So that's 20 lbs of hay PLUS 12 hours of grazing.
The 20 lbs of hay would be enough already.
Grazing muzzle is one possibility. Slowfeeder for the hay, less of it, ditch the hay stretcher, a handful of the Blue Seal when all the others( I assume there are others ) eat, or, maybe better, a handful of oats. Since I don't know the details( fed before pasture, hay, pellets....how? , I can't really tell you when and where to cut. But a cut is needed, definitely.