We have both white and coloured ryelands and at this time of year some of them do start to look very tatty. The mums lose it because of the strain of lambing (I think) and some of the yearlings lose big handfuls. Mind you, trying to get through the hedge doesn't help!!
We had skin scrapes done last year in case it was parasites but nothing found. I'm starting to think to is a lot to do with the weather - heat (or cold), wet (or dry) - take your pick. I've stopped worrying about it now! We shear our probable show sheep as early in January as we can (not allowed to shear before Jan 1st if I remember correctly) so that they have a bit of wool and start to look tidy in time for the Spring festival (Royal Welsh) in May. The rest of them just wait (and get a bit tattier as they go) until everyone gets sheared in June (we have Greyfaces as well, and believe me they are pretty glad to be rid of their fleece by then. Carrying 10kg of wet fleece around in the sunshine cannot be too easy, but it does keep them warm in a cold snap - just like nature intended!