It's been done for over 100 years, millions of head of cattle fed, and not one has gotten sick. In the same time period, tens of thousands of cattle have gotten sick from grazing on the wrong type of wildflower. Technically, there is some minute risk, but it's a lot less risky than flowers.
Something tells me you are entirely unfamiliar with agriculture. Go spend 10 minutes in any kind of ag facility and then tell me what the FDA should be doing is jacking around with something that's proven safe. You might become a vegetarian for a while after you see your foods rolling around in it's own poop, but you'll definitely realize that feeding human-grade grain to cattle is NOT what anyone should be worried about.