Pruning Maple Trees?

I just pruned my 4 year old maple trees that looked like bushes, because of the deer. I am halfway through, I'm wondering if this is the wrong time of year and did I prune off too many branches. Will these trees survive and do I need to give them any fertilizer, because I cut off a lot of branches?

Hardiness Zone: 3b

By TraceyAnn from Brainerd, MN