I'd start with a probiotic. Something like probios is cheap and it may help her system regulate the grain better if she gets poops on hi fat grain. Aside from that I've used alfalfa (both hay and cubes), flax seed, rice bran, beet pulp, oil, cool calories, etc. I've had the best success with high quality grass hay 24/7 with alfalfa at feeding time along with purina Ultium, beet pulp and probios. Ill add fat supplements on top as necessary. The most dramatic difference I saw was with starting probios.

You may also look into her dental health. I second the fecal count and also think about ulcers. All those things can lend to a horse being a hard keeper.