A large part of the property is actually "fenced off" by columns of impenetrable San Pedro. I've read that in Bolivia, farmers use Trichocereus bridgesii (related to San Pedro), also known as Bolivian Torch or achuma, to protect livestock and homesteads.
Basically any clustering spiny columnar cactus that grows higher than 7 or 8 feet. Prickly pear is another option if you'd like to add some depth to your natural fence.
