When working out a balanced diet it is easiest to break it down in to two components.
1. Ensure that your horse has ample supply of roughages (pasture/hay) to supply the fibre base level which is vital for digestive function.
2. Concentrates (pellets/grain) is to ensure that your horse is provided with enough energy, fat and protein to meet the elevated needs when they are required due to exercise, growth, and reproduction.
Happy riding and have a magical day!