I am confused after reading this thread, exactly what Poolfield you are doing? First I thought you were just digging couch out of soil, now I wonder if you are digging it out of a six inch mulch of manure?
The thing about couch is that it comes many variables of quantity and vigour. Yes a six inch mulch of manure/compost will smother and suffocate most of it if there is not too much. But if a lot of roots are there…. enough leaves will poke through to need quite regular removing until roots are exhausted. I am in that position in parts of my new garden with some thick and vigorous couch that will take more than a year to be rid of. It may even poke through landscape fabric whereas Mypex is a little thicker.