I'm not a chemist,but I think you will get several answers to this telling you that it can definately spontaneously ignite,and I have heard of it happening.I knew a guy that had his stacked in a pile underneath a lean-to and it caught fire from this.

That's scary.But I worry more about damp hay becoming mouldy and killing my horses.

OK as the chemist here I'll chime in on spontaneous combustion.

It is rare for bales to ignite completely on their own.  It can happen.  Your most probable outcome is moldy hay and sick horses.  I've found wet/hot bales before but busting them open and letting them dry flake by flake out in the sun will save the investment.  Once the hay flakes are dry you can feed them, usually.  You could also feed them directly to cattle, as cattle are true ruminants and horses aren't, they can eat sub-par rations.

The worst thing to do would be stacking the hot bales deep in the stack and not allowing them to cool or breathe.  This could cause the combustion but more likely would mold a large chunk of valuable hay.

I have several methods of putting my mind at ease over this subject.

1.  I store my hay on the North side of the barn, it gets more breeze in my location and cool early morning sun, not hot evening sun.  If you enter in the North gate the first thing you come to is the hay storage bay, this way the firemen could get to it first while horses were taken out the South and East sides.

2.  For the first week you bring home the hay crawl in it and feel it daily.  Stick your arm in chest deep and feel for any hot spots.  Also smell the hay, sounds crazy but you'll know when it doesn't smell right.

3.  Don't stack your hay in one large pile.  I stack hay in long rows that form an L in the hay bay (nothing more than 4 widths deep or 2 lengths).  Don't stack to the ceiling either.  This allows more air circulation and less problems with hot bales and mold.

Things I'd never do with hay:

1.  Put moist bales in the stack or in the attic area of any barn, attic or loft areas usually have bad air flow and thus can allow temperatures and mold levels to rise.  My neighbors ruined a whole hay crop by doing this.

2.  Leave large stacks of hay out in the blistering sun covered by a tarp.  This is the only time I've ever been really scared of combustion.  The hay was black inside and so hot it burned a hole in the tarp.

Final bit of advice (the only one that you'll really need):

When in doubt throw it out!  If you've got a bale you don't like don't feed it or stack it in your barn!

Good info there.

Most "old" farmers place their square hay in barn that has no sides or vented sides.  After placing hay in our hay barn we always leave the doors open for a few days for ventilation.  There is never 100% dry hay.  If we are out picking up hay out of the field and the dew has hit the top of the bales we always sprinkle rock salt on the top of each layer in the barn as we are stacking it.  The salt absorbs the little bit of moisture.  Fresh hay ALWAYS has the potential for the "combustion" since it is freshly cut.  As we unload, we are subconciously checking for heat  or "heavy" bales as we are loading or unloading.  These are set aside and double checked for unusual conditions.     By the way....we bale our own hay on our farm.

"OK as the chemist here I'll chime in on spontaneous combustion.

It is rare for bales to ignite completely on their own. "

Having been raised on a dairy farm and also having horses my entire life (which is a long one) I will also chime in.

That I am aware of in my county and just over into the next county, we have had:

4 round bale hay fires.  One was to lightening and the barn was also destroyed.

The other three were to spontaneous combustion.  One was a fire in an "open barn" built to house round bales, the round bales were outside in the other two fires.

All the hay was lost because the bales were butt up against each other or stacked, so my county now has compounded our drought hay shortage by the nearly 1,000 that were lost in these four fires --- three being caused by spontaneous combustion.

The bales were not rolled "wet" - heck we haven't had any appreciable rain "since Moby Dick was a minnow", but we have been way hotter than normal since mid-May.  These are all experienced hay farmers who gave the hay ample chance to dry out before tetering.

That being said, there is no such thing as a totally dry bale of hay when it comes right out of the field.

Hay needs time to "cure", a/k/a finish drying out.

I put 330 bales of hay in my barn right out of the field.  I put three tub fans, and several house floor fans on that hay.

I work so the fans were all on heavy-duty outdoor timers, set to come on in early afternoon and go off right about supper time.

That isn't to say that, come next January, I still might find one or two bales in the center on the bottom, that molded or partially molded.  I scatter that entire bale on my yard and then hit it with the lawnmower --- might just as well give it back to nature as to throw in the burn pile