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I raise Black Angus and Herefords. The muscular development of Beef cattle is different among breeds. A brisket is some of the toughest parts of the steer and takes a long time at a low temperature to be good. Grades of beef are based on the fat marbling within the carcass measured at the fifth rib I think. Could be the seventh.. Only less than 3% of cattle grade out as prime then another 20% or so grade as choice and the rest are some grade of select. Old bulls and very lean beef ends up as hamburger. Black steers have a better chance of being prime.
You really need to look at marbling when you buy beef. Sometimes you can get cheaper grades with good marbling. Briskets need good marbling so Angus or Choice are a safe bet.