Cattle love to destroy trees. Whether it is some deep instinct to create more grassland, I wonder sometimes. If there is a large object, be it tree tube,
water trough, whatever, they will push and grind on it remorselessly. For protecting young trees, it takes a large and stout wire cage with strong metal posts, or electric fencing. I have tried and tried to get young trees and cows to co-exist, and myself I have lost a lot of trees, and am just about at my wit's end.
If you really want to, you could take a 16 foot cattle panel, cut it into four 4 foot sections. Then pound four metal T-Posts and attach the wire panels. Problem is cost (for me about $50 per tree), and also and future maintainence access to the trees, such as weeding in particular.
Electric
fence is really effective, and you can easily move it for maintainance operations. I would reccomend using two strands of electric string. Just gotta check the voltage daily to make sure you havent lost power.
good luck!