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?
Thank-you for the info! They aren't my horses, I just take care of them for their owner, but I have contacted the owner and she is gettting them dewormed today. I will add some oil to the grain I feed them too.
My gypsy vanner used to get this but it was because his main was so thick that if you didnt brush it every day or every other day he would start to get dandruff. I agree with travlingypsy its nothing serious. I hope your problem clears up