How can I prevent my horses from escaping?

I have several old race horses, but they always tried to escape by jumping over my fence or breaking through it. I've tried using an electric fence, but it wasn't powerful enough, and I've also tried barbed wire, but one horse got her leg stuck in it and bled to death. What should I do? Please answer soon, as I make good money selling my horses to various slaughterhouses in Mexico and Canada