The more grain you give a goat the more it will stay where you want it to stay. But then you have the problem with overfeeding and weight. A goat can easily clear a 6 foot fence if it wants to. Right now my goats don't want to, but at certain times of the year that changes. At a minimum I would go with a 48" fence. Right now I am looking at 72" no climb horse fence for my buck pen. I would spend the money right now no questions asked if I knew that it would keep the bucks in at Rut, but it is not a guarantee. As your goat gets older the fence should get higher. Get Nubian does and feed them treats at a particular time every day and they will always be there at the proper time and place!
Chris
Chris