It might be a lot less grief to get hold of your county Ag person and ask them if they would please come out and help you recognize the plants in the pasture where your mare is affected.
Either that or your local Co-op if you have a co-op that really is a co-op - lol lol
My county's Co-op is very well informed when it comes to things like this but the more suburban and metro counties north of us aren't worth the powder to blow them to Chattanooga and back - lol
One of my horses is also allergic to oats/corn/soy which is not fun because soy is the protein source in 99.9% of feeds. He also has environmental allergies.
I had him on Tri Hist for a couple years (seasonally). It amped him up and made a nut case out of him.
He's been on "Allergy Herbal Blend" from www.herbs4horses.com
for several years. It not only does a better job controlling his environmental allergies than Tri-Hist ever did, it also mellows him out.
Your mare sounds like something totally different, however. With her heavy breathing as part of the reaction, I would want to know the specific cause and then go from there regarding controlling it. I'd hate to give her the wrong thing and end up with a fatal reaction.
This just doesn't sound like your run-of-the-mill mold/dust/pollen/runny eyes & nose.
I hope there's a nearby Ag or Co-op person that can help you identify what's in that pasture