By it he meant the cow's diet I think. Hormones might show up, but just as you can't chemically tell the difference between an organic vegetable and one raised solely on Miracle grow, you probably can't distinguish the meat of a free-range cow from one that ate nothing but corn.

I don't know that corn makes cows fatter, it's just a cheaper source of food. I'm pretty sure it allows you to grow significantly more cattle on a given amount of land. I could be wrong though. Given that I can hear them mooing every morning when I step outside you'd think I would know more, but I don't.