Jumping in here.. I hope you don’t mind. We’ve done both hay and straw and we find that hay, if not old and rotting, tends to spill a lot of seeds into the garden and we end up growing hay. If you want to use hay... if you can, unroll a bale and let the chickens scratch i it for a long time before covering the garden with it and they’ll eat the seeds for you. Straw... the only issue we have is making sure it hasn’t been sprayed with chemicals before putting it on the garden.