Thanks, yup, i'm reffering to the spent substrate (after you collect several mushroom flushes from it you'll get waste). Indeed, that's a good idea to reuse that as spawn -this particularly works for oyster mushrooms,but what do with it if you grew lion'mane or shiitake on it. For these mushrooms you would need mold/bacterial free spawn (not the case with oysters).
In general I would just chime in that this is most definitely not a waste product. I pick up truckloads of spent grow bags from a local mushroom farm several times a year, it is a great addition to compost, great food for worms (it almost always already has a bunch of wigglers in it), and I like to use it as an addition to mulch. As Redhawk said it could also serve as the basis for another round of mushroom cultivating. When I built my mini hugels I incorporated a whole bunch of spent shitake blocks (including ones with blue/green mold) throughout the mounds. I haven't gotten any shitakes but I also haven't seen any fungal issues for any crops and there is a ton of hyphae as I've been digging around in the garden this summer.
This year I've been experimenting with incorporating mycelium into sheet mulch. The ground is frozen for about 6 months of the year, and I've noticed things decay slowly, compost takes forever to finish off etc.
I've been adding organic matter to my soil, so I'm looking to sheet mulch not only to suppress unwanted vegetation, but also as a means of composting in place. I got some spent grow logs and I've been building sheet mulch using different combinations of newspaper, hay, straw, used coffee grounds, and the grow logs sliced into discs.
I'm using between 5 and 15 gallons of used coffee grounds per log by the way.
I noticed an explosion in the slug population which is par for the course in sheet mulching. But they don't eat my plants. It appears their preferred food is the wet newspaper. I've been observing these slugs on a regular basis. They tend to leave the mycelium alone, and they avoid the used coffee grounds.
In the beds where the mycelium is covered by a thin layer of newspaper I discovered a neat dynamic. The slugs graze on the newspaper but not in a random fashion. They tend to eat the newspaper in spots until they eat right through, and then they move on to a fresh area. So I'm seeing fissures appear in the newspaper. These fissures in turn are allowing oxygen into the sub layer of mycelium and used coffee grounds, and the mycelium react by fruiting up through these holes.
Rather than having an explosion of mushrooms in my garden beds everywhere and all at once like I was expecting, I'm getting a controlled production thanks to my little workers.