I would say something along the lines of,
"Thank you for mowing our field, but I think I prefer the way it looks when it's left to grow naturally, so let's leave it from now on. I also thinking about what you said about cutting down the trees, and I realized I'm pretty attached to them and I'd rather not cut them down. By the way, I really appreciate your plowing the drive!"
You may want to consider the possibility that the overgrown field makes good shelter for animals that eat his crops. I know that for me, personally, when my neighbor lets their yard get out-of-control, it makes great shelter for rabbits, which in turn come into my yard and eat my garden.
"Thank you for mowing our field, but I think I prefer the way it looks when it's left to grow naturally, so let's leave it from now on. I also thinking about what you said about cutting down the trees, and I realized I'm pretty attached to them and I'd rather not cut them down. By the way, I really appreciate your plowing the drive!"
You may want to consider the possibility that the overgrown field makes good shelter for animals that eat his crops. I know that for me, personally, when my neighbor lets their yard get out-of-control, it makes great shelter for rabbits, which in turn come into my yard and eat my garden.