When beef carcasses are presented for quality grading, USDA graders examine visible indicators
of physiological maturity (i.e., size and shape of the ribs and ossification of the bones and
cartilages along the vertebral column of the split carcass, together with the color and texture of
the lean at the 12th-13th rib interface), which are used to classify carcasses into maturity groups
designated A through E (USDA, 1997; Table 1). Approximate chronological ages corresponding
to each physiological maturity classification are: A – 9 to 30 months, B – 30 to 42 months, C – 42
to 72 months, D – 72 to 96 months, and E – older than 96 months (Tatum, 2007).