How to store hay so it doesn't go mouldy (and what mould does to a horse)
Hay goes mouldy when it is baled too wet or gets wet afterwards. Here are the moisture limits at baling, how to stack and cover bales, and what mouldy hay does to a horse's lungs.
Short answer: hay goes mouldy when it is baled too wet or gets wet afterwards. Bale it at no more than 20% moisture for small squares, 18% for round bales and 15% for large squares (North Dakota State University Extension), keep it off the ground with air moving around it, and keep rain and snow off it. In a shed, round bales lose about 5% of their dry matter; net-wrapped and left on the ground, about 23% (Illinois Extension). For a horse the bigger risk is the lungs: organic dust in hay, including mould spores, is the most common trigger of heaves (Merck Veterinary Manual; University of Wisconsin Extension).
We run horses on our own place outside Sherwood Park. Below is how the extension and veterinary people say to store hay, with their numbers, and what to look for when you break a bale.
Why hay goes mouldy
Whatever water is left in a bale feeds microbes. NDSU Extension says baling above about 25% moisture lets aerobic microbes grow, and they feed on the hay and make heat. University of Wisconsin Extension adds that mould can grow in hay without a preservative at moisture above 14% to 15%.
Heat is how you find out. A bale at about 38°C (100°F) after baling is normal (NDSU); one that reaches 66°C (150°F) and stays there signals a problem, and Purdue Extension says to check it twice a day. Illinois Extension says temperature usually peaks 7 to 10 days after baling, and that bales above 77°C (170°F) are in serious danger of combusting.
Moisture at baling
| Bale type | Maximum moisture at baling (NDSU Extension) |
|---|---|
| Small square | 20% |
| Round | 18% |
| Large square | 15% |
Treat these as ceilings, not targets: the Wisconsin mould threshold of 14% to 15% sits below the ceiling for small squares and round bales. If you buy hay, ask when it was baled and at what moisture. If the seller cannot say, break a bale from the middle of the load and smell it before anything comes off the truck.
Stacking and airflow
- Get bales off the ground. Pallets, coarse rock or timbers stop them wicking moisture out of the soil (Illinois Extension).
- Leave gaps. Do not stack against side walls, leave gaps in a large stack, and keep at least 60 cm (2 ft) between the stack and the rafters (Illinois Extension).
- Watch new hay for a month. Illinois Extension says to monitor temperature for up to 30 days; under 49°C (120°F) after that is safe.
- Do not store it over the stalls. Penn State Extension says hay stored over stalls is a fire hazard, carries allergens and blocks air circulation.
Tarps, sheds and round bales outdoors
Illinois Extension lists typical dry matter loss for round bales, citing University of Kentucky work (Collins et al.). The trial was in Kentucky, so read the numbers as a ranking, not a forecast for your yard.
| Round bale storage | Typical dry matter loss |
|---|---|
| Shed | 5% |
| Plastic wrap on the ground | 5% |
| Tarp on a pad | 7% |
| Bale sleeve on the ground | 8% |
| Elevated stack on the ground | 10% |
| Net wrap on the ground | 23% |
Weathering hits the outside first, and the outside is a big share: one third of a 1.7 m (5.5 ft) round bale sits in the outer 15 cm (6 in). If bales have to sit outside, Illinois Extension recommends a well-drained site away from trees, bales up on pallets, rock or timbers, flat ends butted together, and rows running with the slope. A tarp costs less than a shed, but it only works if it is secured and the bottom of the bale stays dry. Wisconsin Extension warns that a tarp laid directly over a stack in the field traps moisture.
How to tell if hay is mouldy
Start with your nose, then your eyes, then give a flake a shake.
- Smell. In a 2025 study of hay samples from the University of Bern (Bouverat et al., Animals), an abnormal musty smell was the strongest single predictor of high dust levels. University of Georgia Extension describes mouldy hay as having a “mousey” smell or giving off visible dust or spores when disturbed.
- Dust. Hay that throws a cloud of dust when you shake it out, or keeps throwing it after you stop, is too dusty to feed to a horse (University of Georgia Extension).
- Colour. Discolouration on the surface, or black sooty mould, points to wet hay (University of Georgia Extension).
University of Wisconsin Extension is blunt: if hay is mouldy, do not feed it to horses at all.
What mouldy hay does to a horse
The main risk is the lungs. The Merck Veterinary Manual describes heaves (recurrent airway obstruction) as a common allergic lung disease of horses, similar to asthma in people. Signs include a chronic cough, nasal discharge, flared nostrils and laboured breathing out. Episodes tend to happen when susceptible horses are stabled or fed hay, and the most common culprits are the organic dusts in hay. Merck says round bale hay is particularly allergenic, and that about 12% of mature horses have some degree of allergen-induced lower airway inflammation.
Wisconsin Extension adds that moulds can also make mycotoxins, so a horse with unexplained symptoms after mouldy hay needs a veterinarian. For the footing side of the same problem, see Arena dust and your horse’s lungs.
Feed good hay without wasting it
Once hay is dry and stored properly, keep it out of the mud. University of Minnesota Extension measured waste at 57% for a round bale fed on the ground with no feeder, 13% to 33% from an open ring and 5% to 6% from a netted feeder. The numbers and what they cost a winter are in How much hay does a horse waste?, ring-and-net setups are in Round bale feeder with a net, and everything we carry is in the hay feeders and slow feed nets collection. One caution: a net slows a horse down, it does not clean up bad hay. If you would not feed the flake loose, do not put it in a net.
Uncle Jeeter says: “Put your nose in the middle of a bale before you pay for the load. If it smells like a wet basement, it’s not a bargain at any price.”
Questions people ask
Is hay with a little mould on the outside still safe to feed?
Throw out any flake with visible mould, black sooty patches or a musty smell. Wisconsin Extension says mouldy hay should not be fed to horses at all, and some horses are highly allergic to certain mould spores.
Does soaking fix dusty hay?
Soaking for 5 to 30 minutes cuts the dust in hay that is merely dusty (Wisconsin Extension), and Merck says it may control signs in mildly affected horses but is not effective for highly sensitive ones. It does not remove mycotoxins, so it is not a way to rescue hay that is already mouldy.
Is a tarp enough, or do I need a shed?
A shed lost about 5% of round bale dry matter and a tarp on a pad about 7% (Illinois Extension), so a good tarp gets you most of the way for less money. Good means tied down, on a pad or pallets, with the bottom of the bale kept dry.
Are round bales worse for a horse with heaves?
Merck says round bale hay is particularly allergenic and a common cause of treatment failure for horses with heaves that live on pasture. It lists hay cubes and hay silage as lower-allergen forage. Ask your veterinarian what suits your horse.
Sources: NDSU Extension, “Properly curing hay is crucial for forage quality” (2024); Illinois Extension, “Hay Storage” (citing Collins et al., University of Kentucky); University of Wisconsin Extension, “Moldy Hay for Horses”; Purdue Extension, “Is Your Hay Too Hot?”; University of Georgia Extension, “Moldy Hay”; Penn State Extension, “Horse Stall Design”; Merck Veterinary Manual, “Recurrent Airway Obstruction (Heaves) in Horses”; Bouverat et al., Animals (2025); University of Minnesota Extension, “Round bale feeder design affects hay waste”.
Next step: already own a poly hay ring? The NAG Bag Slow Feed Net Topper is the net only, fits 5 to 8 ft rings and starts at $219.95 CAD. Not sure which mesh? Message the barn and we will help you pick.
