I just wanted to point out that 4-5 lbs of rice bran really scares me!
Some rice bran has added calcium, but not all rice bran does. If your rice bran does not supply extra calcium, you are going to end up with a really screwed up calcium to phosphorous ratio. Meaning a horse with weak bones because her body is being robbed of calcium.
I personally would cut wayyyy back on the rice bran. Maybe a pound or two max, and only then if it has added calcium. She is growing and you sure don't want her to have weakened bones. (Although calcium/phosphorous ratio is important in a horse of any age).
I know everyone out there hates alfalfa, but here in Arizona we feed LOTS of it and the horses are all fat and shiny. I personally would add some alfalfa pellets or hay to her diet. I think a 50/50 mix of alfalfa to grass hay is ideal.