Hi Dizzychicken
I don’t know exactly how old the horse manure I used was, but it was well rotted enough to grow good potatoes – I sowed and fed using the Maria Thun biodynamic calendar and successfully grew Desiree (3.5lb/tuber in 2014 and just under alb/tuber in the very dry spring and early summer of 2015), Charlotte, Estima and first earlies (Arran Pilot, Red Duke of York or Pentland Javelin).
This year I’ve collected a big pile to rot at the bottom of the garden for 12 months so in 2017 there will definitely be very well rotted stuff.