LOL! It's obvious you were not raised on a farm or even have much familiarity with it. Using PBS as your source??? Have they also convinced you to be a vegetarian?
I spent the first raising dairy, beef, and hogs. Your "mostly grain" comment would kill any ruminant. After I left the fact m I owned and managed a feed and soil testing lab and in that capacity recomended feed rations. I can assure you that "mostly grain" is not what cattle a e fed until the last month before they go to market. Ruminant (which cattle are) would grow very poorly and be very sickly on "mostly grain". They need the " mostly roughage" for their digestive tract to function properly and keep them healthy. It's only a healthy animal that grows well. A ruminant fed "mostly grain" is not a healthy animal. So forget your PBS nonsense.
“Research is proving that the health concerns long associated with eating beef result not from eating beef, but rather from eating corn-fed beef.”
“Feeding cattle on corn fundamentally changes the meat they produce, greatly increasing levels of unhealthy Omega-6 fatty acids and decreasing levels of healthy Omega-3 fatty acids. This change greatly impacts the healthiness of meat for human consumption.”
http://www.globalaginvesting.com/news/blogdetail?contentid=1479