Spontaneous combustion of hay
Our volunteer fire department responded and sprayed the bales with water. The farmer pulled the bale pile apart with a tractor to allow the bales to cool. The smoke from the fire could be smelled for a mile or more, and the bales smoldered for several days.
Hay fires like this are all too common, and they are usually caused by baling and/or storing hay before it is fully cured (dried). The hay can also self-heat and combust if it becomes wet in storage.
The problem is that bacteria and mold grow on wet hay, causing it to ferment and producing flammable gases and heat. Also, as the hay dries, it goes through a natural chemical process called "sweating" in which it releases moisture and heat.
In a stack or pile of hay, the heat from fermenting and sweating cannot escape. The internal temperature can increase to the point that the hay will blacken, smolder, or even burst into flames.
The hay is spoiled even if it just warms up and turns a little brown. It loses most of its nutrients, and of course, livestock prefer not to to eat it.
This sort of combustion can occur in a hay pile of any size. Some of us have seen this in small scale with green lawn clippings or a compost pile.
This farmer lost a lot of hay, but at least he didn't lose a barn. I remember a barn fire that was caused by wet hay bales when I was a child. I was with my mother when she noticed smoke coming from a neighbor's barn and alerted him. The men from nearby ranches gathered and fought the fire, but the barn burned down. (This was on the Ray Ranch at Rose, Nebraska, in the late 1950s or early 1960s when Jay and Martha Hixson were running it.)
Read more on the web:
Cooperative Extension System bulletin "Spontaneous Combustion in Hay Poses Danger"