I am from a very small island made up of at least 80% farms, I have never heard of this. I store our hay in a dog food bin next to the bunnies home area and I have never had a problem. I would be more worried about making sure that they hay doesn’t dry out too much, my bunnies wont touch hay that has dried out even a little hence the dog food bin which seals pretty well

Apparently there is a risk, if the hay is moist when you store it in a barn. Big quantities of moist hay will get warm, and there is a risk of fire. But that’s BIG quantities of hay, and it needs to be quite moist, like 35% moisture. Good quality hay should have a moisture-content of 15%

I’d very much doubt this is a risk when you store 50 lb/23 kg hay in a dry indoors environment. A little bit of hay scattered on a floor could never get warm enough to combust. You could perhaps store it in separate smaller sacks indoors. And make sure it’s dry.
I’ve texted my brother and asked now. He has friends that are farmers. I’ll let you know when he gets back to me.

Hay can spontaneously combust… but, as Bam said, only in large quantities. I doubt a single bale, even if it is moldy, would be able to produce enough heat. And if it’s moldy, then you should throw it out anyway.

Hi there, yes spontaneous combustion of hay is a thing. I believe single bales can do it under the right conditions, however as mentioned, it is much more common with large quantities stacked together. This is why farmer’s often have more than one storage shed for their hay bales, because keeping it all together is too much of a risk, if one catches, you can be sure that all the rest are going too.

When hay is baled at the correct moisture content, it shouldn’t combust. However sometimes given weather conditions etc hay is baled at a higher moisture content which is when it is more prone to combusting. Moist bales can get a lot of heat in them and increased heat or movement of that bale can lead to combustion. If a bale is going to be prone to combustion, you will feel the heat from it. I’ve only had one bale that I was concerned about, it was very freshly baled, very green and quite moist. I believe when I stored it I kept the lid to my storage container open to ensure that heat could escape. With bales like that, you can feel the heat in them so you will know if it’s a risk. Hay baled at the correct moisture content can also combust however if it gets wet due to improper storage or rain. Storing inside is the easiest solution to prevent moisture getting into the bale, sometimes if you are using a sealed plastic container the hay can sweat, but it’s more likely to lead to mould and not spontaneous combustion. You can avoid this by putting some vents in the sides of the container.

Here’s an info site on combustion with hay bales: http://agriculture.vic.gov.au/agriculture/dairy/pastures-management/haystack-fires-spontaneous-combustion

All that being said, spontaneous combustion in single bales is very rare, and highly unlikely in the types of conditions that people store them in, and most bales sold for pet/horse use aren’t going to have that kind of heat in them anyway.

Panda, thank you for making us aware of this phenomenon

My brother’s friend basically said what Az already has said now. Wet hay starts to break down, and that’s an exothermic process – like in a closed compost. The core of the bale can dry out because moisture moves from the inside of the bale and out, and when it’s dry enough inside, it can catch fire due to the heat surrounding it. This is why hay shouldn’t be stored in big quantities in a stable but in a building some distance away.

Azerane’s tip about feeling the bale for heat is really useful, a spontaneous hay-fire is not sth I’d ever have thought of.