Half-sib heritability estimates were derived from ewe lamb progeny of the Woodlands Romney selection project. Ultrasonic subcutaneous fat depths (C) were measured with a Delphi probe at 5, 6, 8 and 14 months of age. Heritability estimates at each age were 0.31 (SE=0.12), 0.45 (0.11), 0.29 (0.06) and 0.28 (0.09) respectively. A Toshiba Sonolayer SAL-22A B-mode ultrasonic scanner was used to measure muscle width (A), depth (B) and GR tissue depth at 14 months of age and resultant heritability estimates were 0.07 (0.05), 0.26 (0.09) and 0.39 (0.11), respectively. Genetic correlations among ultrasonic fat depths at different ages ranged from 0.81-0.95 (SE 0.06-0.12) with higher values for those of closer temporal separation. Phenotypic correlations were consistently about 0.3 below the corresponding genetic correlations. Ultrasonic GR had a high heritability estimate and a high genetic correlation with fat depth at 14 months of 0.84 (0.08),but its genetic correlations with fat depths at earlier ages, were lower ranging from 0.55 to 0.72. The genetic correlation of hogget fleece weight with fat depth at 8 months of age was 0.09 (0.15), and the genetic correlation between 8 month liveweight and fat depth was 0.38 (0.13); similar relationships were found for fat depths measured at other ages. The heritability estimate for muscle depth B was moderate and close to that reported for carcass measurements. In contrast, the estimate for muscle width A was lower than reports for carcass measurements perhaps reflecting limitations in the ultrasonic technology used. The results indicate that ultrasonic measurements are useful selection criteria in breeding programmes to alter carcass traits.
Proceedings of the New Zealand Society of Animal Production, Volume 53, , 299-302, 1993
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