If raised properly grains should be fed to cattle only to help fatten them up. Mixtures of grains (oats, barley, wheat, corn) are preferable. On our farm we always called it 'chops'. This was in addition to letting the cattle wander around and eat grass. We never fed them ONLY grain.
Then when raised correctly, as Adonis says eating the meat doesn't make a difference. Raised correctly being the key word here. What Food Inc. shows is the ABUSE of the animals as they are fed ONLY corn, no exercise, and no grass. This is because corn is cheap and it fattens better than anything else. Of course, when you get paid by the pound you not concerned with quality.
Worse yet cows are given rbGH (cow growth hormones), steroids like finaplix, and several other chemicals. Plus cows are not designed to eat nothing but corn. This introduces more risk of e-coli bacteria because the natural chemicals of grass + the cow stomach acids mostly neutralize e-coli. If fed ONLY corn/grain then the risk increases.
Food Inc. then goes on to show how (keep in mind high risk cows for e-coli) are standing knee deep in crap. Gee where does most bacterial like that live? Hmmm. The larger companies then try avoid this bacteria by washing and processing the meat with ammonia. Yum! So in short you are eating lots of chemicals.
Just imagine if they raised the animals correctly. Then there would be no need for chemicals, no roids, no GH, etc. Sadly Food Inc. does point out that some 70% of processed beef comes from these places that raise the cows in an incorrect manner.