I was supplementing my mares with Mare Plus until last year when I had several mares that made Ultra Rich milk that gave the babies horrible belly aches and diareha. The vet said that I already feed a very good food and my hay is very nice, there was no need to supplement. I feed Purina Strategy 12 % pelleted along with Beet Pulp and Orchard Grass Hay. Hope that helps. Oh, and I do increase the feed in the last trimester. And, as Jill said, it is so important to keep up with the deworming and immunizations.