Not sure about resp specifically but I use Emmune by equine America teamed up with Propell plus which also has echinacea in it. Touch wood my horse hasn't had any lurgies since I have been using this combo. He is like the kid at school that gets every snivel going.
Oh and Bluechip as balancer as well.
You can also have six monthly or yearly bloods to check what is going on and for peace of mind. I just do yearly now.
Ooh and coff-less by Equine America. Haven't used it myself but others swear by it. It is apparently good for immunity and soothing the resp system-not just for coughing.
Let me know how you find the Equinatural stuff if you try it.
Hi my mare has a low immune system - red cell is awesome stuff but my mare can not have this as she has ems and redcell has a lot of sugars so i have imuplus which is a herbal form - she did and does really well on this x