Keep in mind, there is a time period 3-4 weeks after winter, when horses lose some additional weight. The amount depends on horse breed/condition and can vary from 20 to almost 150 pounds. If the horse is not too skiny (ribs with no fat between them) then I would not worry. With proper nutrition, they should be fine by the end of April.
I agree with the Calcium/Phosphorus ratio problem when feeding Rice Bran. If you need some more energy, thus gain weight, consider using Fat Acids (Oil) as this is pure energy source and doesnt ruin your nutrient composition. 3-4 oz daily should be enough.