Actually, cattle in a feedlot get both grasses and grains! They usually get a combination of alfalfa, clover, corn and other items, like soybean meal. Look at all the hay and alfalfa stacked up around a feedlot. That is mixed with grains and fed to the cattle. It is a full featured diet. For some reason, folks are stuck on a one item or another type of feeding regimen for cattle. It sure isn't like the only thing that is being dumped in front of feedlot cattle is shelled corn. Corn is part of the fattening up regimen as preparation of them to move on to slaughter. Y'all really need to go see a feedlot sometime instead of relying on what some city slicker who writes for some rag says about it. Take the time online and do your research. There are myriads of articles from state university extension offices that go into great detail of a proper feed regimen to provide feedlot cattle a balanced diet.
Dairy cattle usually get a silage that is primarily the entire corn plant, grain and all, chopped. And they get it along with hay, usually alfalfa or clover. Many slaughter beef feedlot operators follow the same regimen as the dairy cattle. My neighbor has a dairy operation and chops over 100 acres of corn for them annually. And he has about 40 acres in alfalfa that he bales up and also some off of another neighbor's hay ground. He averages about 3 cuttings off of that ground per year. On top of that, corn and soybean meal is also part of the feed mix.
Ok, there, feedlots don't provide a pick or choose choice, but a combination smorgasbord.