Selection for ultrasonic muscle depth; direct and correlated response in a Norwegian experimental sheep flock

A selection experiment on sheep was carried out from 1993 to 1998, based on ultrasonic muscle depth on lambs measured at the third lumbar vertebra as the only selection criteria. Thirty ewe-lambs and three ram-lambs were selected each year, based on phenotypic muscle depth, adjusted for environmental factors. Ewes were kept in production for 2 years. A randomly selected control line was kept, also consisting of 30 ewes. A single trait animal model including direct effect and maternal ability, was used to predict breeding values for ultrasonic muscle depth at weaning, as well as for correlated traits. An average breeding value was calculated within line and year. The results showed a significant and increasing difference in ultrasonic muscle depth between the lines throughout the entire period. The average difference between the two lines was 2.1mm by the end of the selection period. A correlated response in pre-weaning weight was observed. This difference was particularly large on maternal ability. A significant positive response was found for fat depth pre-weaning, but this response was less clear at weaning. A decrease in lamb weights and litter size was observed in the selected line during the first 3 years, but the lines were not significantly different by the end of the experimental period. The ewes did not change in body weight as an effect of selection for ultrasonic muscle depth. © 2002 Elsevier Science B.V. All rights reserved