Cattle and trees are a poor mix. To have much of a chance of long term success I think you will need to fence off part of the pasture or at least an area around the trees. Cattle like to rub on posts, especially outside corners, so you will need a 'real' fence.
Soil compaction by livestock is also a problem for new or existing trees. You didn't mention what kind (determines the final size) you are planting but to protect the roots you should be at least 10 feet past the ultimate drip line. On a mature, full sized tree that will be quite a large area.
Tree planting is a rewarding activity. I've planted thousands. Periodically I go back and check on trees I've planted, some over 30 years ago. It is amazing to look at them while remembering how small they were as a seedling. Being an optimist I fully intend to see the trees I planted last year when they are 30 years old. In fact with a little (or a lot) of luck I'd like to bowhunt out of last years planting, 30 years from now (like I said, I'm an optimist.
So plan ahead, plant, sit back and enjoy!
Old Buck
(PS Does anyone know any bowhunters who are 86 or older or will I be the first?)