Nix on the methane hypothesis.
Hay bales can catch fire in the middle of a field, or loaded on a trailer. Serious problem everywhere.
The temps can get up to the 400 degree range rather quickly, which is high enough to ignite.
http://news.okstate.edu/articles/agriculture/2020/stotts_braums-fire.html
http://www.youtube.com/watch?v=zRNCrHqD0zE
Quick search on compost, yes - but not as common as hay.
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