 The testing method for protein in beef cattle feed has grown into a science over the post-war years.The standard text in the United States, "Nutrient Requirements of Beef Cattle", has been through eight editions over at least seventy years.The 1996 sixth edition substituted for the fifth edition's "crude protein" the concept of ""metabolizeable protein"", which was defined around the year 2000 as "the "true protein" absorbed by the intestine, supplied by microbial protein and undegraded intake protein".