hi Everyone--I know we're not allowed to advertise here and Im really NOT doing that, but this gnat problem the original poster described is most likely the cullicodes gnat that causes sweet itch and other severe reactions and rubbing. Not all horses respond badly to it, but for the ones that do, life can be misery, especially out in the field at dawn and dusk.
Best Friend Botanicals has an all natural incredibly effective balm for this--its called Midline Magic Balm and its MUCH better than using vaseline ot desitin or whatever you grab at the drugstore--because it not only repels these obnoxious little things very effectively but also heal the sores, bites, rubbing and leathery skin horses get under their bellies. The formula was vet supervised in the creation and its part of their whole line of repellents and shampoos.
Its safe on udders and sheathes and muzzles too. My mare had the worst sweet itch you can imagine and now she can graze so happily plus her belly is all healed, hair has grown back and her skin is all soft and smooth again.