Are you trying to omit or add high protein. Dont know if this will help but if you are wanting to add protein to the diet, Baileys do a Stud balancer which has a good % of protein in it. I use it on my mare, slightly cheaper than the Lo Cal balancer.
Sorry if that wasnt the answer you wanted lol gave me chance to ramble.
Horses with damaged livers need a very low protein diet, but since all animals need some protein, what they have has to be of very high quality. Like only_me, I'd class anything from 12% up as high levels if you have a liver damaged ned. Scour the feed companies websites Guildford - most have information on the make up of their feeds. Follow up with a phone call as most manufacturers are happy to give advice and send a sample. There is a table of ingredients etc on most feed bags but that's not much help if you have to buy one before you can read it! If your ned has a damaged liver, D&H are brilliant - they worked up a tailored diet for my rescue ned Angel who had major problems following ragwort poisoning.