A divergent selection experiment for blood serum IGF-1 concentration has been ongoing since 1989 using 100 spring-calving and 100 fall-calving beef cows. To date, high line (H) sires have averaged 104 + 10 ng/ml more IGF-1 (P < .001) than low line (L) sires and H replacement heifers have averaged 11+3 ng/ml more IGF-I (P < .001) than L replacement heifers. Thus far, realized h2 of IGF-I in the spring- and fall-calving replicates has been .21 and .27, respectively. Means for preweaning and postweaning weights and gains of the most recent H and L progeny were not significantly different. Residual correlations of on-test IGF-I concentrations with preweaning and postweaning weights and gains ranged from .21 (P > .10) to .39 (P < .05). Correlations of IGF-I concentration at d 28 of the postweaning test with postweaning weights ranged from .11 (P < .10) to .18 (P < .05).
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 19. Selection and quantitative genetics; growth; reproduction; lactation; fish; fiber; meat., , 99–102, 1994
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