales are extremely hot to the touch

• You suspect smouldering inside a stack

Do not pull apart or move smoking hay without emergency guidance. Exposing hot hay to oxygen can make fire worse.

Veterinary Emergency

Call a vet urgently if horses or cattle fed suspect hay show:

• Colic signs

• Refusal to eat

• Depression

• Fever

• Diarrhoea

• Coughing or respiratory distress

• Laboured breathing

• Nasal discharge

• Neurological signs

• Tremors or weakness

• Multiple animals affected

• Sudden milk drop in cattle

• Abortion or reproductive losses after mouldy hay exposure

• Severe weight loss or poor intake

For horses, coughing or colic after dusty or mouldy hay should be taken seriously. For cattle, multiple animals affected, poor intake, abortions, or sudden production changes need investigation.

What Should You Do With Rain-Damaged Hay?

1. Separate the Batch

Do not mix questionable hay with your best hay. Keep it identifiable by field, cutting, date, and weather event.

2. Let It Dry Properly Before Baling

Do not bale wet hay as dry hay. If conditions do not allow safe drying, talk to a forage specialist about baleage or other high-moisture options. These need correct wrapping, fermentation, and storage.

3. Check Moisture Before Storage

Use a hay moisture probe where possible. Do not rely only on feel.

4. Monitor Temperature After Baling

Check high-risk bales for heat, especially during the first six weeks after baling. Nebraska Extension recommends checking wet bales with a long-stem thermometer or pipe method and states that bales at 170°F or higher should be closely monitored. (beef.unl.edu)

5. Store High-Risk Hay Away From Other Hay

If hay was baled wetter than ideal, store it separately, preferably outside and away from buildings until you know it is stable.

6. Inspect Before Feeding

Open bales and check inside. Look, smell, and feel.

7. Test the Hay

Submit representative samples for nutrient testing. If mould, mycotoxins, nitrates, or other risks are possible, request the relevant additional tests.

8. Match the Hay to the Animal

Save the safest, cleanest hay for horses. Use lower-quality but safe hay only where it fits the animal’s nutritional needs and risk level.

9. Balance the Ration

If rain reduced energy or digestibility, cattle may need protein, energy, or mineral supplementation. Horses may need a ration balancer or alternative forage source, depending on test results.

10. Watch Animals Closely

After introducing rain-damaged hay, monitor:

• Appetite

• Manure

• Coughing

• Respiratory effort

• Body condition

• Milk production

• Behaviour

• Colic signs

• Feed refusal

• Herd sorting

If animals refuse it, do not assume they are being fussy. They may be telling you something useful.

Common Mistakes Owners Make

Assuming All Rained-On Hay Is Ruined

Some rained-on hay is still useful, especially if the rain was brief, early, and the hay dried properly before baling.

Assuming All Rained-On Hay Is Safe

The opposite mistake is just as dangerous. Rain-damaged hay can be mouldy, dusty, nutrient-poor, heat-damaged, or unsafe.

Feeding Questionable Hay to Horses

Horses are not cattle with prettier legs. They are much more sensitive to mould and dust. Do not feed mouldy hay to horses.

Skipping Forage Testing

You cannot see protein, digestible energy, fibre, minerals, or sugar levels by staring at a bale with determination.

Ignoring Fire Risk

Wet-baled hay can heat and burn. A hay stack that smells caramelised, musty, or hot needs temperature monitoring.

Feeding Poor Hay to High-Demand Animals

Late-gestation cows, lactating cows, growing calves, broodmares, foals, and performance horses need better nutritional planning.

Hiding Bad Hay in a Mixed Ration

Mixing poor hay with better feed may reduce refusal, but it can also force animals to consume hay they were trying to avoid. Be careful.

Prevention: How To Reduce Rain Damage Next Time

You cannot control the weather, but you can reduce the damage.

Useful prevention steps include:

• Watch the forecast before cutting

• Cut during a realistic drying window

• Use conditioning equipment where appropriate

• Spread windrows to improve drying

• Use tedders carefully to reduce drying time

• Avoid excessive raking once leaves are brittle

• Bale at safe moisture levels

• Use preservatives where appropriate

• Consider baleage if dry hay is not possible

• Store hay under cover

• Keep bales off wet ground

• Keep high-risk hay separate

• Test questionable hay before feeding

• Reserve best hay for horses and high-demand animals

• Keep records by field, cutting, and weather event

The goal is not perfect hay every time. The goal is knowing which hay is horse-safe, which hay is cattle-usable, which hay needs supplementation, and which hay belongs nowhere near an animal.

FAQ

Can horses eat hay that has been rained on?

Yes, if it dried properly, is clean, smells fresh, is low in dust, has no mould, and is nutritionally appropriate. Rained-on hay should be tested where possible. Horses should not be fed mouldy, musty, dusty, damp, or spoiled hay. (extension.umn.edu)

Can cattle eat rain-damaged hay?

Often, yes, depending on the level of damage, mould risk, nutrient value, and class of cattle. Dry cows may tolerate lower-quality hay better than lactating cows, growing calves, or pregnant animals. Test the hay and supplement where needed.

Does rain lower hay sugar?

Rain can leach soluble carbohydrates from cut hay. This may make some rained-on hay useful for horses prone to laminitis, but only if the hay is otherwise safe and tested. (extension.umn.edu)

Is mouldy hay safe if I shake it out?

No for horses. Shaking hay may reduce some loose dust, but it does not remove mould risk or mycotoxin concern. Mouldy hay should not be fed to horses.

Can wet hay catch fire?

Yes. Wet hay can heat after baling and may combust. Hay above safe moisture levels or internal temperatures above 170 to 175°F should be treated seriously, and emergency services should be contacted if fire risk is high. (extension.umn.edu)

Final Thoughts

Rain-damaged hay is not automatically useless, but it is never something to feed blindly.

For horses, the safety threshold is strict: dry, clean, low-dust, no mould, no musty smell, and preferably tested. For cattle, some lower-quality hay can still be used, but the ration must be balanced and high-risk groups need better forage.

The real decision is simple: test what can be tested, inspect what can be inspected, and do not feed hay that your nose, eyes, or animals are already warning you about.

A rained-on bale might still be useful. A mouldy bale is a problem. A hot bale is a fire risk. Knowing the difference protects your animals, your feed budget, and your shed.

If you are unsure whether rain-damaged hay is safe for your horses or cattle, ASK A VET™ can help you work through the risk, testing options, animal-specific concerns, and when veterinary care or forage analysis is needed.