PRELIMINARY-RESULTS ON BETWEEN AND WITHIN TWIN SET VARIATION IN INSULIN-LIKE GROWTH FACTOR-I (IGF-1) AND SOME RELATIONSHIPS WITH PERFORMANCE TRAITS IN IDENTICAL TWIN HEIFERS

Blood samples were collected from eight sets of Simmental crossbred identical twin heifers at 291, 328, 254, 656 and 684 days of age and were assayed for serum concentrations of insulin-like growth factor I (IGF-1). The between twin set intraclass correlation for IGF-1 at these ages was -0.08 +/- 0.38, 0.25 +/- 0.35, 0.62 +/- 0.23, 0.68 +/- 0.21 and 0.54 +/- 0.27, respectively. Residual correlations of serum IGF-1 concentrations with weights taken at the same ages ranged from 0.29 (P > 0.10) to 0.74 (P < 0.01). Heifers with higher serum IGF-1 concentrations at 291, 354, 656 and 684 days of age tended to be taller at 291 and 656 days of age with correlations ranging from 0.25 (P > 0.10) to 0.61 (P < 0.05). Correlations of IGF-1 with weight gains were variable in sign and generally nonsignificant. Results indicate that serum concentrations of IGF-1 are positively related to body weight and hip height of beef heifers.