A NOTE ON CONSEQUENCES OF SINGLE-TRAIT SELECTION FOR INSULIN-LIKE GROWTH-FACTOR-I (IGF-1) IN BEEF HEIFERS

Eight sets of identical twin heifers and their first three calf crops were used to determine effects of single-trait selection for blood ser um insulin-like growth factor 1 (IGF-1). Twin heifers were offspring o f Simmental sires and Hereford X Simmental or Hereford X Angus dams. A ll 16 heifers were mated with the same Simmental bull in each year of the study to produce calves born in 1989, 1990 and 1991 (no. = 16, 13 and 10, respectively). Although differences were noted in performance of progency from the four twins with the highest v. the four with the lowest IGF-1 concentrations, they were not large enough to recommend s ingle-trait selection for blood serum IGF-1 in beef heifers.