Agree with others that you don't want to 'beef her up'. Grain(I know it's traditional) is not generally a healthy feed for horses and they don't cope with high starch/sugar rations. If you are going to feed it, it's vital to feed little & often - tiny meals at least 3-4 times daily, which will help her get the most out of it and minimise the health risks associated with it.
Yes, young horses tend to need more protein than older horses, but too high protein is a big prob too. Depending on what you feed her & where you are/what is in the pasture/hay she gets, she *may* be getting adequate balanced nutrition without a supplement, but then again, despite what it may say on the packaging, she may not.
...So on those 2 notes especially, I agree with those who recommend you get onto a good equine nutritionist or such. I personally subscribe to a fantastic service & program called FeedXL.com that I can't recommend highly enough! They are fantastic value, esp. Their nutritionists on call, ready to answer any queries, and have actually saved me money, along with frustration, confusion & ensuing headaches on the tricky subjects of nutrition & feeding.