Genetic and phenotypic relationships between insulin-like growth factor-I (IGF-I) and net feed intake, fat, and growth traits in Angus beef cattle

Insulin-like growth factor-I (IGF-I) concentration measured in the blood plasma of 6520 seedstock Angus beef cattle ( 3622 bulls and 2898 heifers) from eastern Australia between 2002 and 2004 was used to estimate the heritability of IGF-I and phenotypic and genetic correlations with net feed intake (NFI) and other production traits. The average concentration of IGF-I was 314 ng/mL measured at the average age of 242 days. A moderate heritability of 0.35 was estimated for IGF-I. IGF-I was further defined as being measured either at, or prior to, weaning ( average age of 201 days) or post-weaning ( average age 310 days). The genetic correlation between IGF-I recorded at the different ages was 1.0 +/- 0.04. IGF-I and NFI were found to have a genetic correlation of 0.41 +/- 0.21. IGF-I had positive genetic correlations of 0.22 +/- 0.14, 0.19 +/- 0.14, and 0.26 +/- 0.15 with ultrasound-scanned subcutaneous fat depth at the rump (P8) and 12/13th rib (RIB) sites and intramuscular fat % ( IMF), respectively. Corresponding phenotypic correlations were 0.14, 0.13, and 0.12, respectively, for P8, RIB, and IMF. IGF-I had low to moderate negative genetic correlations with growth traits. Direct genetic correlations for IGF-I of -0.22 +/- 0.08,-0.17 +/- 0.09 and -0.10 +/- 0.14 were estimated with birth (BWT), 200-day (WT200), and 400-day (WT400) weights, respectively. Genetic correlations between the direct component of IGF-I and maternal components of BWT and WT200 were 0.15 +/- 0.13 and 0.31 +/- 0.11, respectively. Phenotypic correlations of the direct component of IGF-I with the direct components of BWT, WT200, and WT400 were -0.10, 0.06, and 0.16, respectively. Ultrasound-scanned eye muscle area (EMA) and IGF-I had genetic and phenotypic correlations of -0.22 +/- 0.15 and 0.13, respectively. This study showed that IGF-I is heritable and genetically correlated with important production traits. The genetic correlations indicate that selection for lower IGF-I concentrations would result in cattle that have lower NFI (i.e. more feed efficient), are leaner, with increased growth, and possibly decreased maternal weaning weight.