Genetic analysis of live weight and ultrasonic fat and muscle traits in a hill sheep flock undergoing breed improvement utilizing an embryo transfer programme.

Genetic parameters for pre-weaning live weights and ultrasonic scanning measurements were estimated from a flock of Scottish Blackface sheep undergoing an embryo transfer programme. Maternal environmental effects could be evaluated without confounding with maternal genetic effects because embryos were transferred to unrelated recipient ewes. The data for the study were collected over a 7-year period (1993-1999) and related to a conventional hill farming system. The data were from 1465 lambs, the progeny of 60 sires, 263 donors and 784 recipient ewes. The only exception to the conventional farming system was at mating time when embryos were collected from selected donor ewes, following superovulation, and transferred to unrelated recipient ewes. Maternal environmental effects were important for birth weight (BW), 4-week weight (W4) and weaning weight (WW) but of less importance for ultrasonic fat depth (UFD), muscle depth (UMD) and muscle width (UMW). The heritabilities of the pre-weaning weights were moderate (0.17 to 0.23). The heritabilities of UFD and UMD were 0.44 and 0.27 respectively and were higher than found in previous similar studies. The heritability of UMW was low, 0.06. The genetic and phenotypic correlations among the pre-weaning weight traits were positive and moderate. There was a positive genetic ( + 0.25) and phenotypic ( + 0.24) correlation between UFD and UMD. The phenotypic and genetic correlations between BW and W4 and the scanning traits (UFD, UMD and UMW) were close to zero, and the correlations of WW with UFD, UMD and UMW were positive. The results of this study clearly demonstrate the importance of maternal environmental effects for lamb weights up to weaning and also suggest that genetic improvement for carcass composition in some populations of extensively reared hill sheep could be achieved more rapidly than previously thought possible.