The United States developed a uniform national style when federal grading standards for beef were introduced in 1927 (see box), with the highest “Prime” grade reserved for young, fine-textured meat with abundant marbling. Purebred Angus and Hereford beef were the model for three decades. The shift in consumer preference to lower-fat meat brought revisions of the USDA grades to allow leaner meat to qualify for the Prime and Choice grades (see box). Nowadays, U.S. beef comes mainly from steers (males castrated as calves) and heifers (females that have never calved) between 15 and 24 months old, and fed on grain for the last four to eight months. Recent years have brought a new interest in beef from cattle raised exclusively on grass, which is leaner and stronger in flavor than mainstream beef.