Where I live there are a lot of pecan trees. That leads to issues with trespassers coming onto private property, and picking up pecans, or even shaking them down off of trees to gather them. Of course, it’s without permission.
Maybe something like blackberries next to a fence line would be a good way to keep people away from the horses, except when the berries get ripe, and people think they can help themselves.
One boarding barn where I worked near had some pecan trees in the middle of the fenced paddock, that usually had horses turned out in it. You would not believe how many people would march out in a herd of horses, shooing them away, and using those little cage things to knock down pecans and pick them up. THis was in broad daylight, and the people would actually argue with armed officers that told them to get out of the horse paddock. They couldn’t keep the pecans either.