Rain-Damaged Hay for Horses and Cattle
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Rain-Damaged Hay for Horses and Cattle: What It Really Means and What to Do Next
By Dr Duncan Houston
Rain on cut hay is one of the most frustrating and misunderstood problems in livestock feeding.
On the surface, hay may still look usable. Bales are made, stored, and fed. But underneath, the nutritional value can be significantly reduced, and in some cases, health risks increase.
In practice, the biggest mistake is assuming that “a bit of rain” does not matter. It does. The impact depends on timing, forage type, and how the hay is handled afterward.
This article explains exactly what happens when hay is rained on, how much quality is lost, how it affects horses versus cattle, and how to make correct feeding decisions based on testing rather than guesswork.
Quick Answer
Rain-damaged hay loses nutrients, digestibility, and dry matter through leaching, respiration, leaf loss, and microbial activity. The extent depends on when the rain occurs and how long the hay remains wet. Always test rain-damaged hay before feeding, and adjust supplementation accordingly. Moldy hay should never be fed to horses and should be used cautiously in cattle.
What Actually Happens When Hay Gets Rained On
Rain damage is not one single process. It is a combination of four key mechanisms that reduce quality.
1. Leaching: Nutrients Washed Away
Rain removes soluble nutrients from the plant.
This includes:
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sugars
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proteins
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vitamins
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minerals
The effect is worse when:
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hay is already partially dried
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rain is prolonged rather than brief
What matters most is that energy is lost first.
This means the hay may still look bulky, but its feeding value is reduced.
2. Respiration: Energy Burned Off
After cutting, plant cells continue to respire until moisture drops below about 30 percent.
During this time:
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stored energy is used
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digestible nutrients are lost
Rain delays drying, which extends respiration and increases losses.
3. Leaf Shatter: The Best Parts Are Lost
Leaves contain the highest concentration of:
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protein
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energy
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digestible nutrients
When hay is rehandled after rain:
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leaves become brittle
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they break off during raking and baling
This is particularly important in legumes.
4. Microbial Breakdown: Mold and Decay
Wet hay encourages:
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mold growth
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fungal activity
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microbial degradation
This leads to:
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reduced dry matter
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lower digestibility
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potential toxin production
How Much Is Actually Lost?
Losses vary, but they are often underestimated.
Typical Losses
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5 to 22 percent dry matter loss per inch of rain
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up to 44 percent loss in some legume hays after repeated wetting
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10 to 40 percent reduction in digestibility
Severity Framework: How Bad Is Your Hay?
Low Damage
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brief rain
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early-stage cutting
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minimal handling afterward
Action:
Monitor and consider mild supplementation.
Moderate Damage
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partial drying followed by rain
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visible leaf loss
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reduced intake
Action:
Test hay and adjust ration.
High Damage
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repeated wetting
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delayed drying
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clear mold risk
Action:
Use cautiously, supplement heavily, or restrict to lower-risk animals.
Critical
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visible mold
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musty smell
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heating or spoilage
Action:
Do not feed to horses. Use extreme caution or discard.
Grass vs Legume Hay: Why It Matters
Not all hay responds the same way to rain.
Grass Hay
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more structurally resilient
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lower leaf-to-stem ratio
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generally loses less dry matter
Legume Hay (Alfalfa)
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higher leaf content
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more nutrient-dense initially
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more vulnerable to leaf loss
Losses can be severe if rain occurs after partial drying.
Practical Insight
Alfalfa often looks worse after rain because the most valuable part, the leaf, is what is lost first.
Timing of Rain: The Most Important Factor
When rain hits matters more than how much rain falls.
Lowest Risk
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immediately after cutting
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before significant drying
Highest Risk
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after partial drying
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before baling
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during slow, prolonged rainfall
Why Slow Rain Is Worse
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more time for leaching
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more nutrient loss
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extended microbial activity
Short, heavy showers often cause less damage than long, light rain.
Horses vs Cattle: Critical Differences
Horses
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highly sensitive to mold
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higher risk of respiratory disease
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risk of colic and toxin exposure
Never feed visibly moldy hay to horses.
Cattle
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more tolerant of minor mold
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may sort or refuse poor-quality hay
However:
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mold toxins still pose a risk
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intake and performance can decline
Do not assume cattle are “safe” just because they will eat it.
When Is This an Emergency?
Treat hay as unsafe if:
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it smells musty or moldy
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animals refuse it
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horses show respiratory signs
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cattle show reduced intake or digestive upset
Remove the hay immediately and reassess feeding.
Test Hay. Do Not Guess.
Visual inspection is not enough.
What to Test
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dry matter
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crude protein
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energy (TDN)
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fiber
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mold and microbial load
Why Testing Matters
Rain-damaged hay can look acceptable but be:
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low in energy
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low in protein
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poorly digestible
Testing allows:
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accurate supplementation
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correct ration balancing
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better performance outcomes
What To Do If You Have Rain-Damaged Hay
Step 1: Test It
Always start with lab analysis.
Step 2: Match It to the Right Animals
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lower-quality hay → maintenance animals
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higher-risk animals → better-quality feed
Step 3: Supplement Correctly
Depending on results:
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add protein
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increase energy
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balance minerals
Step 4: Monitor Intake
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watch for refusal
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check manure consistency
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track body condition
Practical Management When Rain Cannot Be Avoided
Sometimes, weather does not cooperate.
Reduce Risk During Harvest
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use conditioners and tedders
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improve drying speed
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avoid cutting before poor forecasts
Storage Matters
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keep bales off the ground
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protect from further moisture
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ensure ventilation
Consider Alternative Strategies
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bale at higher moisture for silage systems
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delay harvest if conditions allow
Common Mistakes
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assuming all hay is equal
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feeding moldy hay to horses
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not testing damaged hay
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overestimating energy content
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not adjusting supplementation
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ignoring intake changes
Prevention: Plan Before the Weather Turns
The best approach is proactive:
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monitor weather closely
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plan cutting windows carefully
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use proper drying equipment
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store hay correctly
FAQ
Is rain-damaged hay useless?
No, but it often requires testing and supplementation.
Can cattle eat moldy hay?
They can tolerate some mold, but it still affects health and performance.
Why is alfalfa more affected?
Because nutrient-rich leaves are more easily lost.
Does all rain cause damage?
No, but timing and duration determine severity.
Should I always test hay after rain?
Yes, especially if quality is uncertain.
Can I still feed it safely?
Often yes, but only with proper management.
Final Thoughts
Rain-damaged hay is not automatically a disaster, but it is rarely “business as usual.”
The key is not whether it rained.
The key is:
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when it rained
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how much quality was lost
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how you respond
If you test, adjust, and monitor properly, you can still use affected hay without compromising herd health.
If you guess, you risk poor performance, health issues, and lost productivity.
If you want help analysing forage results, adjusting rations, or deciding whether hay is safe for your horses or cattle, ASK A VET™ can guide practical decisions and help you optimise feeding strategies based on real data.