SPEED OF ULTRASOUND IN HEREFORD BULLS AND ITS CORRELATION WITH CARCASS COMPOSITION

Measurements were made of the speed of ultrasound transmission through sites in the hind limbs of 125 pedigree Hereford bulls. Twenty-five of these were measured twice at weekly intervals on three occasions prior to slaughter to assess the short-term repeatability of the measurement and the magnitude of long-term changes. Analyses of variance of the means of the measurements at two sites showed that the residual standard deviation (within animal and occasion), was 0.01-mu-s/cm. There was a decrease of 0.01-mu-s/cm in the group mean over the 2-week period and a significant time X animal interaction. This showed that lipid concentration at the measurement sites decreased as the bulls adjusted to their new surroundings following delivery to the Institute's farm. When a separate group of 64 bulls was measured at the farm at which they were being reared an increase in the group mean of 0.006-mu-s/cm was recorded over a 30-day period, indicating an increase in lipid concentration. The residual standard deviation for that group was 0.007-mu-s/cm, similar to that recorded above. The mean of the reciprocal speeds at the two sites, when used in a multiple regression with live mass, yielded a residual standard deviation in predicted proportion of lean in the side of 20.0 g/kg and in total fat proportion of 22.1 g/kg. These corresponded to population standard deviations, adjusted for live mass, of 29.7 and 34.1 g/kg respectively. It was concluded that the measurement of ultrasound speed in the hind limbs of Hereford bulls could be used to predict lean proportion in the carcass. The method does not require subjective interpretation and responds equally to subcutaneous and inter- and intra-muscular fat.