Well I washed my Clyde yesterday & used the Quick Silver shampoo, it didn't get it all out but then again some parts were pretty dirty, they are all soft and clean tho!! Another bath with focus only on her feathers will (should) get her feathers beautiful. I recommend you keep a few bottles in your barn, or some sort of whitening shampoo for her feathers. Be sure you wash your hands afterwards tho, these shampoos are purple and they turn your nails purple.
My horses get fed twice a day (like most horses do) and we fly spray them at both feedings, I take a brush into Evie's stall and brush her feathers, since I'm in there anyway why not? That may be another idea for you, even if you just do it once a day.
Idk about your place but a SMALL part of their pasture has some burr bushes and OF COURSE the horses LOOOVE to graze in them so I constantly clean burrs out of manes, tails, forelocks and now Evie's feathers. Since her feathers are clean and soft it made the burrs slip right out, thank gosh.
Horses, horses, horses....
16 year old TWH Mare named Ginger
3 year old APHA Paint Gelding named Fox.
RIP Evie (Clydesdale) 7.29.09 & Magic (OTTB) 2.23.10