It's illegal to give antibiotics via feed. It is also illegal to give antibiotics to lactating dairy cows without observing withdrawal periods for that animals milk. Meaning, any milk produced while that animal is on antibiotics can't be sold. Consequently, farmers don't just throw antibiotics around all will-nilly. If the milk can't pass testing, the farmer doesn't get paid. Simple as that. There is no advantage to sub-therapeutic antibiotic use. Antibiotics aren't exactly cheap either.

Also, many people are confused about dairy cows and corn. Dairy cows are fed primarily corn SILAGE, which is the entire corn plant chopped up and ensiled. Meaning, it's mostly green, leafy stuff, very much like grass. And this isn't all the cow eats. A significant portion of the cows diet will also be made up of alfalfa or grass hay or silage. They are fed some corn grain because it's actually better for them to do so. A cow's digestion of fibers in hay and haylage is primarily done by bacteria living in the cows rumen. These bacteria need energy in the form of starch or sugar for energy so they can digest the fiber. Alfalfa (and grass) is bred largely for protein content these days, and are lower in starch. Farmers compensate by adding starch in the form of grain. Pasture grass often has a lower protein content and higher starch content, that's why those cows don't need grain, though they are often supplemented with grain anyway. Far more consideration goes into a dairy cow's diet than I think you assume. For example, here's a diet I formulated for a pregnant dairy cow that would be giving birth soon http://i.imgur.com/XiYqX.jpg . A lot of time, effort, and science (no wai!!!) goes into diet formulation, they aren't just shoving them full of corn.

A cow's metabolism will not allow it to be "milked to death." The homeorhetic regulation of the cows energy use acts through bST and IGF (bovine somatotropin - yes, the same bST people freak out about when buying milk is produced naturally in EVERY SINGLE COW. EVER. Naturally) to regulate nutrient utilization across different physiological states (puberty, pregnancy, lactation, etc.). It's metabolism shifts to support lactation, not to produce milk at the expense of the cow's health. I could go into extreme detail about this, but here is fairly simple paper, amongst many scientific papers on it: http://www.ansci.cornell.edu/bauman/envir_impact/BaumanDomAnEndo.pdf .

In modern day dairy farming a huge emphasis is put on cow comfor, because the fact is comfortable cows produce more milk. Healthy cows produce more than unhealthy cows. That's why barns are equipped with fans, sprinklers, cow mattresses, etc..

Are they able to live to their maximum potential life span? No. Definitely not. Is it because they get sick and die early? No. Definitely not. Dairy cows have a shorter lifespan because they are humanely slaughtered for beef. It is more efficient to replace older cows with younger springing heifers, and to slaughter them while they can still yield quality meat. If a cow can't walk into the slaughterhouse on it's own power, it cannot legally be slaughtered for meat. Meaning, sick, dying, injured cows cannot be sold for beef. You might consider looking into humane slaughter and the laws that surround it as well.

I'm not saying anybody has to support animal agriculture, but it would be nice if every person who bashes it didn't assume everyone in the dairy industry is an idiot. Universities worldwide research this stuff every day. If you want to base your opinion on your emotional response alone, that's your choice, but it doesn't hurt to look at the scientific research. If you're gonna argue about something it's better to know what you're talking about anyway.

And if you wonder where I got this information from, I studied dairy science at an Ivy League college, have papers full of scientific articles on dairy cows, I have worked on a dairy research project studying effects of minerals on pregnant cow metabolism, and I also worked for a plant breeding research project for 1.5 years that bred new cultivars of alfalfa, grass, and clover forage crops. And when I say bred, I mean selective breeding, not genetic modification... that was an entirely different department in the university. I also worked as a calf feeder on a large dairy farm.

Feel free to argue with me, but I must warn you, I like to arm myself with science.