I think there is more to it than just OM. Keeps land fit and healthy then added benefit of the sheep putting the nutrition back in. We usually follow them with fodder beet which is again grazed then in with spring barley. Helps to keep grass weeds at bay.I don’t have a lot of muck to apply, as the sheep are out on roots, spreading their own. If you are spreading muck, you are only importing OM or moving it around the farm.
I dare say that if you were to chop the straw AND keep sheep on roots, it would have the same effect as putting muck on.