No round bales, not even with a net over it. He needs to eat with his head down. It’s only going to get worse over time and he’ll start reacting more frequently and each episode will likely be worse. Each time he reacts will scar his lungs further making it harder to breathe over time.
I keep my MIL’s horse and he became symptomatic about 3 years ago. Tried soaking hay, steaming hay, supplements, etc to no avail. Even on the best hay I could find he still had issues. Changed his environment. NO stall time and no round bale or any kind of ‘baled’ hay. There is a 2 sided shed I let him have access to if raining. It has mats and no shavings.
Hay cubes are expensive but they were the only thing (in addition to the environment changes) that have kept him symptom free. Had a couple of times that he flared up the past three years but could tie it back to access to baled hay (tree fell on fence which let him into my horse’s lot, my horse drug her hay pillow over to the shared fenceline). However end of summer (not on hay yet) he started reacting to something again. We can only guess dust/pollen is setting him off now. Vet checked out his lot and his two sided shed and couldn’t see anything else I could do differently. Went back on dex for 5 days to get him back under control and now doing a step down plan to see how far I can reduce the dose before he starts showing symptoms again.