Some will burn the fields post harvest and most of the ash will remain in that field. If they are not burnt off clean the material it will be cultivated into the soil and eventually break down and decompose but it’s a longer process than most grass.
There were a couple of mills encouraging harvesting unburnt cane that was run through an extractor fan to remove shucks and lighter fine material for a potential biomass resource but that didn’t pan out as hoped.
