Butterball – this would be a good show name for a chestnut hunter lol
Hay and Grass
I agree with the grazing muzzle for turn out and comments about quality of hay/amount of hay given. (about 2 pounds of hay per 100 pounds of body weight).
Up the work load
You can also incorporate working in hand, lunging exercises or free lunging exercises. I love this book for lunging tips and working in hand. I’ve found an extra 20-30 mins of any of these type of exercise as a supplement/in addition to riding will make a difference in fitness and ridability. But ease into it and make sure they are warmed up well. Stretching and a solid warm up helps prevent injury while increasing work load.
Lungeing: The German Riding and Driving System
I hope this is helpful