Why not let them dry out inside a barn? I’d imagine leaving them out, exposed to the elements, would allow for a higher likelihood of fodder becoming mouldy?
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Oddly enough, it’s *because* some of the hay is rotting.
Decomposition is an exothermic reaction, and it causes the hay bale to warm up and eventually auto-ignite. When left out exposed in a field, the breeze can carry away some of the moisture, and if it does auto-ignite, it’s not going to burn down the barn.
Eventually, the hay will dry out enough that it doesn’t support enough bacterial life, slowing down the decomposition reaction enough that the hay won’t heat up anymore.
It’s very similar to the effect that can cause oil soaked rags to auto-ignite when left in a pile without air circulation to carry off heat released from decomposition.
They use to pile lose hay into the barn. But too much moisture and it can spontaneously combust.
Nowadays, with big machinery it’s easier and cheaper to just make a few piles outdoors and cover it with a tarp. Or just leave it as a big pile because, properly stacked, it will withstand the elements for a couple years with minimal spoilage anyway.
Remember, putting things inside a building requires labour… Often manual labour. As a farmer you do everything possible to cut out manual labour because there’s already more than enough of that on a farm. Worse, there’s no time.
Hay bales ferment up until they’re **dry**. There’s a ton of bacteria that wants to eat that hay, and they get hot as they digest it. The thing is, hay has a lot of surface area; there is a ton of **air** and space in those bundles for that bacteria to sit on. So there can be quite a lot of **super hot** bacteria going into a feeding frenzy…
TLDR: Those bales can actually burst into flame if they get too hot, and that will happen if too many of them are stored too close together. It’d be incredibly dangerous to store them together until fermentation slows.
We don’t leave our hay out, really. It’s not about drying, we don’t bale until the hay is dry. You don’t wanna bale wet hay. By the time we are all finished on a field, I reckon the bales have been out for long enough for heat to not be a problem, as other commenters have said.
So by the time we are done baling, we immediately start moving hay bales starting from where we began and working out to where we ended on the field.
In addition to all of the other responses, I was told as a kid that frequently landowners grow and bale hay on the land and then leave it there to rot, because they have no use for it but if they “use” the land it’s a tax write-off. Coming from the land of much generational wealth and inheritance, it makes a lot of sense to me.
There’s an actual machine for chucking hot bales out of barn lofts if a fire seems likely, used one stacking bales as a kid
I sell Hay as one of my business, typically all hey is cut and raked and allowed to dry before being baled. Where it gets different is what type of animals are going to be eating the hay. For horses, they have to be stored inside, so it doesn’t mold etc which can cause problems for a horse to colic. Typically the round bales you see stored in the field are left in the field to feed cows, and they typically do that throughout the winter. But horse quality hay is 100% stored in barns where is allowed to finish Curing and is safe to store and feed. Hay also heats up tremendously during the drying process as well
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