My 17yr old mare started coughing quite a while ago after an extremely hard ride by someone else that borrowed her. I currently have her on hay cubes and the cough has been relieved. However, she expels white foam mostly with lite riding. I haven’t seen it when idle. She is short of breath with extended canter.  Example: I rode her yesterday and in the first few minutes she had foam in her mouth and then it cleared up and the rest of the ride her mouth was clean. I am thinking heaves but I have never heard anything about the foam. Can you give me an idea of the why on the foam? Thank you.