I live in Idaho as well and my horses have gotten this spot as well.
Do you have no see ums in your pasture? They’re little flying bugs that are blood suckers. They cause some itching. If your horse gets lines of bites across her chest, you have them.
If you flood irrigate, you probably have them. They’re annoying and I have only found 1 spray that will repel and kill on contact. It’s by Farnam, and it’s in a blue bottle….spray and wipe.
What I do is clean it up, and put swat on it.
It doesn’t bother them, that I’ve noticed. I’ve ridden and the girth doesn’t seem to make it worse or anything.
"Take the time it takes, so that it takes less time."
"Expect a lot, accept a little, reward often."