I don't know if it's the same for dogs but with my own grass allergy it's a very specific grass. Hatefully, it's bermuda grass that gets me, which is what I have a pasture full of and is nearly the only quality hay that I can get. It's not just a nasal 'spring' allergy either, it's a contact allergy too, filling hay bags is torture for me! I have to do allergy pills, cover up with long sleeves/pants and then IMMEDIATELY wash the exposed parts of my body after I come into contact. Even then I'm itching for at least an hour after I put hay.
You might have your vet run specific allergy tests to see if it's a certain type then try to limit her exposure to it as much as possible? Or try to make a 'dry lot' type of area for her to potty and exercise? I know that some kennels mulch their potty areas or even use a pea gravel type of base. Maybe make her an area like that so that she's not constantly exposed to it?
Just some thoughts....