Rain-Damaged Hay for Horses and Cattle
In this article
Rain-Damaged Hay for Horses and Cattle
By Dr Duncan Houston
If your hay gets rained on before baling, the question is not just “Is it still usable?” It is “What has changed, and what risk does that create?”
Rain-damaged hay is common, especially in unpredictable seasons. Some of it can still be fed safely. Some of it becomes a nutritional problem. And some of it becomes outright dangerous, particularly for horses.
Understanding the difference is what protects your animals.
Quick Answer
Rain-damaged hay loses nutrients, especially protein and energy, and is at higher risk of mold and spoilage. It can often still be used for cattle with proper supplementation, but moldy or poor-quality hay should not be fed to horses. Testing is the only reliable way to know how safe and useful it is.
What Actually Happens When Hay Gets Rained On?
Rain affects hay in four main ways:
1. Nutrient leaching
Water washes out soluble nutrients such as:
• Sugars
• Proteins
• Vitamins
• Minerals
This is most significant when hay is partially dried and then gets wet again.
2. Continued plant respiration
If moisture remains high:
• The plant continues metabolic activity
• Energy is consumed
• Digestibility drops
This means the hay is literally losing value before it even gets baled.
3. Leaf loss
Leaves contain the highest nutrient concentration.
Rain-damaged hay often requires more handling:
• Raking
• Turning
This leads to leaf shatter and loss of protein-rich material.
4. Microbial growth
Moisture creates the perfect environment for:
• Mold
• Bacteria
• Yeast
This is where the real risk begins, especially for horses.
How Much Quality Do You Lose?
As a rough guide:
• Around 5 percent dry matter loss per inch of rain
But what matters more than quantity is quality.
What is lost first:
• Protein
• Energy
• Digestibility
So even if the bale looks acceptable, it may no longer meet nutritional needs.
Is Rain-Damaged Hay Safe?
For cattle
Often yes, with management.
Cattle can tolerate:
• Lower-quality forage
• Some level of mold (within reason)
But performance will drop if not supplemented.
For horses
This is where you need to be far more cautious.
Decision checkpoint:
If hay smells musty, feels damp, or shows visible mold, do not feed it to horses.
Horses are highly sensitive to:
• Mold spores
• Dust
• Toxins
This can lead to:
• Respiratory disease
• Colic
• Reduced performance
Severity Framework
Low Risk
• Slight discoloration
• Mild nutrient loss
• No mold or smell
Action:
Can be fed with nutritional adjustment.
Moderate Risk
• Noticeable quality loss
• Reduced palatability
• Slight odor
Action:
Feed cautiously, test, and supplement.
High Risk
• Musty smell
• Visible dust or early mold
• Reduced intake
Action:
Avoid for horses, limit use in cattle, reassess.
Critical
• Visible mold
• Heating or spoilage
• Strong odor
Action:
Do not feed.
When Is This a Problem?
You should be concerned if:
• Animals refuse the hay
• Intake drops
• Weight or body condition declines
• Horses develop coughing or nasal discharge
• Mold is visible or smell is strong
Rain damage is not always obvious. Sometimes the issue shows up as gradual weight loss rather than immediate illness.
What Should You Do Right Now?
If you have rain-damaged hay:
-
Inspect visually and by smell
-
Separate affected batches
-
Test hay before feeding widely
-
Introduce gradually
-
Monitor intake and condition
Time-based guidance:
• Reassess body condition every 2 to 4 weeks
• Adjust supplementation early rather than late
Testing: The Most Important Step
The only way to truly understand rain-damaged hay is testing.
Basic analysis should include:
• Moisture
• Crude protein
• Fibre
• Energy
• Mold or fungal load
Decision checkpoint:
If you are unsure whether hay is safe, testing is cheaper than guessing wrong.
Feeding Strategies
For cattle
• Mix with higher-quality forage
• Add protein supplementation
• Monitor body condition regularly
• Adjust ration as needed
For horses
• Use only if no mold is present
• Supplement with higher-quality forage
• Introduce slowly
• Monitor closely for respiratory signs
In practice, many marginal hays are acceptable for cattle but not for horses.
Common Mistakes
• Assuming appearance equals quality
• Feeding moldy hay to horses
• Not testing affected hay
• Not adjusting supplementation
• Feeding poor hay to high-demand animals like lactating or growing stock
The most common mistake is treating all animals the same. Horses and cattle respond very differently.
Prevention
During haymaking
• Monitor weather closely
• Bale at correct moisture levels
• Reduce handling to limit leaf loss
After baling
• Store under cover
• Keep off the ground
• Protect from further moisture
Feeding management
• Rotate hay batches
• Label affected lots
• Plan supplementation early
Practical Decision-Making
Ask yourself:
• Does it smell clean or musty?
• Is there visible mold or dust?
• Is intake normal?
• Are animals maintaining condition?
If the answer raises doubt, reassess before continuing to feed.
FAQ
Can rain-damaged hay recover if it dries later?
No. Nutrient loss has already occurred and does not reverse.
Can cattle eat moldy hay safely?
They tolerate some mold better than horses, but heavily moldy hay should still be avoided.
Why are horses more sensitive?
Their respiratory system is more reactive, and mold exposure can quickly lead to disease.
Should I always test rain-damaged hay?
Yes, especially if feeding high-value animals or relying on it long-term.
Can I fix low-quality hay with supplements?
Yes, to a degree. But supplements cannot fix mold contamination.
Final Thoughts
Rain-damaged hay is not automatically useless, but it is no longer a consistent or predictable feed.
The real risk is not just reduced nutrition.
It is hidden variability.
Some batches are safe.
Some are borderline.
Some should never be fed.
The difference comes down to testing, inspection, and management.
If you are unsure whether your hay is safe, need help interpreting feed analysis, or want to adjust your feeding plan to protect condition and performance, ASK A VET™ can help guide those decisions so you are not guessing with your herd’s nutrition.