Back Fat Thickness
Back fat is an estimate of external fat, which is the most important factor in determining retail yield percentage. It is measured at the 12th rib, perpendicular to the outside fat at a point three-fourths of the length of the ribeye muscle from the backbone. As external fat increases, the percentage of retail product decreases.
This measurement is often subjectively adjusted at the time of data collection to reflect unusual fat distribution of the carcass. The adjusted back fat thickness is then reported on an age-constant basis.
A contemporary 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.
Back fat thickness is generally included in a multiple-trait model along with its ultrasound indicator and other carcass-fat-related traits (e.g., marbling and ultrasound percentage of intramuscular fat). 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. Only direct genetic effects are fitted.
Back fat thickness is the primary driver of yield grade, the actual economically relevant trait. If EPD for both fat thickness and yield grade exist, the yield grade EPD should be used.