re: Burger Kings new ad is tasty

Only feeding grain with no roughage isn't a thing unless you are using an intake limiter which is very uncommon today (and those intake limiters aren't antibiotics, oils usually). There are plenty of cattle fed finishing diets without antibiotics. To add to that, there isn't some massive amount of antibiotics being fed regardless of what program they are on. And no finishing cattle are switched over to a grass based diet prior to slaughter.

Grain-based diets are also far more beneficial for the environment if you want to look at methane emissions as well. The amount of methane produced per unit of meat or milk is far less than in a purely grass based system.