This counts for very little, but I once transported a dairy farmer (Paramedic) between hospitals and had about an hour or so to pick his brain about the industry. As I recall, Corn has too much sugar in it, and if you give your cows too much corn, your feed will be too 'hot' and it fucks the cows up somehow. There's a LOT of work that goes into balancing the nutrient intake for dairy cows. Also, the antibiotics are used for promoting growth in meat cattle, because something about the antibiotics makes the cows grow bigger faster.