Wheat straw. It might not be easily available in metropolitan areas and areas outside the mid West. In my area, it runs about $3-$4 for a 50-60 lb bale picked up at the farmers barn. Try Craigslist searching the farm category in your area.
I’m sure others have tried other mulches. I just don’t know what else is used. I would think that hay would rot and be moldy if used.
Thank you for the info. God bless.