You will need tree guards around the tree trunks. Bark chewing is just something sheep cannot avoid doing, particularly if they are deficient in minerals like copper. Cheapest solution is chicken netting (1.2m). The southdown is one of the worst bark chewing breeds I have come across in our orchards but just love their short stature. Once you have protected your trees you can sleep at night knowing they will do their job and leave your trees alone. Good luck.