Mosey is a 20yr old IR/EMS Foxtrotter.  He has been off work thru the winter and another few weeks after a problem trim.  

Off work = 1/10 acre dry lot.

Work = trail riding: hilly, rocky wild horse trails  2-3 times a week.   

His hay:  ESC+starch=8%, protein=36.5g/lb. balanced per Dr.K’s NRC Plus class.

His insulin is maintaining at 43 (range 10-40) on 5000mg Metformin twice a day.  I usually ride him about 3 hours after his AM Metformin dose.

I’m finding that he fatigues to the point of shutting down after about 30 minutes on the trail, especially if it involves a hard uphill walk.

Is it simply a fitness issue?  (He is NOT fit, but not fat).   I feed extra Timothy Balance Cubes the night before and day of a ride to bring his DE up to “light work” requirements, but his CP is always 125-130% of NRC requirement.  PLUS I’m feeding extra aminos (Top4Aminos from MyBestHorse).  

I stumbled on this article:

https://practicalhorsemanmag.com/health-archive/nutrition-report-amino-acids/. ““Excess protein may also hinder performance in the horse, …It contributes to what we call acidosis—a lowering of the pH within the horse’s body—which causes him to fatigue faster and not have as good a performance.” Consequently, in recent years, equine nutritionists have begun recommending that performance horses be fed a lower percentage of dietary protein to avoid the protein excesses that often accompany the extra calories they consume.” 

Is he getting too much protein?  Extra aminos not necessary?  How to address this?