I know that it is possible for a horse to have allergies but can horses develop asthma as they grow older? Since it has gotten hot here in Texas my 9 year old mare has had a bit of wheezing when breathing. Any ideas or solutions?

Couldn't stand the weather when I lived there. I would try wetting down your horses hay and the shavings in its stall. You may also want to have your vet listen to your horses lungs and make sure there is not something more serious going on. It may be as simple as putting him on a bronchodilator.