Beef, the meat of an adult (over 1 year) bovine, wasn't always as popular as it is today. America has had cattle since the mid-1500s, but most immigrants preferred either pork or chicken. Shortages of those two meats during the Civil War, however, suddenly made beef attractive and very much in demand. Today's beef comes from cows (females that have borne at least one calf), steers (males castrated when very young), heifers (females that have never borne a calf) and bulls under 2 years old. Meat packers can request and pay for their meat to be graded by the U.S. Department of Agriculture (USDA). The grading is based on three factors: conformation (the proportion of meat to bone), finish (proportion of fat to lean) and overall quality. Beginning with the best quality, the eight USDA grades for beef are Prime, Choice, Select, Standard, Commercial, Utility, Cutter and Canner. The meat's grade is stamped within a purple shield (a harmless vegetable dye is used for the ink) at regular intervals on the outside of each carcass.