How Rain-Damaged Hay Affects Horses and Cattle

By Dr Duncan Houston

Rain on cut hay is not just an inconvenience. It can change the feed value, the digestibility, the safety, and the way that hay should be used.

This matters because once hay has been cut, it has lost the protection of the living plant. If rain hits during curing, the damage is not something the forage can simply recover from. Nutrients can leach out, plant respiration can continue longer than it should, leaves can shatter during extra handling, and microbes can begin breaking the hay down before it ever reaches the feed room.

For horses, this is especially important. They are less forgiving of mold, dust, and nutritional inconsistency than cattle. Cattle can often make use of lower-quality hay more effectively, but even for them, rain damage can still reduce intake, performance, and feeding efficiency.

This article explains what rain actually does to hay, how it changes feeding value, when the hay may still be usable, and what to do before feeding it to horses or cattle.

Quick Answer

Rain-damaged hay can lose soluble nutrients, suffer more leaf loss, develop lower digestibility, and become more vulnerable to mold and microbial damage. Some rain-damaged hay can still be fed, but it should not be judged by appearance alone. Horses need more caution than cattle, especially if there is dust, mold, heating, or reduced nutritional value.

Quick Decision Guide

Light rain shortly after cutting, with fast redrying and no visible mold → hay may still be usable, but quality may be reduced

Heavy or repeated rain on curing hay → meaningful nutrient and dry matter loss is likely

Hay was rained on, baled wet, feels warm, smells musty, or shows mold → do not feed until properly assessed

Healthy beef cattle may tolerate lower-quality rain-damaged hay better than horses → species matters

Performance horses, respiratory horses, laminitic horses, youngstock, and broodmares → require much more caution

Why Rain on Cut Hay Is Such a Problem

Once forage is cut, it starts changing immediately.

The goal of curing is to dry the hay fast enough to preserve nutrients and prevent spoilage. Rain interrupts that process and creates multiple problems at once.

The main issues are:

• nutrient leaching

• prolonged plant respiration

• leaf shatter

• microbial breakdown

The mistake many people make is thinking rain only affects color or smell.

It affects feeding value.

What This Usually Turns Out To Be

When rain-damaged hay becomes a feeding issue, the situation usually falls into one of these:

• hay lost quality but is still usable with proper ration adjustment

• hay has become dusty or moldy and is no longer suitable for horses

• hay has lost enough energy and protein that animals underperform on it

• hay was baled too wet after rain and becomes a microbial or heating risk

The biggest mistake is feeding it blindly because “it still looks like hay.”

Hay can look acceptable and still be nutritionally disappointing or risky.

Nutrient Leaching: What Rain Washes Away

One of the first things rain does is remove soluble nutrients from the cut forage.

These include:

• sugars and other soluble carbohydrates

• some water-soluble vitamins

• certain minerals, especially those more easily lost in water

The longer the hay is exposed, and the more drying and rewetting cycles it goes through, the greater that loss can become.

What matters most here is timing.

A brief shower soon after cutting may do relatively little damage compared with prolonged rain after the forage has already partially dried. Once plant cells have begun drying, rainfall can wash more soluble nutrients out of the tissue.

Respiration Continues Longer Than It Should

Freshly cut forage continues to respire until it dries down enough.

That means it keeps burning stored carbohydrates.

Normally, that process slows as moisture drops. But if rain re-wets the hay before curing is complete, respiration can continue longer, and more of the forage’s energy is lost before the bale is even made.

This is why rained-on hay often tests lower in energy than expected.

It is not just that nutrients were washed away. Some were also burned up.

Leaf Shatter: Losing the Best Part of the Hay

Leaves are usually the most nutrient-dense part of the plant.

They carry more protein and more digestible nutrients than the stem. After rain, hay often needs extra handling to try to get it dry enough to bale. That means more tedding, more raking, more turning, and more opportunities for fragile leaves to break off.

This matters a lot in legume-rich hay and in any forage where the leaf fraction is an important part of quality.

The result is simple:

more stem, less leaf, lower quality.

Decision Checkpoint

If the hay has clearly lost leaf and become stemmy after repeated rain and handling, the feeding value has dropped even if the bale is not moldy.

Microbial Breakdown and Dry Matter Loss

Rain-damaged hay stays wet longer, and that invites microbes and fungi to start working on it.

As they grow, they break down plant tissue, reduce dry matter, and create conditions where hay may become dusty, moldy, or less palatable.

This is one of the hidden costs of rain damage.

You may not just be losing nutrients. You may literally be losing feed volume.

Dry matter loss matters because it reduces how much usable forage is left to feed, even before you account for lower nutritional value.

Why Horses Need More Caution Than Cattle

This is one of the most important practical points.

Cattle can usually tolerate lower-quality forage better than horses. They are often more flexible when it comes to moderate reductions in hay quality, and their digestive physiology makes them better able to use rougher forage.

Horses are different.

They are more sensitive to:

• mold and dust

• airway irritation

• lower digestibility

• sudden changes in forage quality

• nutritional imbalances in more demanding classes of horse

This is why hay that might still be acceptable for some cattle may be a poor choice for horses.

Special Risks of Rain-Damaged Hay in Horses

For horses, the main concerns include:

• respiratory irritation from mold or dust

• lower energy value leading to weight loss or poor performance

• poorer digestibility

• greater feeding inconsistency

• possible mold-related or spoilage-related complications

This is even more important in:

• horses with heaves or airway disease

• performance horses

• older horses

• insulin-resistant or laminitic horses

• broodmares

• growing horses

What matters most is not whether the hay is technically edible. It is whether it is appropriate for that horse.

Rain-Damaged Hay in Cattle

Cattle can often use rain-damaged hay more effectively than horses, but that does not mean there is no consequence.

Lower-quality hay can still lead to:

• lower growth rates

• reduced body condition

• poorer feed efficiency

• lower intake if smell or texture is off

• more need for supplementation

For cattle, the question is often not “can this be fed at all?” but “what class of cattle can it be fed to, and what does the ration need to make up for it?”

That is still an important economic and nutritional decision.

Severity Framework

Hay Analysis Is the Smart Move

If hay has been rained on, testing becomes much more valuable.

A forage analysis can help assess:

• moisture

• crude protein

• fiber levels

• energy value

• soluble carbohydrate levels

• mold or mycotoxin risk when relevant

This matters because visual inspection only tells part of the story.

Two bales can both look weathered but differ dramatically in nutritional value and feeding safety.

What Vets and Nutritionists Care About Most

• is the hay safe

• is the hay digestible enough

• what class of animal is being fed

• what needs to be supplemented if the hay is used

Testing turns guessing into planning.

What Should You Do Right Now?

If your hay was rained on:

1. Inspect it carefully for odor, dust, mold, color change, and heating

2. Separate clearly damaged bales from better ones

3. Test representative samples before feeding valuable or sensitive animals

4. Avoid feeding suspect hay to horses with respiratory issues

5. Be ready to supplement if energy or protein is lower than expected

6. Do not assume cattle and horses can be managed the same way on the same damaged forage

Simple checkpoint:

weathered but dry hay may still be usable with testing

musty, moldy, or heated hay is a different problem

Common Mistakes Owners Make

Some of the most common mistakes include:

• feeding rained-on hay without testing it

• assuming rain only affects appearance

• overlooking leaf loss and digestibility changes

• feeding questionable hay to horses with respiratory issues

• using damaged hay for high-demand animals without ration adjustment

• confusing “not moldy” with “still good quality”

The biggest mistake is treating all rain damage as minor.

Sometimes it is.
Sometimes it is not.

When Is This an Emergency?

Rain-damaged hay itself is not usually an emergency until it is fed and animals react badly.

Urgent veterinary attention is needed if animals develop:

• coughing or labored breathing after feeding

• refusal to eat with signs of illness

• colic signs

• sudden deterioration after consuming moldy or spoiled hay

For horses especially, respiratory fallout from poor-quality hay can become a major welfare and performance issue very quickly.

Prevention: How to Reduce Rain Damage Next Time

The best protection is still planning.

Useful prevention steps include:

• cutting with a strong dry weather window

• using conditioners to speed drying

• limiting unnecessary handling to reduce leaf loss

• baling only at appropriate moisture

• storing hay in a dry, ventilated area

• checking bales closely after difficult harvest conditions

Prevention is always cheaper than trying to rescue poor-quality hay afterward.

Frequently Asked Questions

Can horses eat rain-damaged hay?
Sometimes, but it depends on how badly the hay was affected. Horses need more caution than cattle, especially if mold, dust, or nutritional loss is significant.

Does rain always ruin hay?
No. Light rain with fast drying may only cause mild quality loss. Repeated or prolonged rain is much more damaging.

Why is rain-damaged hay often lower quality?
Because nutrients can leach out, respiration continues longer, leaves shatter, and microbes begin breaking the forage down.

Should rain-damaged hay be tested?
Yes. Testing is the best way to judge whether it is safe and nutritionally useful.

Can cattle handle rain-damaged hay better than horses?
Often yes, but lower quality still has consequences and may require ration adjustments.

Final Thoughts

Rain-damaged hay is not automatically worthless, but it is never something to judge casually.

For horses especially, the margin for error is smaller. Dust, mold, reduced digestibility, and lower nutritional value all matter more than many people realize. For cattle, the hay may still be useful, but only if you understand what quality has been lost and what the ration now needs.

The key is simple:

do not guess what the rain did to the hay.
Find out.

If you want help reviewing forage results, deciding whether rain-damaged hay is suitable for horses or cattle, or working out what supplementation may now be needed, ASK A VET™ can help you think through the next step clearly and practically.