It is important that when you are in the early training phases that you provide enough protein that will allow your horse the ability to develop muscles in response to the exercise. This is especially important if the horse is young as they will already be using quite a bit of protein for growing.
There are a lot of feeds out there that are high in protein. You just have to establish what else you require in the horses diet to find the right feed that works for you.
Here at MEA we supplement extra protein in to the horses diet using Coolstance Copra Meal. It is high in protein but does not contain a lot of extra energy as a lot of our horses do not require the extra energy in their diets.