Their complex ruminant stomach is not designed at all to handle anything other than grass, so these grains (95%+ GMO corn and soy) make the cattle very sick.  This is where antibiotics come in, because the feedlots are sacrificing the cows' health for quick fat gains and big profits from unassuming customers.
 

Depressing?  Yes.  If you want more information, I think this article from Animal Welfare Approved is great.  But I don't like presenting problems without solutions.  

This is why we like local food and farm tours.  Labels are crap.  Even "Pasture Raised", the one we use most often, has been bastardized lately and is not overseen by any regulatory bodies.  Local food holds you accountable.  You can come see it for yourself, meet your farmers, and ask any questions you would like.  This MATTERS.

While we are not certified grassfed, we hold ourselves to a far, far higher standard.  Our animals live outside, on pasture, 24/7/365.  They get a species appropriate diet of 100% pasture ONLY for the ruminant animals (cows and sheep), and the omnivores (chickens and pigs) are supplemented with a certified organic, soy free/GMO free feed.  Our animals are NEVER in confinement.  We participate with nature, rotating the animals daily to build healthy soil and create a vibrant ecosystem that will get better and better every single year.