Rash?

[Roxannemc]

Some of my hens were over mated by too many roos backs are bare.Put the 7 in with just one Not too zealous Roo but their backs seem really red almost rash like solid red.Its been 3 weeks.no feathers. Nor less red.What could this be.
its molt with most of mine right now also.Rest of the hens not blocked from flock although bare backs..not red.
One hen went in full plumed and still is.

 

[Ol Grey Mare]

Have you absolutely eliminated parasites?
Can you photograph the area and maybe one of the other birds who are bare but not red for comparison?
The exposed skin is being irritated by all manner of things - sun exposure, parasites, continued mating by the one roo even if he is not too zealous, possible attention from others in the flock in the form of some light pecking/scratching, etc -- you might want to consider hen saddles to give that skin some protection until such time that the feathers are able to recover and provide the protection they are designed to be.