Wet hay creates heat through a chemical process that will cause a fire if the hay is wet enough.
https://www.ag.ndsu.edu/news/newsreleases/2011/july-25-2011/don2019t-risk-hay-fires#:~:text=Excessive%20moisture%20is%20the%20mo st%20common%20cause%20of,hay.%20The%20wet%20condit ions%20also%20impede%20hay’s%20dry-down.
Baling hay ahead of a thunderstorm and pushing the envelope a bit is interesting. You get a wet bale into the barn and you can feel the heat radiating from the hay. It feels a lot like an overheated cell phone or laptop that’s about to blow its battery. You can feel the heat right through your gloves. Except no one really wears gloves baling. Wet hay is also very heavy. No one wants to throw a wet hay bale.
Dry hay is pretty hard to burn. It’s like paper. If you get it started and have enough oxygen it will go like a blast furnace, but it needs to be loose and dry to do anything.
Wet hay also molds and either loses nutrients or gets full-on toxic snd kills anything that eats it. You need to get hay as dry as possible as fast as possible so it doesn’t get rained on before it gets put up. Any rain on cut hay really hurts the quality. So anything you can do to crimp the hay and get it dry fast is valuable. In the Upper Midwest we don’t have as large of fields and probably have more grass in our alfalfa so we don’t use teddys. We use haybines or “crimpers” that kind of combine the fluffing process with mowing and leaves the hay in rows. Then we go right to raking, then bailing.