Low fences are just for cattle, who most of the time are to lazy to jump over them. Lots of smaller animals, like pronghorns can jump over them, but nearly always crawl under them, low fences are going to be no hinderance to blackbuck either way. Some of the ranches in TX are so big anyway, even if they have a high perimeter fence animals in the center may never see it and from what I have seen in Australia, and Africa, even the high fences only keep in the animals that don't want to make the effort to get to the other side. I watched a rusa stag jump an 8' fence everyday in both directions to a water hole, when he could have walked 50 yards to the open gate.