I won't banter the breeds too much, but just FYI, our most efficient commercial cows are the AngusXTarentaise, and AngusXHereford. Almost all of the AngusXTarentaise wean 50% of themselves year in/year out, and a couple push 60% every year. One is 9, the other is her 7yr old 3/4 Angus daughter.
A couple of our AngusX Milking Shorthorns push the bar every year, too, and they'd have to stand on a pail to kick a duck in the @$$. 1000lbs and milk like fools of course. In our herd though, type is the key factor. Weights not so much because 2 cows can be the same height, but 200lbs apart. That good-footed, thick, fleshy type has to be there. The slab-sided, long rangy types always seem to weed themselves out, if you pay attention and let them.