Data from a selection experiment for residual feed intake (RFI) were used to estimate genetic correlations between measures of efficiency and performance traits with juvenile IGF-I, and to demonstrate direct and correlated responses to selection. The heritability of IGF-I was 0.28 and genetic correlations of IGF-I with feed intake (0.26), backfat (0.52), and feed conversion ratio (0.78) were moderate to large. The estimated and realized genetic correlations between RFI and IGF-I were 0.63 and 0.84. In contrast, genetic correlations between IGFI and lifetime or test period growth did not differ significantly from zero (0.06 and −0.19). Selection for decreased RFI produced a direct response in RFI, as expected, and was accompanied by downward correlated responses in ADFI, juvenile IGFI, backfat, and growth traits, listed in order of decreasing relative magnitude, and an increased loin muscle area. The correlated response in IGF-I to selection on RFI demonstrates that this physiological measure is genetically associated with efficiency, and is thus useful as an early information source to estimate genetic merit for efficiency before performance testing. Decreased juvenile IGF-I is associated with leaner, more efficient animals.

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