Depending on what the trees are, you may want to think about leaving them for shade. I'm not as familiar w/poisonous pasture plants for cattle as I am for horses, and I can't see the trees good enough from the pic to be able to tell what they are. But, if you have any questions about any of them, post a better pic, and I'll see if I can ID them for you.
Cattle actually like trees. They provide appreciated shade & wind block for the animals.