My 9 year old Peruvian Paso mare is sweating a lot more than normal after very light work. In the past months, I have cut down on her grain and wonder if this is causing a nutritional deficiency. I am adding selinium and vitamin E to her diet, and she does have a salt block in her stall. She is otherwise healthy, with a shiney coat and bright eyes. She just seems to be easily out of breath and lathering up after 15-20 minute gentle rides. Does anyone have any ideas?