Four inexperienced operators used a Smith's Essem Metatest ultrasonic grader and a Philips Sonatest instrument to estimate the thickness of tissue overlying the M. longissimus dorsi at three sites over the tenth intercostal space of live steers. They also measured skin fold thickness over the central site using a micrometer. Suprafascial (selvedge) and total carcass fat were measured at corresponding sites on the chilled carcasses.
The residual standard deviations from multiple and simple regressions of suprafascial and total carcass fat thickness on the ultrasonic readings of different operators, using both instruments over the three sites with and without a measurement of skin thickness were compared.
The following conclusions were drawn. Use of a constant distance from the midline of the live animal to locate the site for ultrasonic point measurement would lead to errors in estimating fat thickness at a specific anatomical location. If the edge of the M. 1. dorsi is palpated to locate the measurement site, estimates of carcass fat thickness with a residual standard deviation of 2·5 mm are possible by relatively inexperienced operators using single point reading ultrasonic instruments. Fat deposition between the fascial sheath and the underlying M. 1. dorsi is a source of inaccuracy when estimating carcass fat cover particularly at sites closer to the midline and on fatter animals.
Using the Metatest instrument, fat cover could bo estimated in the live animal with an accuracy similar to that reported by other workers using the Scanogram. Using the Sonatest instrument the estimates were generally slightly less accurate especially at the most lateral site. Inclusion of a measurement of skin thickness gave a very slight but consistent improvement in accuracy of estimation of carcass fat cover.