Metal barn siding protection from horses

I just added 20 some feet to my existing horse barn to store hay. Its the typical pro panel steel siding and metal frame. I plan to install 3/4 ply on the inside (CDX or T&G to protect the siding as I stack hay and store equipment, but outside is a different problem.

 

One side is exposed to the horse paddock. I am looking for ideas on how to protect. I live in Colorado. I was thinking of using 2x6 or 2x8 ceder spaced off the sheeting with sleepers. But that material is pricy. The existing horse stalls appear to be sided in doug fir, and it seems to be holding up. Would that material work. If I go with doug fir, is there a treatment that would be safe for horses?

 

Thanks for any suggestions.

Metal barn siding protection from horses