Like bwire said, the rash is caused by the cover and it seems to be worse earlier when that cover is greener. It should get better as the cover dies or gets beat down by the weather.
I have a hard charging english pointer with the same trouble. The chest protector may work, but it can also rub and cause its own problems. I wait until I get home and scrub her with mild soap or cleanser(I use Cetaphil) and a rough washcloth. If the rash gets any puss-filled spots the next day, I wash it again. I found that giving a benedryl right after we hunt/train in heavier cover can also help. Applying an ointment like A&D, first aid cream or even vaseline to your dogs chest and abdomen before your hunt helps to lessen the rash/scratches too.
Good luck and if you find a way to stop the problem, let us know!
If you'd go to your vet they will probably tell you it's Contact Dermatitis. You could go skid plate, another option would be salmon oil on her food. The omega's in it should thicken her coat up a bit if it's a little think underneath.