Anyone know of some good supplements for a horse with moderate equine asthma?

For asthma I would go straight to usiong a nebulizer with albuterol. 

If it is allegeries  that is causing the breathing problems I have had good luck with the Airway EQ from MVP 

I would figure out what causes the problem and eliminate it. My horse had problems that hit him the third week in June two years in a row. Figured out it was a yellow flower, all over the pasture. Keep the pasture mowed and never had the problem again. Dusty hay?  Dusty stall?  

Where are you and how many years has the horse had symptoms?

Wisconsin. He had pheumonia this winter with all our crappy weather. Then it progressed to this.

Gotcha. If this is damage from disease, I wouldn't bother with supplements. Albuterol will be your friend. I had a mare who couldn't breathe. I did absolutely everything. Nothing worked. Turns out she was allergic to Bahia grass. She's living her best beautiful life in Kentucky without Bahia. Her breathing is fine now. I tried every supplement out there but none made one bit of difference in her ability to breathe.

Ive used all the supplements and by far the ONLY one i saw immeidate results with is CEP Daily lung pellets! I was having to nebulize with albuterol prior and sometimes Dex but not any longer. And my mare who would rub her tail out and need steroids applied daily has not needed ANY of that. 

We have a horse that gets a slight cough in the spring and fall...I call it planting/combining cough because it seems like it is caused by dust and pollens from farming season.  We give him Equipulmin daily for a few days and then when they are roping on him.  It seems to resolve the problem.  I do agree that I have had success with Albuterol for more serious problems.