I've been using grazing rye as an over-winter green manure for over 20 years now, latterly in a mix with winter tares / vetch. As a sign of being a harmless old gentleman with too much time on his hands, I will often leave a fringe to grow on to full height, some 6 feet or so, with seed heads as long as your longest finger. Then in July or August collect the seed heads (one at a time) into a large box or bag, then re-sow them on to bare ground in early autumn, just by rubbing the whole head through my gloved hands. Financially not a cost-effective way of spending one's time, but therapeutic on a fine warm afternoon, and curiously satisfying.
I find they germinate rapidly, being about 4 inches high in a week or 10 days. Then the rye forms a thick green blanket some 8-10 inches high by November, when it pauses, before growing rapidly in March / April. As I tend to do rather large blocks of green manure over the 4 plots I work, and as my old back can't stand too much digging, I tend then to either strim or hover-mow most of it, except for the fringes I leave to grow for seed. Then, as soon as the ground is suitably dry, I rotovate it in. It's only been in the last few years that I've known about the inhibiting effect it has on the growth of other seeds, but I find it's possible to plant seedlings in that ground almost straight away - but leave 2-3 weeks before sowing seeds.
