the reason antibiotics are given to cattle most of the time is because they feed them corn in most all feed lots in the USA. The benefit of corn feeding is that you get a very large, fat, cow in a shorter amount of time, thus it goes to slaughter quicker so you can produce more money faster. It is a business. Cattle really can't eat corn and it will kill them without antibiotics, Acidosis is the problem. So in order not to kill the cattle they give them antibiotics to combat the problem. So when we consume the meat we get lots of antibiotics too. 3/4 or more of all the prescribed antibiotics in this country are used for livestock. That is a major reason we get immune to antibiotic resistant bugs because we are eating antibiotics all the time and resistance develops. I saw a show on PBS that told of this whole problem and it was very scary.
Here is the show and if you have not seen it you should. Very eye opening and interesting. Called King Corn.
http://www.pbs.org/independentlens/kingcorn/cows.html