Ultrasound back fat thickness
The ultrasound rib fat thickness measurement can be made from the same image used to estimate ribeye area. Fat thickness at the 12th-13th rib is measured at a point three fourths of the distance from the medial end of the longissimus dorsi muscle (12-13th rib interface) and perpendicular to the surface. Ultrasound scanning protocol requires the collection of an image made between the 12th-13th ribs using a linear-array transducer and standoff guide that conforms to the curvature of the animal’s back. Ultrasound technicians and image processing labs should be certified or accredited by the Ultrasound Guidelines Council.
Ultrasonic back fat thickness is adjusted to a common age (e.g., 365 days of age), and in multiple-breed analyses, is also adjusted for breed composition.
Animals of the same sex, reared and managed together up until the time of scanning form a contemporary group. Scanning contemporary group definition includes the following: herd code, weaning date, weaning management group (pasture, creep, non-creep, etc.), scanning date, and post-weaning management group designation. For animals scanned at a central test, the contemporary group definition for an animal should include its herd of origin and other birth and weaning contemporary group information.
Given the ultrasound measure of back fat is an indicator of the economically relevant trait of carcass back fat, it is generally included in multiple-trait models that include carcass back fat and potentially other carcass-fat-related traits (e.g., percentage of intramuscular fat, marbling). There are no maternal effects fitted for ultrasound or carcass traits given the maternal influence declines post-weaning.
Ultrasound back fat is an indicator trait that is measured on seedstock animals. It is used to help predict the genetic merit of animals for carcass back fat. The majority of beef breed organizations do not publish an ultrasound back fat EPD but rather publish a back fat EPD on a carcass basis that is the product of a multiple-trait model including the ultrasound indicator trait.