We fence off the whole area we have planted up with new trees except for one copper beech - that is right in the middle of a pasture and ultimately will provide shelter for sheep to lie under. While it's establishing we have built a hexagonal 'pen' for it, 11 or 12 feet across, with a top rail, round stobs and sheep mesh. This might seem excessive, but sheep can stretch a remarkably long way over the top of a fence to rip off leaves, standing on their back legs and shoving hard. It also provides space to plant wildflowers around the base. Having lost trees in the past to both sheep and cattle (as well as rabbits, hares and voles) we would rather err on the side of wider separation than risk losing expensive plants.
You don't mention [member=212417]Farmboss64[/member] what livestock you have, nor how many trees, and whether the trees are within a grazing area or alongside one. That information would help us provide an appropriate answer.