Fences are generally as much to keep out non welcome breeding bucks that for some reason others let live, with inferior genetics, as much as to keep theirs in and to keep poachers out.....
The other reason people's put up high fences. Say you own a 5000 acre ranch. People buy small ranches on your border. Then they kill way more game than their ranch will support, everything that jumps the fence. They get hoggish and they get fenched out. Hasbeen
