If your mare is otherwise said to be healthy by your veterinary, I feed Purina Strategy to my hard keeper. I feed the amount my vet recommended In winter months I add one pound of Grow N Win to the regular measure of Strategy each feeding to add calories and improve muscle per my vet.
There is also a product called Cool Calories you can use rather than the Grow N Win to add fat calories to a hard keeper. You would only use Cool Calories or Grow N Win not both.
I have just started Cool Calories to see how well it works so I can’t comment on it just yet. Grow N Win pelleted feed does a great job.