Hay stored outside loses value every time it rains on it. University extension services have measured how much, and the numbers make the case for a cover better than any sales pitch.
What the research shows
The figures vary with climate, bale type and how long hay sits, but they point the same way:
- West Virginia University research cited in Hay & Forage Grower puts dry matter losses at about 5% for hay stored in a barn, versus about 33% for hay stored uncovered on the ground.
- A University of Georgia summary lists about 5 to 10% loss for tarped twine-tied bales, versus 20 to 40% for twine bales outside on well-drained ground.
- Oklahoma State estimates that covering bales and getting them off the ground can bring losses down to around 8%.
The two rules
Cover it, and get it off the ground. The University of Wisconsin points out that even tarped bales lose more when they sit on moist soil, because the bottom wicks water. A pad of gravel, pallets or old tires under the stack matters almost as much as the tarp over it.
Which tarp
- Heavy-duty vinyl for a cover that stays on all season and sheds every drop. Tie it down well; a loose tarp flaps and lets water in at the edges.
- Canvas where the stack needs to breathe, for example firewood that's still drying. It sheds rain and lets moisture out.
- Poly for a short spell of bad weather, not a whole season; sunlight breaks it down.
Firewood
The same thinking applies to a woodpile: keep the top covered so rain runs off, keep the sides open or covered with something breathable so the wood keeps drying, and keep it off the ground.
Wet hay heats up
Hay keeps living after it's cut. Plant cells keep respiring for a while, and microbes in damp hay keep respiring much longer, and respiration releases heat. Extension services agree on the threshold: hay baled at or below about 20 percent moisture settles down as respiration slows; above that, respiration and mold growth continue and the bale heats. Alabama Extension advises lower targets still, around 18 percent, and Kentucky's guidance for large round bales is 15 to 18 percent.
Heat matters because it can run away. Alabama Extension treats about 140°F inside a bale as a fire hazard and 150°F and up as extreme risk. Kentucky's advice is to move bales that reach 150°F, and to call the fire department before moving them if the temperature is above 160°F and climbing. Most heating happens in the first weeks after baling.
A tarp won't fix wet hay. Covering keeps rain off cured hay. It does nothing for hay that was baled too wet, and a tight cover over heating hay holds the heat in. Bale dry, check temperatures in the first weeks, and only then cover for the season.
Sources
- Hay & Forage Grower, "Get the most from your hay with the right storage strategy"
- University of Georgia, hay storage systems (PDF)
- AgWeb, "Under Cover" (Oklahoma State, Huhnke)
- University of Wisconsin Extension, big bale storage losses
- Alabama Cooperative Extension System, hay fires and moisture
- University of Kentucky College of Agriculture, "Don't let hay harvest go up in smoke"
- Alberta Agriculture, heated bales: management and handling FAQ
Written by the TrueTarps team. Questions about your job? Contact us or request a custom tarp.