Our Mare was diagnosed with mild COPD years ago, if she is in she has Respiration Oil ( Super Solvitax) in her feed, rubber matting and shaving and soaked hay or haylage, even if on out during day in at night.
We have kept her out 24/7 this year and she has been much better, no soaked hay but showed a few symptoms on some of the hay (poor batch) ( would normally feed haylage but as she shares with a lami prone and greedy youngster we have had to stick with Hay) so we put her on the oil for January, she's been off it this week and is coping well.