Have they been wormed/vacc? We had a pony who had TONS of dandruff and he was perfectly healthy but he had lice. So the BO just used the powdered lice killer stuff and it cleared it up. But usually horses only get lice when they are ill that's why eveyone at the barn was so shocked and powder was flying every where.
But my horse will get an annoying amout of dandruff and I use veggie oil, it cleared it up made her coat look nice. Just a cap full in her grain did the trick. Hope this helped?
From east to west a travlin gypsy found her prancing pony for now their hearts run as one...into the north