GENETIC-PARAMETERS FOR ULTRASOUND FAT DEPTH AND EYE MUSCLE MEASUREMENTS IN LIVE POLL DORSET SHEEP

The study was conducted to estimate heritabilities and genetic correlations for liveweight, fat depth at the C site and eye muscle depth, width and area measured by B mode ultrasound on live young sheep. Records on 4338 animals were available for analysis, representing 161 sires and 79 management/sex groups. The management groups were allocated to young (5-9 months), medium (10-13 months) and old (14-18 months) data sets for analysis, representing 89, 82 and 19 sires respectively. The heritabilities of liveweight, ultrasonic fat depth and ultrasonic eye muscle measurements were estimated after adjusting for management/sex group, birth and rearing type, age class of dam and age of the animal. The heritability estimates for the three age data sets respectively were: for liveweight 0.19+/-0.06, 0.11+/-0.06 and 0.37+/-0.18; for fat depth 0.01+/-0.03, 0.18+/-0.07 and 0.28+/-0.15; and for eye muscle depth 0.15+/-0.05, 0.11+/-0.06 and 0.16+/-0.11. After adjusting for sex/management group and liveweight within sex (the other factors listed above being no longer significant), heritability estimates were: for adjusted fat depth 0.11+/-0.05, 0.33+/-0.09 and 0.29+/-0.16 and for adjusted eye muscle depth 0.15+/-0.06, 0.15+/-0.06 and 0.19+/-0.12. The heritability estimates for eye muscle area, estimated as the area of an ellipse with dimensions equal to the measured depth and width, were about 10% lower than for eye muscle depth, but the genetic correlations with depth, in each case were 0.94 or higher. The heritability estimates for eye muscle width were less than 0.05 in young and medium age animals and 0.12 to 0.15 in old animals with high genetic correlations with depth which were not different from 1.00. Ultrasound measurements of fat depth at the GR site were available on 2065 lambs under 8 months of age representing 76 sires in 33 groups. The heritability estimates for fat depth adjusted for liveweight, at the GR and C sites in these lambs were both 0.10+/-0.05 with the genetic correlation between sites being 0.76+/-0.19. There is potential for including eye muscle depth in selection indices to improve leanness. Ultrasound measurement of eye muscle width and area will not improve prediction of eye muscle over that attained by measuring eye muscle depth. Predicted responses in liveweight, fat depth and eye muscle depth for five breeding objectives for terminal sires, using the appropriate LAMBPLAN selection indices, are shown and discussed.