Luing cattle are scarce but excel. Lowlines. Galloway. Just about any genetics from south America but you can't get it here in the USA. Murray grays also excel. A few lines of shorthorns, black angus, or red angus could cross well on any cow herd of said breeds. IMO its more about type than breed to be successful in grass based programs. I think a linebred cow base bred to another different breed bull who is also linebred is the way to go to get the quality in the carcass and the growth from the heterosis. Just my two cents.