Tree's in paddocks look great and give stock shade from the heat of the sun.  Getting them established is another matter.

We've watched stock lean over the 40 gallon drum guards and use wire netting guards like a ladder to eat new trees. These are the only stock guards we've found that work!  They're made here in NZ out of knot-free treated pointed pine batons, wired securely together in a flat panel that you then wrap around the tree and hammer in. 

They let air around the trunk, unlike solid plastic pipe guards which keep the bark soft so stock strip it when the pipe comes off.

You can get a pruning saw between the batons to shape the tree as it grows and it's rabbit proof as well.  The guards come in a range of sizes, unstained, the most popular sizes are listed.   The 1.8m x 2m is pictured.