Can i mix this all in a feed?

Ok this goes along with the last question I feed my mare 6lbs. of Ultium made by Purina (3lbs in morning around 10am and 3lbs at night around 8:30pm) She also gets a half pound of alfalfa pellets at night and the original source vit. and min. supplement... Now if I choose to mix in rice bran, do I need to cut out something I am feeding her now?