I feed rice bran to all my horses Equi-Jewel main
I started feeding it to my horse with insulin resistance because the I-R caused him to lose weight.
He and everyone else stay plump on 3/4 cup daily, along with other supplements, grass hay and pasture.
A quality vit/min supplement is needed when feeding rice bran
Just an FYI, she is 20 and is at a prime age to develop metabolic issues such as Equine Metabolic Syndrome, insulin resistance and cushings.
The horse in my avatar was 19 when the insulin resistance exploded on him. He lost about 80# in six weeks and that is when a degreed equine nutritionist that owns a local feed company told me to him on equine rice bran "five minutes ago"
Point-being it is possible your horse is in the early stages of some metabolic issues and feeding her as if she is will not hurt her.
That means no grains or molasses. The less sugars and starches that go into the better.
Giving her a high quality pre/probiotic will also help her digest her food properly.
I feed this to my I-R horse Optamax™: medvetpharm
I buy it thru United Equine Vet Supplies; they have an on-line catalogue.
He gets one ounce daily, the recommended amount for stressed horses.
Hope this helps