Both of our horses get about a cup of rice bran daily. The 26yr old, is a very hard keeper and gets rice bran, senior feed, all the grass hay she'll eat and 2 flakes of alfalfa.
The 8yr old gets rice bran, a small amount of senior feed and grass hay. The vet told us the senior feed for a nutritionally balanced way to pack weight on her (she was getting way more, we've cut back since she's a great weight now, came to us 100+lbs underweight) and rice bran because she had no winter coat and we were already getting snow. She grew in a very thick winter coat pretty quickly between the rice bran, senior feed, unlimited hay and blanketing her when the temp dropped.
We're keeping her on rice bran and a small amount of senior feed, her summer coat is growing in beautifully, it's not sparse and dull like when we got her. She's not eating much hay right now though, we've got tons of grass growing in the pastures and we're rotating both horses around them.