Quote from : The Omnivore's Dilemma: A Natural History of Four Meals Book

In the debate over the use of antibiotics in agriculture, a distinction is usually made between their clinical and nonclinical uses. Public health advocates don’t object to treating sick animals with antibiotics; they just don’t want to see the drugs lose their effectiveness because factory farms are feeding them to healthy animals to promote growth. But the use of antibiotics in feedlot cattle confounds this distinction. Here the drugs are plainly being used to treat sick animals, yet the animals probably wouldn’t be sick if not for the diet of grain we feed them.