Ribeye area is an indicator of muscling. The longissimus or ribeye muscle is measured at the 12th rib by using a grid or a ribeye tracing that is measured with a compensating polar planimeter or an image-analysis system.
Ribeye area is adjusted to an age-constant basis.
A contemporary test group is a set of cattle of the same sex that have been raised together and have received equal treatment up to the point of slaughter. All progeny within a contemporary group should ideally be born within a 90-day period, and male calves must be castrated. A contemporary group up to the time of weaning will be subdivided if some cattle go on feed as calves and others are started on feed as yearlings, and if the cattle are then split into two or more slaughter groups. Birth date, identification of sire and dam, breed of dam (or breed proportions in crossbred dams) should be recorded for all individuals.
Ribeye area is generally included in a multiple-trait model with traits such as ultrasound ribeye area and carcass weight. An early growth trait (e.g., birth or weaning weight) may also be included to account for sequential culling if the genetic covariance is sufficient. Direct effects only are fitted.
Ribeye area contributes to the yield grade equation, but to a lesser degree than does back fat thickness. It is not an economically relevant trait but is an indicator of muscle and an indicator of the economically relevant trait which is yield grade.