John Hume once had an audacious plan. To combat rhino poaching, the South African entrepreneur aimed to create a massive rhinoceros breeding farm where the animals would be kept safe and their numbers could flourish on his vast, privately owned savanna. He’d fund the operation by sawing off and selling his animals’ horns.

With a legal rhino horn trade and a consistent supply from farms like his, he reasoned, horn prices would eventually drop and make poaching less attractive. Made of keratin, the same material as fingernails, rhino horn can grow back at a rate of about four inches per year. So selling horn, mostly to buyers in Asia for traditional medicine, carvings, and jewelry, could be sustainable—if it