It may be the grass - and it may be Nematodirus. When did you worm them? What is the history of the pasture?
If Nematodirus you could lose them if you delay in worming them again. Personally, that's what I'd do, with a different wormer, if you haven't done it since they started scouring on that pasture, unless the pasture hadn't had sheep on it for a year.
If good reason to believe it's not Nematodirus then hay and water, followed by limited access to the grass, as you're planning, should help.