A divergent selection experiment with Romney sheep, using plasma concentration of IGF-1 measured 4 weeks after weaning as the selection criterion, was initiated in 1986. From 111 ram lambs born and reared as twins, 4 with the highest level of IGF-1, 4 with the lowest level and 4 selected at random were used to create three lines (high, low and control). Each ram was randomly mated with 25 ewes. Data from 1986 (number of lambs=111), and two F1 generations (1988, n=241 and 1989, n=151) are presented here. Analysis of variance showed that date of birth, age of dam, rearing rank, and sex significantly contributed to variation in plasma levels of IGF-1. Expression of variation in plasma IGF-1 between years appeared to be influenced by variation in pasture supply. Residual correlations between IGF-1 levels and production traits were positive but of variable magnitude (weaning weight, 0.14 to 0.43; post-weaning growth rate, -0.07 to 0.24; fleeceweight, 0.27). Based on the selection differential generated by initial screening of rams and the average divergency between the high and low lines, realised heritabilities of 0.22 and 0.27 were obtained for plasma IGF-1 concentration in 1988 and 1989, respectively.
Proceedings of the New Zealand Society of Animal Production, Volume 51, , 371-374, 1991
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