Selection responses in New Zealand Romney sheep

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Abstract
Data from a flock selected for high yearling greasy fleece weight and a randomly selected control group, derived from the same base flock, were analysed. Each flock contained 70-80 mixed-age breeding ewes mated to 4 rams each year. Data from yearlings born in the flocks from 1956 to 1979 were analysed. On average, they represented 8.5 generations. The accumulated selection differential in the selection flock was about 5 kg, and this resulted in a genetic response to selection of 0.7-0.8 kg in yearling greasy fleece weight. Estimates of the realised heritability of greasy fleece weight ranged from O. 10 to 0.17. Several significant correlated responses occurred in the yearling data. Relative to control flock means: clean fleece weight, staple length, mean fibre diameter, clean scoured yield, weaning weight, and liveweight increased; whereas quality number and crimp frequency decreased. Mean date-of-birth became earlier. Lifetime averages of the performance of ewes born between 1956 and 1971 indicated several correlated responses in the mature females. Relative to control group means: greasy fleece weight, clean fleece weight, staple length, clean scoured yield, liveweight, number of lambs born, and number of lambs reared increased; whereas quality number, total crimp number, and crimp frequency decreased.

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