Yes. Horses are meant and designed to eat grass. As long as The pasture isn't over grazed, it should be just fine. If you are worried about it, you can add to her feeding regiment. For example, my horse gets 1lb of Buckeye Grow and Win everyday. He is a hard keeper and this provides him with the additional vitamins/minerals and calories he needs to stay healthy. Another good thing you can feed them, if you are worried about calories, is beet pulp. It is loaded with fats that are good for horses and is easily digested. It's also fairly inexpensive. Just make sure you soak the living bejesus out of it. Beet pulp holds a ton of water and if not soaked properly, can cause choking. I use one part beet pulp, three parts water, and let is soak for at least 45mins. I usually let it sit while I ride.
Strength is not defined by physical ability. It is determined by your actions and the compassion of your soul.