Hi there. You will certainly hear from more experienced voices than mine.... but a couple of years back we had really long grass (couldn't find contractor, and tractor with topper was being renovated, so we had grass which was uncut all spring until mid-June. We have small rare breed mountain sheep, so they were well hidden at times! I'd read that it wasn't a good thing for a number of reasons.... bad for feet and poor nutrition.... but no harm came to the flock. Personally, if I could move the flock relatively easily onto a newly mown field then I suspect that I would, but don't lose sleep over it!