Beef from steers and heifers is similar. Depending on economics, the number of heifers kept for breeding varies. The meat from older bulls, because it is usually tougher, is frequently used for mince (known as ground beef in the United States). Cattle raised for beef may be allowed to roam free on grasslands, or may be confined at some stage in pens as part of a large feeding operation called a feedlot (or concentrated animal feeding operation), where they are usually fed a ration of grain, protein, roughage and a vitamin/mineral preblend.