We only finish one a year for the freezer, they get a couple kg of cheap blend for the last few months along with their weeds but we have had one or two that wouldn't eat it so they finish without but we are in no big hurry.As a native breed beef farmer I think I can say this with out being biased, petty much all breeds finish well of grass alone, its the system not the breed.
I use to buy Belgium Blues back in the 80s & several farmers said it will take some corn to finish them, they never seen any on this farm & finished very well.
@Henarar seems to manage quite well with blues on a grass system.
In any case a lot are out of cows with at least some native in them.
The breeding bulls we sell get a couple kg after weaning till they go at 18 to 24 months and they seem to do ok for those that buy them,
If I have his location right one went not far from Som farmer a few weeks ago, that will get him guessing