If you're in the U.S. this allergy herbal blend really works. Allergy Herbal Blend - pelletized
One of my horses developed environmental allergies after we moved to the Tennessee Valley. Tri Hist controlled the allergies but made a nut case out of him.
I discovered the Allergy Herbal Blend in 2005 and he stayed on it, seasonally, until 2013.
I didn't need it last year (nor this year SO FAR) because I discovered on accident, this horse is magnesium deficient
I put him on Ani-Med's "Remission" as he was showing very subtle signs of insulin resistance.
He'd been on Remission for around six weeks when it hit me, his allergies hadn't come to visit. Nor did they ever for the entire 2013 year.
I am not saying your mare is magnesium deficient but, being a mare anyway and Remission is not expensive, it might not hurt to buy a small tub and see how she does.
If her runny nose still doesn't clear up, then maybe try the Allergy Herbal Blend which is really pricey.
I have seen Remission in the Tractor Supply in my county but not in the TSC the next county over. You might check your TSC to see if they carry it, if you're interested