I don't know where your 70% came from, but I know for a fact cattle can live quite well without any grain (they love grain, but it is kind of like feeding your kids an all sugar diet. You can only do it for a little while). Start going to your butcher and ask for grass-fed beef. The more demand there is, the more you will see it.
One part of grain that is used for cattle is the grain shaft, which is used for bedding. Not sure if you have a better use for it, but you sure don't want to eat it...even the cows don't eat that (well, they don't each much of it anyway).
Also, make sure you aren't confusing dairy cattle with beef cattle.
Dairy cattle will often eat small amount of grain over longer periods (they live longer), but often consume a larger portion of hog corn and alfalfa. You can read up on "Total Mix Ration" if you want.
Beef cattle, when brought in for finishing will sometimes eat large amounts of grain for a short period.
Do you also want to eat hog corn and alfalfa? You can each hog corn, but the cooking time is pretty high.