Good luck and best wishes with your mare! I received a Percheron for a birthday present from my husband a year ago.
Since I'm in Fla. Its easy to keep hay in front of her 24/7. I've been feeding her a 4 quart scoop of a 10/10 pelleted feed once a day and she stays fat and happy. I do use a slow feed hay net I made myself, that's a real money saver because she can consume a bale of coastal bermuda hay in one day if its just thrown out there. The slow feed net makes a bale last about 2.5 days. She has free choice mineral block at all times.
One thing I learned about my draft is: She can take a barn apart in a heartbeat. I have done more fence repair and hammering up boards in the last year than I did in 10 with my Arabs! She is a sweetie pie to hang out with and loves children. We trail ride with her every once in awhile when there are people here who need a calm easy going horse.