Belgian Blue cattle are "naturally" myostatin negative. These beasts are 100% muscle 0% fat and though your usual clenbuterol fattened meatstock cannot compete, the heavily abused beta-2 agonist could infact work its muscle building, fat burning magic via very similar mechanisms.
Belgian Blue Cattle....when scientist get a hold of an animal, I just can't favor this kind of thing. I love science, but let the animal be normal. Crossbreeding is fine, unless you are messing with gene implantation, artificially, or adding or removing something. In my opinion, that behavior comes from greed at the animals' expense.